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Alhazen's Theory of Vision and Its Reception in the WestAuthor(s): David C. LindbergSource: Isis, Vol. 58, No. 3 (Autumn, 1967), pp. 321-341Published by: on behalf ofThe University of Chicago Press The History of Science SocietyStable URL: http://www.jstor.org/stable/227990Accessed: 26-07-2015 05:13 UTC

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Alhazen s

heory

o

i s i o n

n d t s

Reception

n

t h e W e st

By

David C.

Lindberg*

I

HE

MOST

SERIOUS

problem facing

the Muslim heirs

of

Greek

thought

was

the

extraordinary diversity

of their inheritance.

Among

theories of

optics,

for

instance,

Muslim

thinkers had the

following

choice:

the

emission

theory

of

sight

of

Euclid and

Ptolemy,

which

postulated

visual

rays

emanat-

ing

from the

observer's

eye;

the older

Epicurean

intromission

theory,

which

reversed the

rays

and

made them

corporeal;

the

combined

emission-intromis-

sion

theories

of

Plato

and

Galen;

and

some

enigmatic

statements

of

Aristotle

about

light

as

qualitative

change

in a

medium.1 These

Greek theories

gener-

ated a

wide assortment of

optical

theories

in

Islam,

two

of

which came

to

dominate. Hunain ibn Ishaq (d. 877), the most prolific translator of scientific

works

into

Arabic,

argued

for

a combined emission-intromission

theory

in

the

tradition

of Plato

and Galen.2 Al-Kindi

(d.

c.

873)

agreed

with Hunain

that

rays

are emitted

by

both the

visible

object

and

the

eye, although

he

couched

his

theory

in

terms of a

general

emanation

of

power

having

Stoic

and

Neoplatonic

origins

and

appropriated

the

geometrical

approach

to

optics

appearing

in

the works of

Euclid and

Ptolemy.3

Avicenna

(Ibn

Sina,

d.

1037)

took

exception

to the

views

of

Hunain

and

al-Kindi,

denying

that visual

rays

are

of

any

use in

explaining

the

process

of

sight

and

insisting

on

a

complete

intromission

theory.4

These and other Muslim

philosophers

made

important

*

University

of

Wisconsin.

A

short

version

of

this

paper

was

presented

before a

joint

meet-

ing

of the American

Association

for

the

Ad-

vancement

of

Science

and the

History

of

Science

Society,

December

1966.

The

paper

is a

product

of research

supported by

the

National

Science

Foundation.

1

Obviously

this

attempt

at classification ob-

scures

a host

of

distinctions. On Greek

optics

see

Arthur

Erich

Haas,

"Antike

Lichttheorien,"

Archiv

fiir

Geschichte der

Philosophie,

1907,

20:345-386;

J.

Hirschberg,

"Die

Optik

der alten

Griechen,"

Zeitschrift

fiir

Psychologie

und

Physiologie

der

Sinnesorgane,

1898,

16:321-351;

Albert

Lejeune,

Euclide et

Ptolemee

(Louvain:

Bibliotheque

de

l'Universite,

1948);

Lejeune,

Recherches sur

la

catoptrique grecque (Brus-

sels:

Palais de

Academies,

1957).

2

The Book

of

the

Ten Treatises

of

the

Eye,

ascribed to Hunain ibn

Ishdq

(809-877

A.D.),

trans.

Max

Meyerhof

(Cairo:

Government

Press,

1928),

pp.

31-39.

3

Graziella Federici

Vescovini,

Studi

sulla

prospettiva

medievale

(Turin:

G.

Giappichelli,

1965),

Ch.

3;

Alkindi,

De

aspectibus,

in A.

A.

Bjornbo

and

Sebastian

Vogl,

"Alkindi,

Tideus

und

Pseudo-Euklid,

Drei

optische

Werke,"

Abhandlung zur Geschichte der mathemati-

schen

Wissenschaften,

1912, 26,

Pt.

3.

4

Eilhard

Wiedemann,

"Ibn

Sina's An-

schauung

vom

Sehvorgang,"

Archiv

fiir

die Ge-

321






DAVID C. LINDBERG

contributions

through

their

criticisms of

Greek theories and

their

syntheses

of

disparate

elements of Greek

thought;

moreover,

their

influence on Western

optical

thought

was

far from

negligible.

Yet,

none of them

created an in-

clusive optical system to rival that of Ptolemy; they dealt with but one or

another

aspect

of

sight,

usually

in

the

space

of a

few

paragraphs

or a

few

pages.

The first

comprehensive

and

systematic

alternative to Greek

optical

theo-

ries was

formulated

by

Alhazen

(Ibn

al-Haitham,

d.

c.

1039),

a

figure

of

im-

mense

importance

in

the

history

of

optics.

Alhazen

leveled a

devastating

attack

at

prevailing

optical

theories and formulated a

grand

and

viable

alter-

native. Moreover, he had a

profound

influence on

the West:

his

principal

work on

optics

(Kitab

al-manazir,

cited

by Western

authors

as

De

aspectibus

or

Perspectiva)

5

was

translated

into

Latin late

in

the

twelfth

or

early

in the

thirteenth

century

and

dominated Western

optical thought

until

early

in

the

seventeenth

century.

Modern

optical

thought

issues,

by

direct

descent,

from the

work of Alhazen

and his immediate

followers.

The central feature of Alhazen's

system

is

its

theory

of direct

vision,

and

with

this

topic

the

Perspectiva

opens.6

Alhazen

notes, first,

the

effect

of

bright

lights

on the

eye.

"We

find,"

he

says,

"that when the

eye

looks

into

exceed-

ingly

bright lights,

it suffers

greatly

because of them and

is

injured.

For when

an

observer looks

at the

body

of the

sun,

he

cannot see it

well,

since

his

eye

suffers

pain

because

of

the

light."

7

Clearly

this

implies

an

action of

bright

bodies on the

eye,

for

injury

is

something

inflicted

by

an

agent

on

a

recipient

and could not, in the case of the eye, result from emission of the eye's own

ray.

The

phenomenon

of

the

afterimage

supports

the same

position:

schichte

der

Naturwissenschaften

und der

Technik, 1913,

4:239-241.

This

is

a

German

translation

of a short

work on

physics by

Avi-

cenna.

Avicenna

expresses

similar

views in

many

other

works.

5

The

only printed

edition

of

this work is

Opticae

thesaurus

Alhazeni

Arabis

libri

septem,

nunc

primum

editi a

Federico Risnero

(Basel,

1572);

the title

Opticae

thesaurus

was

given

to

the book

by

Risner.

Manuscripts

of the Arabic

text have

recently

been discovered

(see

Max

Krause,

"Stambuler Handschriften

islamischer

Mathematiker,"

Quellen

und Studien

zur

Geschichte

der

Mathematik,

Astronomie

und

Physik,

1936,

Abt.

B, 3:476,

which

lists

five lo-

cated

in

Istanbul);

an

edition is

being

pre-

pared

by

A. I. Sabra.

My

study

has

been

limited

to the

Latin

text;

but it is

abundantly

clear-

from

a

comparison

of

English

translations

of

the

same

section made

from both

Latin and

Arabic

texts

(Stephen

L.

Polyak,

The Retina

[Chicago:

Univ.

Chicago

Press,

1941], pp.

109-

111),

from

the

recent

study by

Matthias

Schramm (Ibn al-Haythams Weg zur Physik

[Wiesbaden:

Franz

Steiner,

1963]),

and

from

a

comparison

of

the

Opticae

thesaurus with Al-

hazen's shorter tracts

on

optics

translated

from

Arabic

by

Wiedemann, Baarman, Winter,

and

others-that

the

Latin text

faithfully

repro-

duces the

substance of

Alhazen's

ideas. If it

can

be shown that there are

significant

differ-

ences

between

the

Arabic and

Latin

texts,

then

I

must be content with

elucidating

the

Latin

tradition,

which was influential

in

the

West.

I

have

repeatedly

checked the

Risner text

against

earlier

Latin

manuscripts

(British

Museum

Royal

MS

12.G.VII

and

Bruges

MS

512)

and

find no

differences in

substance.

The

best

secondary

works on Alhazen's

op-

tics are Vasco Ronchi, Histoire de la lumiere,

trans.

J.

Taton

(Paris:

Librairie Armand

Colin,

1956),

a

translation of his

Storia

della

luce

(Bo-

logna:

N.

Zanichelli,

1952);

H.

J. J.

Winter,

"The

Optical

Researches of

Ibn

al-Haitham,"

Centaurus,

1954,

3:190-210;

Vescovini,

Studi,

Ch.

7;

Schramm,

Weg

zur

Physik;

and

Leopold

Schnaase,

Die

Optik

Alhazens

(Stargard:

A.

Muller,

1889).

There

is also

a

two-volume

study

by

a

contemporary

Egyptian

physicist,

Mustafa

Nazif,

Ibn

al-Haitham:

His

Optical

Researches

and Discoveries

(Cairo:

Nuri

Press,

1942-1943).

6

I.e.,

the

Latin

text,

which seems to lack

the

first few brief chapters of the Arabic text. Cf.

Eilhard

Wiedemann,

"Zu Ibn

al-Haitams

Op-

tik,"

Arch. Gesch.

Naturw.

Tech.,

1910,

3:4.

7

Opticae

thesaurus,

I,

Sec. 1.

p.

1. All trans-

lations are rendered

from

this edition.

Space

has not

permitted

inclusion of the

Latin text.

322






ALHAZEN'S THEORY

OF

VISION

. . .

when an

observer

looks

at

a

bright

fire

and

allows

it to

linger

in

his

vision for a

long

time,

if

he then

transfers

his

gaze

to a

weakly

illuminated

place,

he

will

[continue

to]

see

the same

thing

[i.e.,

the

brightness].

. . .

Fi-

nally this fades away and vision returns to its normal disposition.8

After

overwhelming

his reader

with

this and similar

data,

Alhazen

concludes,

"All

these

things

indicate that

light

produces

some effect

in the

eye."9

So far Alhazen

has confined

his

argument

to

light

from what

we

would

call

self-luminous

objects.

Such luminous

rays,

filtering

through

mist or

dust,

had been

recognized

since

antiquity,10

but

Alhazen

has more

in

mind than

these. He

argues

that

every

visible

object

is

seen

by

the

emission of its

own

light,

though

illumination

by

a

self-luminous

body

is

a normal

prerequisite:

It

has

been demonstrated above

that

light

issues

in

all

directions

opposite

any

body

that is illuminated with

any light.

Therefore when the

eye

is

oppo-

site

a

visible

object

and the

object

is

illuminated with

light

of

any

sort,

light

comes

to

the

surface of the

eye

from

the

light

of

the visible

object."

However,

an

observer

perceives

the color of the

visible

object

as well as

its

light.

This is a similar

process

and

always

accompanies

the

perception

of

light:

It has

been shown

already

that the

form of

color

of

any

colored

body,

illumi-

nated

by

any

light

whatsoever,

always

accompanies

the

light

emanating

from

that

body

to

any

region

opposite

the

body.

. . .

Therefore the

form of

the

color of a

visible

body always

accompanies

the

light

coming

to

the

eye

from

the

light

of the

body.

And since

light

and color come to the surface of the

eye simultaneously,

the

eye perceives

the color

of

the visible

object

on account

of the

light

coming

to it

from

the

object.

It

is

proper,

therefore,

that

the

eye

should not

perceive

the color of the visible

object except through

the form

of

color

accompanying

the

light

to

the

eye;

and

the form of color is

always

mixed

with the

form

of

light

. .12

Light

and

color,

the first

twenty-two

visible

intentions

identified

by

Al-

hazen,

are

perceived

by

sense alone without

the

support

of

any

process

of

ratiocination. The

remaining twenty

visible

intentions-including

such

things as remoteness, position, shape, magnitude, motion, rest, and beauty -

are

perceived

visually,

but

only by

processes

of

recognition,

distinction,

and

argumentation performed

by

the

virtus distinctiva.

Light

and

color

re-

main

the

primary

visible

intentions,

and the

others

are perceived

through

their mediation.13

Six conditions

must

be fulfilled if the

forms

of

light

and color

(issuing

8

Ibid.

nately; consequently,

I

have

made no

attempt

9

Ibid.

to

distinguish

between them in

my English

translation. In the

passage

quoted,

Alhazen

lOE.g.,

Galen,

De

usu

partium,

X, 12, in speaks

of

light,

but

elsewhere

(e.g., p.

14) he

(Euvres anatomiques, physiologiques et medi- speaks of the form of light

cales de

Galien,

trans. Charles

Daremberg,

Vol.

12

Ibid., p. 7.

I

(Paris:

J.-B.

Baillire,

1854),

p.

639.

13

Ibid.,

II,

Ch.

2, pp.

34 if.

(chapter

desig-

11

Opt.

thes., I,

Sec.

14,

p.

7.

Here and else-

nations

go

back

to

the

Arabic

original;

see

where,

the

Latin

text of

Alhazen's treatise

em- n.

23

below).

On the

meaning

of "intention"

ploys

the terms

lux

and

lumen

indiscrimi-

see

Vescovini,

Studi,

pp.

64-69,

80-85.

323






DAVID C. LINDBERG

from an

object

in

all

directions)

are

to enter the

eye

of the observer and

bring

about

perception:

14

(1)

There must be a certain distance between

the

eye

and

the visible

object.

(2)

The

visible

object

must

be

directly

in

front

of

the

eye,

that

is,

within the visual field.

(3)

The

object

must be either self-

luminous

or

illuminated

by

light

from another

body. (4)

The

visible

object

must

possess

magnitude;

that

is,

lines

drawn

from

the

extremities of the

visible

object

to the

center

of the

eye

must

intercept,

on

the surface of

the

glacial

humor

(crystalline

lens),

"a

segment

of sensible

magnitude by

comparison

with

the

whole surface of

the

glacial

humor."15

(5)

The medium

or

media between the

object

and

the

observer

must

be

transparent.

(6)

The

object

must be

dense and solid.

This

final

requirement

is instructive

regard-

ing

the nature of

light.

In

the

first

place,

the

object

must be dense and solid

because

only

dense and

solid bodies have

color and

part

of

the act

of vision

is

perception

of the

color

of the

body. Secondly,

if

the

body

were not dense

and

solid

(i.e.,

if

it

were

transparent),

the

light

by

which

it

is illuminated

would

pass

through

without

opposition.

In

this

event,

there

would

be no

light

in

the

surface of the

body

capable

of

emanating

its form to

the

observer:

When

there

is a

transparent

body

opposite

the

eye

and it is

illuminated

by

light

from the direction of the

observer,

the

light

passes

through

it

and

is

not

fixed in its

surface;

and

thus in

the surface

of the

body

opposite

the

eye

there

will

be

no

light

from which a

form

can come

to

the

eye.16

Evidently

the

light

(or

the form of

light)

issuing

from

a

nonluminous

body

is

not

its

own but

has

been

deposited

there

by

an

illuminating

body.

The

foregoing

discussion

makes it clear

(1)

that the

forms

of

light

and

color

issue

in all

directions from

self-luminous or illuminated bodies

through

transparent

media and

(2)

that

the

forms

of

light

and color make

an

impres-

sion

on the

eye.

But

it

might

still be

argued

that

the forms of

light

and color

do

not

issue

from the

visible

object

unless

triggered

by

rays emanating

from

the

observer's

eye;

that

is,

one could

still claim

that

visual

rays

play

a neces-

sary

role

in

vision.

In order

to

demonstrate

the

futility

of this

hypothesis

of

visual rays, Alhazen undertakes a long and tightly knit argument. Let us

suppose,

he

says,

"that

rays

issue

from the

eye

and

pass through

the trans-

parent

body

[between

the

eye

and the

object]

to the

object

of

sight

and that

perception

occurs

by

means

of those

rays."

17

Either

these

rays

take

something

from

the

object

and

return

it

to the

eye,

or

they

do not. If

they

do

not,

the

eye

cannot

perceive

the

object

by

means of them. But

this is counter

to the

original supposition

that

rays

issue

from

the

eye

to

perceive

the

object.

Con-

sequently,

it must be concluded

that the

rays

do transmit

something

from

the

object

to the

eye:

14 Opt. thes., I, Ch. 7, pp. 22-23. World, Alhazen embraced the emission theory,

15

Ibid., I,

Sec.

40,

p.

23.

but

I

am

making

no

attempt

in

this

article

to

16

Ibid.,

Sec.

42,

p.

23.

examine the

development

of

Alhazen's

theories;

17

Ibid.,

Sec.

23,

p.

14.

Here

Alhazen assumes

cf.

Eilhard

Wiedemann,

"Zur Geschichte der

that which

he

proposes

to

disprove.

In

his Lehre vom

Sehen,"

Annalen

der

Physik

und

earlier

treatise,

On the

Configuration

of

the

Chemie,

1890,

neue

Folge,

39:473.

324






ALHAZEN'S THEORY

OF

VISION

Therefore

[according

to

the

emission

view]

those

[visual]

rays

that

perceive

the

visible

object

transmit

something

to the

eye,

by

means

of

which

the

eye

perceives

the

object.

And

since the

rays

transmit

something

to

the

eye,

by

means of which the eye perceives the object, the eye perceives the light and

color

in

the visible

object

by

no other means than

through

something coming

to the

eye

from the

light

and color

in

the

object.

..

.18

Thus,

even

in the

emission

theory, sight

is

ultimately

achieved

by

communi-

cation

of

something

from the

object

to

the

eye.

Since it has

already

been

demonstrated that the forms of

light

and

color

emanate

in all directions

from

the

visible

object

without the

hypothesis

of visual

rays,

of what

advantage

are

the

visual

rays?

As

Alhazen

expresses

this

impressive

argument,

. .

.

sight

occurs

only

as

something

of

the visible

object

comes

from the

ob-

ject [to the eye], whether or not rays issue from the eye. Now it has already

been

declared that

sight

is

achieved

only

if

the

body

intermediate between

the

eye

and

the

visible

object

is

transparent,

and it

is not

achieved

if

the

medium is

opaque.

.

. . Since

.

. . the forms of

the

light

and

color

in

the

visible

object

reach

the

eye (if

they

were

[originally] opposite

the

eye),

that

which comes

from

the

visible

object

to the

eye (through

which

the

eye

per-

ceives

the

light

and color

in the visible

object

no

matter

what the situation

[with

respect

to visual

rays])

is

merely

that

form,

whether

or

not

rays

issue

[from

the

eye].

Furthermore,

it

has been

shown that the

forms

of

light

and

color are

always generated

in air and

in

all

transport

bodies

and

are

always

extended

to the

opposite

regions,

whether

or

not the

eye

is

present.

There-

fore the

egress

of

rays

[from

the

eye]

is

superfluous

and useless.19

Not

yet

satisfied that

he has

disposed

of

the

theory

of

visual

rays,

Alhazen

launches

a further attack. If

it

is

assumed,

once

again,

that

sight

is

due to

something issuing

from

the

eye,

either that

thing

is

body

or

it is not.

If it is

body,

it

follows

that when one

looks at

the vault

of

the

heavens,

body

flows

from the

eye

to

fill

the

entire

space

between the

heavens and

the

eye,

yet

without

destroying

or

diminishing

the

eye

in

any

way.

Since

that

is

obviously

impossible,

that which

flows

from the

eye

is not

body.

But

if

that which issues

from

the

eye

is not

body,

it cannot

perceive

the

the

object,

since

"there is

no

perception except in

bodies."20

Thus,

by

means of a reductio ad absurdum,

Alhazen has demonstrated

that a

ray

issuing

from

the

eye

cannot

be

respon-

sible for

sight;

but

he has not demonstrated

that no

ray

issues

from the

eye.

However,

he

concludes

that

if the

rays

are not

responsible

for

sight, they

are not

sensible;

therefore

they

are

conjectural,

"and

nothing ought

to be

believed

except

through

reason or

by

sight."21

Although

it

appears

that

Alhazen

has

completely

discredited the

theory

of visual

rays,

the

obscurity

of the text

has led to

recent

confusion

on

this

18

Opt.

thes., I,

Sec.

23,

p.

14.

require

that

the

ray issuing

from the

eye

be

19Ibid. Note Alhazen's appeal to the prin- material.

ciple

of

economy.

21

".

.

.

et

nihil debet

putari

nisi

per

ra-

20

Ibid. This also

excludes

the

possibility,

tionem vel a

visu"

(ibid.).

The last three words

Alhazen

thinks,

that

something

issuing

from in

this

Latin

text are not found

in

the

Risner

the

eye

could take

something

from the visible edition

but are included

in

British

Museum

object

and return it to

the

eye;

that too would

Royal

MS

12.G.VII,

fol. 7v.

325






DAVID

C.

LINDBERG

point.22

Friedrich

Risner,

editor of

the

only

printed

edition

of Alhazen's

optics

(1572),

divided

the

work into

sections

and

gave

each

a title. Section

24

of Book I

he

entitled:

"Vision

seems

to

occur

through

avvavyetav,

that

is,

rays

simultaneously

received and emitted." 23Alhazen's text

immediately

beneath

this title

begins

as

follows:

It has been asserted

on

account

of this

[i.e.,

the

argument

of the

previous

section]

that

both schools of

thought [presumably

emission

and

intromission]

speak

the

truth

and

that both beliefs

are

correct

and

consistent;

but

one

does

not

suffice

without

the

other,

and

there can

be

no

sight except

through

that

which

is maintained

by

both schools

of

thought.24

A number

of

historians,

deceived

by

Risner's

title,

have concluded

that

in

the

opening

lines

of

Section

24

Alhazen backs

down on

his

denial

of the

existence of visual

rays.25

But,

in

fact,

Alhazen never admits the real existence

of the

rays.

He is

willing

only

to allow

mathematicians,

who

are

concerned

with

a

mathematical

account

of

the

phenomena

rather than with

the

real

nature

of

things,

to use

visual

rays

to

represent

the

geometrical properties

of

sight.

Indeed,

these

rays

or lines

are

indispensable

if

one

is to

understand

how

sight

occurs,

for

through

them one is able

to

visualize

"the nature

of

the

arrangement

according

to which

the

eye

is affected

by

the form

[of

light

or

color]."26

But,

according

to

Alhazen,

all mathematicians

who

postulate

visual

rays

"use

only imaginary

lines

in

their

demonstrations,

and

they

call

them 'radial lines.'

"27

Moreover, the belief "of those who consider radial

22

There was

no

confusion

in medieval

Europe. Roger

Bacon

(Opus

majus,

V,

1,

Dist.

7,

Ch.

3)

and

John

Pecham

(Perspectiva

commu-

nis, I,

Props.

44-46)

were

fully

aware

of Al-

hazen's

uncompromising

opposition

to visual

rays

as

agents

that

go

out

and

seize

something

from the

object

and

convey

it back

to the

eye.

23

"Visio

videtur

fieri

per

avvav,yetav,

id

est

receptos

simul et

emissos

radios"

(Opt.

thes.,

I,

Sec.

24,

p.

15).

Aetius

Placita,

IV,

13,

11

(Her-

mann

Diels,

Doxographi

Graeci

[3rd

ed.,

Berlin:

W. de Gruyter, 1958], p. 404) uses the term

-vva6vyeta

(simultaneous

radiation)

to describe

Plato's double-emission

theory

of

sight (Plato

does

not

use the

term

himself;

see

Haas,

"Licht-

theorien,"

p.

393,

n.

100),

and

it

is

clear

that

Risner

is

comparing

Alhazen's

theory

to

just

such

a double-emission

theory.

It

should be

noted that Risner

added

the

sections,

but the

division of

chapters

goes

back

to

the Arabic

original;

cf.

Risner's

edition

with

the

summary

of

chapters

in the Arabic

text

contained

in Lutfi

M.

Sa'di,

"Ibn-al-Haitham

(Alhazen),

Medieval

Scientist,"

University

of

Michigan Medical Bulletin, 1956, 22, No. 6:258-

259.

24

"Et

declaratum

est ex

hoc,

quod

duae

sec-

tae

dicant verum:

et

quod

duae

opiniones

sint

rectae

et

convenientes:

sed non

completur

al-

tera

earum,

nisi

per

alteram,

neque

potest

esse

visio,

nisi

per

illud,

quod

aggregatur

ex

duabus

sectis"

(Opt.

thes.,

I,

Sec.

24,

p. 15).

The

open-

ing phrase

is

somewhat

ambiguous,

and

the

context

is of little

help.

The

passage

could

equally

well be

translated,

"It has

been

demon-

strated

on

account

of this

. .

,"

which alters

the

meaning

subtly

but

significantly.

The

Arabic text

is

not relevant at

this

point,

since

I

am

concerned

with

the

misleading

character

of Risner's

title and the

Latin text immedi-

ately

following

it.

However,

either translation

is capable of being interpreted as an admission

by

Alhazen that

visual

rays

exist.

25

For

example,

Ronchi

states

that

Alhazen,

"apres

avoir

affirm6

nettement

que

la vision

ne

se fait

pas

au

moyen

de

rayons

emis

par

l'eil,

en

vient

a

une

sorte

de

compromis

et

avance

que

la vision

semble

se faire concur-

remment

par

des

rayons

recus

et

par

des

rayons

6mis"

(Histoire, pp.

39-40).

Following

Ronchi

and

Risner,

I

made

the

same

mistake

in

my

article

"The

Perspectiva

communis of

John

Pecham:

Its

Influence, Sources,

and

Content,"

Archives

internationales

d'histoire

des

sciences,

1965,

18:47-48.

26

Opt.

thes.,

I,

Sec.

24,

p.

15.

27

Ibid.,

Sec.

23,

p.

15.

Cf.

Galen,

De

usu

partium,

X,

12,

in

(Euvres,

trans.

Daremberg,

Vol.

I,

p.

639.

326






ALHAZEN'S

THEORY

OF

VISION

lines

to

be

imaginary

is

true,

and the

belief

of

those

who

suppose

that some-

thing

[actually]

issues from the

eye

is

false."28

Thus

visual

rays

(or

radial

lines)

are mere

geometrical

constructions,

useful

in

demonstrating

the

proper-

ties of sight. They can serve as a mathematical hypothesis, but they have no

physical

existence.

However,

if

these

rays

are

imaginary,

why imagine

them

to issue from

the

eye

rather

than from the

visible

object?

This is

probably

a

concession,

first,

to

traditional

geometrical optics (e.g.,

the

work

of Euclid

and

Ptolemy),

which had

been

combined

with

belief

in

visual

rays;

and sec-

ond,

to

the natural

intelligibility

of a

center

of

perspective

from

which

rays

emanate

to

perceive

visible

things.

An

intromission

theory

of

vision

brings

new

urgency

to the determination

of which ocular

organ

is

the

sensitive one.

Indeed,

the

question

changes

from

"Which

organ

is the source

of

the

rays?"

to "Which

organ

receives

the

rays?";

and this

change

in

question

widens the

scope

of

permissible

answers. In the

visual

ray

theory,

there was little

alternative

to

placing

the source

of

rays

at the center

of the

eye

so

that

the

rays

would

be

unrefracted as

they emerged

and,

consequently,

capable

of

accurately

determining

the

location

of

objects

in

space.29

The intromission

theory,

however,

opens

the

question

to further

investigation.

Nevertheless,

Alhazen's conclusion

was

substantially

the

same

as that

of

antiquity.

Islamic

prohibitions

against

dissection

left Muslim in-

vestigators

with

little choice but to

rely

on

Greek

descriptions

of

the

eye

and

pronouncements

regarding

the sensitive

organ.

Alhazen's

description

of the

eye

varies

only

in minor

details

from the

descriptions

of

Galen and Rufus

of

Ephesus,

and

Alhazen

even

admits that

it

is drawn from earlier anatomical

treatises.30

He

identifies

four

tunics

(consolidativa,

uvea, cornea,

and

aranea)

and three

humors

(aqueous

humor,

vitreous

humor,

and

crystalline lens),31

and

follows

Galen

closely

in

arguing

that

the

glacial

humor

(crystalline

lens)

is

the sensitive

organ:

"If

injury

should

befall

the

glacial

humor,

the

other

tunics

remaining

sound,

sight

is

destroyed;

if the other tunics should be

corrupted,

their

transparency

and the health of the

glacial

humor

being

re-

28

Opt.

thes.,

I,

Sec.

23,

p.

15.

29

Cf.

L'Optique

de Claude

Ptolemee dans

la version latine d'apres I'arabe de l'emir Eu-

gene

de

Sicile,

ed. Albert

Lejeune

(Louvain:

Publications

Universitaires

de

Louvain,

1956),

pp.

148-149.

30

Opt.

thes.,

I, Sec.,

13,

p.

7.

Galen and

Rufus

were the

most

important

ancient

sources

on

the

anatomy

and

physiology

of the

eye,

though

Alhazen's

direct

dependence

on

them cannot be

demonstrated.

The

drawings

of

the

eye

con-

tained

in the Risner edition

(p.

6 of

Alhazen's

Opt.

thes. and

p.

87

of Witelo's

Optica,

bound

with

the

Opt. thes.)

do not

originate

with

Alha-

zen or

Witelo,

but

were taken

from

the

De

cor-

poris humani fabrica (1st ed., 1543) of Andreas

Vesalius;

cf.

J.

Hirschberg,

Geschichte der

Augenheilkunde,

in

Graefe-Saemisch

Hand-

buch der

gesamten

Augenheilkunde,

Vol.

XIII

(Leipzig:

Wilhelm

Engelmann,

1908),

p.

149.

For

a

description

and

reproductions

of

diagrams

taken

from

Arabic

manuscripts

of

Alhazen's

Kitab

al-manizir

and other Islamic

authors,

including one dated 1083 and apparently copied

by

Alhazen's

son-in-law,

see

Polyak,

Retina,

pp.

114-119

and

Figs.

7-12.

31

Galen

(2nd

half

of the

2nd

century)

iden-

tifies the same

three

humors,

but he is

a

bit

ambiguous

on

the tunics.

He

describes seven

tunics,

but

these

appear

to be circles

or

layers

rather

than tunics

in

the

usual

sense.

Cf.

Galen,

De usu

partium,

X,

2,

trans.

Daremberg,

Vol.

I,

pp.

609-614.

On

Galen's

anatomy

of

the

eye

and

theory

of

vision,

see

Hirschberg,

"Die

Optik

der alten

Griechen,"

pp.

347-351;

Polyak,

Retina,

pp.

97-101.

Rufus

of

Ephesus

(1st half of the 2nd century) describes four

tunics,

but

they

do not

correspond

exactly

to

the

four

tunics described

by

Alhazen.

On Ru-

fus,

see

iEuvres de

Rufus

d'Atphese,

ed. and

trans. Charles

Daremberg

and Ch.

Lmile Ruelle

(Paris:

J.

B.

Bailliere,

1879),

pp.

154,

170-172.

327






DAVID C. LINDBERG

tained,

sight

is

not

destroyed."

32

Alhazen

specifies,

further,

that the

sensitive

part

of the

glacial

humor

is its

front

surface,

which

is

concentric

with

the

cornea.33

Thus far a very general account of Alhazen's theory of vision has been

presented:

the

forms of

light

and

color

emanate from

bodies

in all

directions;

they

pass through

the

transparent

cornea

of the

eye

and fall on the front

surface

of the

glacial

humor.

But enormous

obstacles

remain to

be overcome.

Chief of

these is to

show how various

parts

of the same

object

are

distinguished

from

each other.34 The

observer

perceives

not

only

the

presence

of

light

and

color,

but

particular

patterns

of

light

and color.

Clearly,

vision

is more

than

mere

reception

of

forms;

reception

occurs

in

such

a

way

that different

objects

(or

different

parts

of

the

same

object)

perceived

at the

same

time

are

perceived

as

being

distinct

and

in

their true

spatial relationship.

It

appears

as

though

this could be

explained

by

attributing

the

perception

of different

parts

of

the

object,

scattered about

the

visual

field,

to

different

parts

of

the

surface

of the

glacial

humor.

But this raises a

serious

difficulty:

from each

part

of

the

object

the forms

of

light

and color emanate in all

directions;

consequently,

every

part

of

the

glacial

humor

should receive forms of

light

and

color

from

every

part

of

the

object,

and total

confusion should

result.

Alhazen

overcame

the

difficulty by considering

the

visual

field

point

by

point.35

Every

point

on a

visible

object

radiates

the forms of its

light

and

color

in

all

directions,

but

only

the

form

directed

toward the

center

of

curva-

ture of the front surface of the eye is incident on the eye perpendicularly

and enters without

refraction.36

Shifting

to the

language

of

geometrical

lines,

Alhazen

points

out that

from

every

point

on

an

object

there are

infinitely

many

lines

incident

on

the front

of the

eye,

but

only

one

line from

each

point

is incident

perpendicularly

and

is

hence unrefracted. The

forms that

are unrefracted

are most

efficacious

in

vision,

and

refracted forms

yield

only

an indistinct

impression.

By

thus

restricting

himself to

forms or

rays

propa-

gated

rectilinearly,

Alhazen has eliminated all

possibility

of confusion in

the

eye:

there

is

a

single

ray

from

every

point

on the

object,

passing

in

a

straight

line toward

the

center

of the observer's

eye.37

Because

these

rays

are

recti-

32

Opt.

thes.,

I,

Sec.

16,

p.

8.

Cf.

Galen,

De

usu

partium,

X,

3,

trans.

Daremberg,

Vol.

I,

p.

608.

A similar

argument

is

presented

by

Hunain

ibn

Ishaq

(Ten

Treatises

of

the

Eye,

p. 4)

and

by

later authors

such

as Bacon

and

Pecham.

33".

. .

et erit

forma

ordinata,

sicut est

ordinata

in

superficie

rei

visae,

et in

parte

ista

superficiei

glacialis"

(Opt.

thes., I,

Sec.

24,

p.

15).

Cf.

ibid.,

Sec.

40,

p.

23.

"Oportet,

ut cen-

trum

superficiei glacialis

et

centrum

superficiei

visus sint

unum

punctum"

(ibid.,

Sec.

23,

p. 14).

34

In the emission theory, the eye is an

organ

with

directional

sensitivity: rays

issue

forth

in

all

directions,

and the location of

the

object

in

space

is

determined

by

the direction

of

the

ray

terminating

on

it.

In

discarding

the

emission

theory,

Alhazen

gave up

this direc-

tional

sensitivity:

the front

surface

of

the lens

is

sensitive to the

presence

of

light

but

not to

the

direction

from which

it

came.

This

gener-

ates the

problem

of

distinguishing

various

parts

of

the visual

field.

s3

Ibid.,

Sec.

18,

pp.

9-10.

36

It is

uncertain

in this

context whether

"point"

means

"very

small area"

or

"that

which has no

part."

In

Bk.

IV,

Alhazen

argues

that

reflection of a

sensible

ray

must occur

from

a sensible

point, having

a

latitude

equal

to

that

of

the

ray;

although

he

gives

no

indication,

Al-

hazen

might

have considered

the same

analy-

sis applicable in the present case. Cf. ibid., IV,

Sec.

16,

p.

112.

37

Note

that

all

tunics

(or,

more

accurately,

all those before

the

back

surface

of

the lens-

see n.

39)

are

concentric

with

the

cornea,

so

that

rays perpendicular

to the

cornea are

per-

pendicular

to all tunics.

328






ALHAZEN'S

THEORY OF

VISION

linear,

they

maintain a

fixed

order,

and "the

form will be

arranged

on

the

surface

of the

glacial

humor

just

as it is on

the

surface of the visible ob-

ject."38

A

one-to-one

correspondence,

which

insures clear

and

unconfused

perception, has thus been established between points on the object and points

on

the

surface of

the

glacial

humor.39

When the

propagation

of

forms

in

straight

lines

is

expressed

in

geometrical

terms,

one

has

a

pyramid

with

base on

the

visible

object

and vertex

at the

center

of

the

observer's

eye.40

From

each

point

on the

base,

a line

can be

drawn to

the

vertex

in the

eye,

representing

the

path

of

the form

by

which

vision

of that

point

is achieved.

This

pyramid

is a

geometrical representation

of

the

process

of

sight

and aids the

investigator

in

understanding

the

process.41

Alhazen

has

thus

succeeded

in

restoring

the

visual

pyramid

of

Euclidean

and

Ptolemaic

optics

and,

thereby,

the inherent

intelligibility

of

Greek

emission

theories of vision. But,

significantly,

he has done so within an intromis-

sion

framework.

For

the

first

time

an

intromission

theory

of vision

has

be-

come

a

viable

alternative,

adequate

to

compete

on

geometrical

as

well

as

physical

and

physiological

terms

with

the

theory

of visual

rays.

Alhazen's

theory

of

vision,

as

presented

above,

is confined

mostly

to the

first

of

the

seven

books

of

his

Perspectiva.

Book

II contains

his

psychology

of

per-

ception.

Book

III continues

in a

psychological

vein,

dealing

with

the errors of

vision,

including

those

associated

with

binocular

vision. Books IV

and V

are

concerned

with

reflection from

plane

mirrors and curved

mirrors

(both

con-

cave

and

convex)

of

spherical,

conical,

and

cylindrical figure.

In Book VI

Al-

hazen

discusses

the errors

in

perception

resulting

from

vision

by

reflected

rays

(i.e.,

errors

in

number,

location,

and size

of

images).

Book VII

is

devoted

to

refraction

of

rays.

There

is

no doubt

that

Alhazen

contributed

to

geometrical

studies

of

reflection

and

refraction,

but

his

significant

innovations

were

limited

to

his

theory

of

vision.

Although

he

extended

Ptolemy's geometrical

optics

to

new cases

and to

a

higher

level of

sophistication,

it

was

still Ptole-

38

Ibid.,

I,

Sec.

24,

p.

15.

39

In

this

paper

I

have

not

probed

deeply

into Alhazen's

psychology

of

perception.

In

brief, he argues that vision is not completed in

the

glacial

humor

(lens),

but

by

the

virtus dis-

tinctiva

belonging

to the

ultimum

sentiens,

which

is situated

in

the anterior

part

of

the

brain.

The forms

of

light

and

color

penetrate

the

glacial

humor

and,

at

the

interface

separat-

ing

the

glacial

humor and

vitreous humor

(which

is

before

the center

of

the

eye),

are

re-

fracted

away

from

the center

of the

eye

and

never

actually

converge

to a

vertex.

The forms

are

then

conducted

through

the

vitreous

humor

and

hollow

optic

nerve-all the

time maintain-

ing

their

proper

disposition-to

the

optic

chias-

ma, where they join the forms from the other

eye.

They

continue

through

the visual

spirit

to the anterior

part

of the brain.

Cf.

Opt.

thes.,

I,

Ch.

5,

pp.

15

ff.;

II,

Ch.

1,

pp.

24

ff.

For

a

full-length

study

of

Alhazen's

psychology

of

perception,

see Hans

Bauer,

Die

Psychologie

Alhazens

(Beitrdge

zur Geschichte der Philoso-

phie

des

Mittelalters, 1911, 10,

Pt.

5).

The

same

general

scheme

is found

in

the

writings

of

Bacon,

Pecham,

and

Witelo;

on

differences

between the psychology of Alhazen and his

Latin

followers,

see

Vescovini, Studi,

Chs.

4,

7.

40

In

works translated

from

the

Arabic,

the

term

pyramis

is used even

when the

figure

has

a round base and

hence

could

aptly

be

desig-

nated

by

the term

conus. See

Marshall

Clagett,

"The De curvis

superficiebus

Archimenidis: A

Medieval

Commentary

of

Johannes

de Tinemue

on Book

I

of the

De

sphaera

et

cylindro

of

Archimedes,"

Osiris,

1954,

11:298.

41

Opt.

thes., I,

Sec.

24, p.

15.

An earlier

point

can

now

be clarified: Alhazen

is

willing

for mathematicians to talk about imaginary rays

issuing

from

the

eye

in

pyramidal

form

because

if

rays

emanate

from the

center

of the

eye,

the

pyramid

of vision is formed

without further

ado;

there

are

no

rays

not

perpendicular

to

the surface

of the

eye

to

interfere with

the

geometrical

scheme.

329






DAVID C.

LINDBERG

maic

optics

that

he

was

extending.

He

played

the

game

with

more

finesse than

Ptolemy,

but

it

was still

the

same

game.

The

novelty

of Alhazen's

theory

of

vision had no

influence on

traditional

geometrical

optics;

not

only

can

geo-

metrical optics be pursued without commitment to any particular theory of

vision,

but

Alhazen

was

even

willing

to

allow

"mathematicians"

to

continue

to

express

themselves

in

terms of the

discredited emission

theory.

II

The

wave of

translations from

Arabic to

Latin in

the

twelfth and thirteenth

centuries

included Alhazen's Kitab

al-manazir. The name

of

the

translator

and

provenance

of

the

translation

are

unknown,

but the treatise

was

evidently

translated in

the

late twelfth or

early

thirteenth

century.

The earliest

known

Western citation

appears

in a work

of

Jordanus

de

Nemore,42

who flourished

in

the

early

part

of the

thirteenth

century,

but

its

diffusion

in

the first half

of

the thirteenth

century

was not

sufficiently

wide to

have

brought

it to the

attention

of

Robert

Grosseteste,

who wrote

on

optics

in the

first third

of the

century.43

The

full

impact

of Alhazen's

new

optical

theories is first seen

in

the

writings

of

Roger

Bacon,

John

Pecham,

and

Witelo,

all

of whom wrote

on

optics

in

the

1260's

and

1270's.44

The

eagerness

with

which

Alhazen's

optics

was

received

in the

West

was

doubtless

due to its

promise

of

contributing

to

an

already flourishing

optical

tradition. Kept alive (if barely) by the encyclopedic tradition of the early

Middle

Ages, optical

theory

was nourished

dramatically

by early

translations

of scientific

works

into

Latin and

brought

into

prominence

by

Robert Grosse-

teste,

who

revived

optical

and other scientific studies

at Oxford

early

in

the

thirteenth

century.45 Among

the

optical

treatises available to

Bacon, Pecham,

42

See

Marshall

Clagett,

Archimedes

in the

Middle

Ages,

Vol.

I

(Madison:

Univ. Wisconsin

Press,

1964), p.

669.

43

Richard

C.

Dales,

"Robert Grosseteste's

Scientific

Works,"

Isis, 1961,

52:394-402,

dates

Grosseteste's optical works between 1231 and

1235.

Occasionally

al-Bitriji (d. early

13th cen-

tury)

is

quoted

as

having

asserted

that Alhazen's

Perspectiva

was

circulating

in the West

during

his

lifetime:

"Nam

licet

perspectiva

Alhacen

sit

in

usu

aliquorum

sapientium

Latino-

rum.

.

... (Cf.

Lucien

Leclerc,

Histoire

de la

medecine

Arabe,

Vol.

II

[Paris:

E.

Leroux,

1876],

p.

516.)

However,

this is

not a

quotation

from

al-Bitrfji,

but

from a

fragment

of the

Opus

tertium of

Roger

Bacon,

which

(in

Paris,

Bibliotheque

Nationale,

Latin MS

10264,

fol.

186)

is

wrongly

entitled

Liber tertius

Alpetra-

gii; cf. Un fragment inedit de l'Opus tertium

de

Roger

Bacon,

ed. Pierre Duhem

(Quaracchi:

Collegium

S.

Bonaventurae,

1909),

p.

75.

44

On medieval

Western

optics

in

general,

see

A. C.

Crombie,

Robert Grosseteste and

the

Ori-

gins

of

Experimental

Science,

1100-1700

(Ox-

ford: Clarendon

Press,

1953),

and

Vescovini,

Studi.

On

Pecham,

see

my

"The

Perspectiva

communis of

John

Pecham."

All

texts

and

translations

from Pecham's

Perspectiva

com-

munis are

drawn

from

my forthcoming

edition

(Univ.

Wisconsin

Press).

The

best sources

on

Bacon's

optics

are in

Roger

Bacon

Essays,

ed.

A. G. Little

(Oxford:

Clarendon

Press,

1914),

but further work

is

in

order.

However,

Bacon's

theories are

readily

accessible

in

the

English

translation

of

his

Opus

majus

(The

Opus

Ma-

jus

of

Roger

Bacon,

trans. R. B.

Burke,

Phila-

delphia:

Univ.

Pennsylvania

Press,

1928).

On

Witelo,

see

Clemens

Baeumker,

Witelo,

ein

Phi-

losoph

und

Naturforscher

der XIII.

Jahrhun-

derts

(Beitrige

zur

Geschichte

der

Philosophie

des

Mittelalters, 1908,

3,

Pt.

2);

Baeumker

bases

some

of his

argument

on the

conclusion

that

Witelo was

the

author

of

De

intelligentiis,

a

conclusion now clearly recognized as false. On

Witelo,

see

also

my

introduction

to

a forthcom-

ing

facsimile

reprint

of

the Risner edition

(New

York:

Johnson

Reprint,

Sources of

Science).

45

On Grosseteste's

sources,

see

Crombie,

Grosseteste,

pp.

116-117.

330






ALHAZEN'S THEORY OF VISION

and

Witelo were

works

by

Greek,

Muslim,

and Latin authors.

There

were,

of

course,

Alhazen

and

Grosseteste.

In addition to

these

two,

Pecham

cites

al-Kindi, Aristotle,

the

pseudo-Euclidean

Catoptrica,

and a medieval

abridge-

ment of Euclid's Optica; he appears to have known, also, Alhazen's De specu-

lis comburentibus

(a

short work

distinct

from the

Perspectiva),

and he

may

have used

Ptolemy's

Optica.46

Bacon cites

all

these on

optics,

as

well as

Avi-

cenna,

Averroes, Tideus,

Constantinus

Africanus,

and

Augustine.

Though

but one of

many

authorities,

Alhazen

exerted

by

far

the

dominant

influence. Bacon

continually

cites him

by

name,

and

Pecham

and Witelo

consciously

patterned

their

major optical

works

after his

Perspectiva,

respec-

tively

condensing

and

expanding

its treatment.

Pecham

continually

bows

to

the

authority

of

Alhazen,

whom he

cites

as

"the Author"

or "the

Physicist";

and

Risner,

publisher

of

the

printed

works

of Alhazen

(Latin

text)

and Witelo

in a

single

volume,

has indicated their close

relationship

by

elaborate cross-

references. But

aside from

citations

and

format,

the theories of vision

ex-

pressed

by

Bacon,

Pecham,

and

Witelo

are

essentially

the

same as

Alhazen's.47

All

describe

the

anatomy

of the

eye similarly,

with

only

small

differences

in

detail.

Vision,

according

to

Bacon,

Pecham,

and

Witelo,

occurs

through rays

issuing

from

the

visible

object

and

falling

perpendicularly

on

the

surface

of

the

eye

and

the

glacial

humor.

Nonperpendicular

rays

are

refracted

and con-

tribute to vision

only

incidentally. Through

the

visual

pyramid,

consisting

solely

of

perpendicular

rays issuing

from the

object

and

converging

toward

a

vertex

at the center

of

the

eye,48

the

forms of the

light

and color of the

ob-

ject

are

arranged

on

the

surface

of

the

glacial

humor

precisely

as

on

the

sur-

face

of

the

object; consequently

a

one-to-one

correspondence

is

established,

which insures

clarity

of vision. In

order to

establish this

theory

of

vision,

Bacon, Pecham,

and

Witelo even

rely

on the

same

evidence and

the same

arguments

as

Alhazen. All

four,

for

example,

cite the

pain

experienced

by

the

eye

in

looking

at

bright lights

as

evidence that

light

and

color make some

kind

of

impression

on

the

eye,

and

Pecham

(like Alhazen)

begins

his

treatise with

a

description

of such

evidence.49

It

should

be

evident,

then-and historians

have

long agreed

on this-that

the main outlines of Alhazen's theory of sight, as well as his more abstract

geometry

of

image

formation

by

reflection and

refraction

and

many

minor

details,

were

incorporated

in

the

optical

works of

Bacon,

Pecham,

and Witelo.

Alhazen's

theory

was

comprehensive

and

systematic;

it

was

superior

in

almost

every respect

to

anything

the West

had

known

before.

To

some extent it also

46

For a

fuller

discussion of Pecham's

sources,

away

from the

center of the

eye

at

the

interface

see

my

edition

of

the

Perspectiva

communis.

between the

glacial

humor and

vitreous humor.

47

Crombie,

Grosseteste,

gives

Bacon,

Pecham,

This

prevents

inversion

or reversal of the

forms

and Witelo far

too much

credit

for

experi-

as

they

are

conducted

through

the

optic

nerves;

mental

or observational

prowess

and attaches

cf.

n.

39.

too

much

significance

to Grosseteste's influence

49

Perspectiva

communis, I,

Prop.

1. On

Al-

on their

optical

theories. For the most

part,

hazen, see above.

Cf.

Bacon, The

Opus

Majus

Bacon, Pecham,

and Witelo did not

observe,

of

Roger

Bacon,

ed.

J.

H.

Bridges (London:

experiment,

or read

Grosseteste;

they

read Al-

Williams and

Norgate,

1900),

V,

1,

Dist.

5,

Ch.

1,

hazen

and

other Islamic

and Greek

authorities.

Vol.

II,

pp.

30-32;Witelo,

Opticae

libri

decem,

48

As in

Alhazen's

theory,

the

rays

do not

ed.

Risner

(bound

with

Opt.

thes.),

III,

pp.

actually converge

to

a

vertex,

but are

refracted

87-88,

91.

331






DAVID C.

LINDBERG

filled

an

intellectual

void;

the works of

Aristotle, Euclid,

Ptolemy,

al-Kindi,

and

Grosseteste

said little about the anatomical

and

physiological

details

of

sight.

These

treatises dealt

primarily

with

geometrical optics,

on

which

they

were in substantial agreement with Alhazen. Conflict was therefore at a mini-

mum.

Where earlier

authorities had

spoken,

Alhazen

agreed;

where

they

had

been

silent,

he

provided

a

theory

that was

overwhelmingly

systematic

and

comprehensive.

Nevertheless,

strong

reservations have been

expressed

about the assimila-

tion of

Alhazen's

optics

in

the

West,

particularly

with

regard

to

the

properties

of

the

emanated

entity.

Vasco

Ronchi,

one of the

most articulate and

prolific

writers

on

the

history

of

optics

in

recent

years,

has

repeatedly

characterized

medieval Western

optics

as a

corruption

of Muslim

views and a return

to the

less

satisfactory

theories

of

Greek

antiquity.

According

to

Ronchi,

Alhazen

had set the

theory

of

sight

on the road

leading

to

Kepler

and modern

optics

by

decomposing

"l'objet

visible en

elements

punctiformes,

ce

qui

faisait

perdre

a

la

vision

de

l'objet

lui-meme le caractere d'une

operation

globale

qu'on

ne

pouvait

scinder."

50

By treating

the

object

a

point

at a

time,

Alhazen

had

escaped

the

absurdities of

Epicurean

eidola,

while

still

operating

within

an intromission

framework.

"Les

'eidola' et les

'ecorces,'"

Ronchi

writes,

"sont mortes."

51

And

so

they

were. The

question

is whether

they

stayed

dead.

Ronchi thinks

not.

Eidola and

rinds

reappeared,

he

insists,

in

the

"species"

of

Western

optics.

Scholastic

scientists,

with a few

exceptions, attempted

to

reconcile eidola

(poorly disguised

in

new

terminology)

with

Alhazen's revo-

lutionary

contributions. The

result,

Ronchi

insists,

was

"une

construction

grotesque."52

Simply put,

Ronchi

feels that

species

are eidola.

He sums

up

this

viewpoint

most

forcefully

when,

after

describing

the

diffusion

of

Al-

hazen's ideas and

the

inability

of Western

scientists to

understand

them,

he

writes,

What ensued was

an

indescribable

atrophy

of

thought.

Ideas tended

more

or

less

to cluster about

the

doctrine

of

species,

a

new edition of

the ancient

eidola. These

species,

however,

were

produced

by

the

lumen,

when

it im-

pinged upon

a

body,

and

they

moved

along

the observer's visual

rays

as

though along rails guiding them toward the eyes. During this motion they

contracted

in

order

to

be able

to

enter the

pupil.

The contraction no

longer

constituted

a serious

obstacle

[as

it had

in

ancient

times]

because

Ibn al-

Haitham's

mechanism had

provided

a sort

of

justification

for it.

In

other

words,

an effort

was made

to

combine the

classical

with

the

new.

The

merger

was a

monstrosity,

with which the

philosophers

and

mathema-

ticians

of

the later Middle

Ages

tried

to

reason when confronted

by

optical

problems.53

An

important question

remains: To

precisely

which

Western

thinkers

is

Ronchi's

analysis

to be

applied? According

to

the

most

sympathetic

interpre-

tation, Ronchi recognizes only three Western philosophers who assimilated

Alhazen's

revolutionary

theories

regarding

the

process

of

sight:

Bacon,

Pe-

50

Ronchi, Histoire,

p.

38.

53

Ronchi,

Optics,

The

Science

of

Vision,

51

Ibid.,

p.

37.

On

eidola,

see below.

trans.

Edward

Rosen

(New

York: New

York

52

Ibid.,

p.

55.

Univ.

Press,

1957), p.

32.

332






ALHAZEN'S

THEORY OF

VISION

cham,

and Witelo. On the

other

hand,

he

singles

out Dante

Alighieri

(d.

1321),

Gregorius

Reisch

(d.

1525),

and

Giambattista

della Porta

(d.

1615)

as

examples

of men

who

failed to

grasp

Alhazen's

theory,

and

from

an examina-

tion of their works purports to demonstrate the universal (or almost uni-

versal)

failure

of scientists of

the

late Middle

Ages

to

understand

or

adopt

Alhazen's

important

innovations.54

In

Ronchi's

view,

then,

Alhazen's ideas

were assimilated

by

Bacon, Pecham,

and Witelo

in the thirteenth

century,

but

afterwards

forgotten,

rejected,

or misunderstood

until the time of Mau-

rolycus

and

Kepler

at

the end

of

the

sixteenth

century.

But

Ronchi has

often been

misleading,

and this

"sympathetic

interpreta-

tion" of

Ronchi's views

does not

jump

out at

the

reader of his works.

In

his

main

work

on

the

history

of

optics

Ronchi

gives

not the

slightest

indication

that Bacon and Pecham

had

anything

to do

with the

diffusion of Alhazen's

ideas.55 Moreover, Ronchi's condemnation of the idea of

species

is

sweeping;

in no

work does he admit

any

exceptions

to the

grotesqueness

or

acknowledge

any

varieties of the idea.

Thus

he

leaves the

clear

impression

that

the

species

concept

in all its

forms was a

departure

from the

teaching

of

Alhazen;

and

this

impression

is

reinforced

by

the

fact that

in

most of

Ronchi's works

only

Witelo,

who

never

used the

term

"species,"

is

recognized

as a

faithful follower

of

Alhazen.56

Finally,

in

his

most

explicit

statement of the

period

during

which

this

"atrophy

of

thought"

occurred,

Ronchi

writes,

...

pendant

quatre

ou

cinq

siecles

les

idees

d'Alhazen

n'ont

eu aucune

consequence appreciable. Si, pendant ce laps de temps, on recherche dans

les

oeuvres le

plus

remarquables

du monde

occidental

quelles

etaient

les idees

predominantes

au

sujet

de la

lumiere,

on

retrouve celles de la

period

grecque.57

Since

the

light

of

understanding

dawns

again

toward

the end

of

the

sixteenth

century

with

the

printing

of Alhazen's

Perspectiva,58

four or

five centuries

take one back

to the

eleventh

or twelfth

century-before

the

translation of

Alhazen's

work

into Latin.

Consequently,

the

passage

must

be

interpreted

as

asserting

either

that

Alhazen's

theory

was

not

adopted

by

Bacon, Pecham,

54

Histoire,

pp.

45-49,

57-73.

These

three

men are hardly the best representatives (with

the

possible

exception

of della

Porta)

of medie-

val

or even late-medieval Western

optics.

Against

Ronchi one

might

even

argue

that

since

Dante, Reisch,

and

della

Porta failed

to

grasp

Alhazen's

theories,

they

could

not

have

been serious students

of

optics.

At

any

rate,

Ronchi's thesis would be more

impressive

if

it

could

be

demonstrated

with

respect

to

such

competent

natural

philosophers

and

optical

theorists as Theodoric of

Freiberg,

Blasius of

Parma,

and Friedrich

Risner.

55

Ronchi's main work on the

history

of

optics

is Storia della

luce,

translated

into

French

as

Histoire

de

la

lumiere.

The

only

work

(to

my knowledge)

in which

Ronchi

mentions

Bacon and

Pecham as followers of

Alhazen

is

his

Optics,

The

Science

of

Vision,

p.

31,

and

there

they

receive but two

sentences.

56

Histoire,

pp.

44-45;

Ronchi,

"Sul con-

tributo di Ibn-al-Haitham alle teorie della vi-

sione e

della

luce,"

in

Actes du VIIIe

Congres

International d'Histoire

des Sciences

(Jerusalem,

1953),

p.

520.

57

Histoire,

p.

45.

58

Ronchi

(ibid.)

asserts that

Risner,

publisher

of the

1572

edition,

was

also its

translator

from

Arabic

to

Latin.

Elsewhere

he

identifies

Witelo

as

the

translator

("Complexities,

Advances,

and

Misconceptions

in the

Development

of the

Science

of

Vision: What is

being

Discovered?"

in

Scientific

Change,

ed. A.

C.

Crombie [New

York:

Basic

Books, 1963],

p. 546).

Actually,

neither is

correct. The

book was rendered

into

Latin

by

an unknown

translator late

in

the

12th

or

early

in the

13th

century,

more

likely

the

latter.

Risner

merely

edited

the medieval

trans-

lation,

as he

relates

in

the

preface

to

his

edi-

tion;

Witelo

had

nothing

whatsoever to

do

with

it.

333






DAVID C. LINDBERG

and

Witelo or

that

its

incorporation

in

their works was not an

"appreciable

consequence."59

But,

in

fact,

Bacon,

Pecham,

and

Witelo

fully

assimilated

the

optics

of

Alhazen, and their works had a very appreciable influence on succeeding

generations.

The

unusually

large

number

of

manuscripts

and

printed

editions

of

Pecham's

Perspectiva

communis

(as

least

47

extant

manuscripts

and

10

printings)

and

Witelo's

Perspectiva (numerous

manuscripts

and 3

printings)

testifies to

the diffusion

of Alhazen's ideas

throughout

the later Middle

Ages.

Nor were these

a matter

of

reproduction

without

understanding;

for

example,

the

1542

edition

of Pecham's

Perspectiva

communis was

altered

extensively

by

its

editor,

the

mathematician and instrument-maker

George

Hartmann

(1489-1564),

with

no

distortion of the essential ideas.

Moreover,

Pecham's

Perspectiva

communis

was the

subject

of a

number

of

commentaries,

the

authors of which did not fail to understand the contents.60 Eventually Pe-

cham's book became the standard

optical

text

in the

medieval

university,

so

that even

an

elementary

education

in

optics

provided

an

understanding

of

Alhazen's

theories. If late

medieval

writers on

optics

did not

preserve

Al-

hazen's ideas on vision in

their

works,

it

was not because

they

failed

to

comprehend,

but because

they

were

asking

different

questions.6'

Nevertheless,

Ronchi's

work has

raised

the

question

of

the

relationship

of

medieval

species

to

Alhazen's

forms and

Epicurean

eidola;

and

if

I

disagree

with

Ronchi's

answer,

I

at least endorse his

question,

for

it

is most

important,

deserving

a detailed

and

unambiguous

answer.

I

will devote the

remainder

of this essay to it, restricting myself for the present to the concept of species

in

its

thirteenth-century

form;

62

it will

be

my

thesis

that from an

optical

standpoint

the Western

idea

of

species

is

indistinguishable

(with

one

impor-

tant

exception)

from Alhazen's

concept

of form.

Since

the

standard of

com-

parison,

as Ronchi has

posed

the

problem,

is the

Epicurean

eidolon,

let

us

first

consider

the

latter.

Epicurus

describes eidola

in his Letter to Herodotus:

"particles

are

con-

tinually

streaming

off

from the surface

of bodies.

. . . And those

given

off,

for

a

long

time

retain

the

position

and

arrangement

which their atoms

had

when

they

formed

part

of the solid bodies."63

Sight

occurs as these

eidola

enter

the

observer's

eye:

We

must

also consider that

it

is

by

the entrance of

something

coming

from

external

objects

that we

see their

shapes

and

think of

them. For external

59

Ronchi

has

already

admitted,

on the same

larizers

or amateurs like Dante and Reisch.

For

page,

that

Witelo

faithfully

followed

Alhazen.

example,

Theodoric

of

Freiberg,

in his

De

luce,

However,

more

recently

Ronchi has

written

was concerned not

with

geometrical

optics

but

that

"at

the

end

of

the

thirteenth

century

with

a causal account

of

light;

cf.

William

A.

A.D

. .

.

[the

work

of

Ibn

al-Haitham]

had as

Wallace,

O.P.,

The

Scientific

Methodology

of

yet

had

no

impact

on the

Western

world" Theodoric

of

Freiberg (Fribourg,

Switzerland:

(Scientific

Change, p. 545).

Univ.

Press,

1959), pp.

152-161.

60For a discussion of the medieval commen-

6

I plan to deal with the later development

taries

on the

Perspectiva

communis,

see

my

A

*

v

aries

onhe

Perspectiva

communis,

see

my

of

the

concept

of

species in

a

future

paper.

edition,

n.

44 above.

The

influence

of Al-

hazen's

Perspectiva

is

explored

in more detail

63

Diogenes

Laertius,

Lives

of

Eminent

Phi-

in

my

introduction

to the

forthcoming

reprint

losophers,

X,

48,

trans. R. D. Hicks

(London:

of the

Risner

edition. William

Heinemann,

1925),

Vol.

II,

pp.

577-

61

I

have

in

mind

serious

scientists,

not

popu-

579.

334






ALHAZEN'S THEORY OF

VISION

things

would

not

stamp

on us

their own nature

of

colour

and

form

through

the

medium of

the

air

which is

between them and

us,

or

by

means

of

rays

of

light

or

currents of

any

sort

going

from

us

to

them,

so

well as

by

the entrance

into our

eyes

or

minds, to whichever their size is suitable, of certain films

coming

from

the

things

themselves,

these films

or

outlines

being

of the same

colour and

shape

as the external

things

themselves.64

Thus eidola

(material images

or

skins)

are

stripped

from the outer surfaces

of

objects

and

propagated

through

space

as

coherent

units.

Because

they

are

of the

same

shape

and color

as

the

object,

they

faithfully

communicate

the

latter

to the

observer. But

the

fatal

objection

to

the

Epicurean

theory,

be-

sides

its

generally

simplistic

character,

is the

problem

of

shrinking

the

eidola

of

large

objects,

so

as

to

squeeze

them

into the observer's

eye,

without

altering

them in

shape

or

color-and this for all

possible

distances between

the

object

and the

observer.

Now

it

is

clear that

Alhazen's forms

have

little in common

with

Epicurean

eidola. In

the first

place,

Alhazen is

willing

to consider the form

of each

point

(or

small

part)

of the

object independently:

he

refers,

for

example,

to

"the

form of the

light

and

color

that

comes

from

any

point

of

the

visible

object

to the surface

of

the

eye."

65

Since

he is

dealing

with

an

image

of

each

point

rather than a

single

replica

of the

whole

object,

squeezing

images through

the

pupil

is no

problem. Secondly,

Alhazen's

forms

are

not

images

in the

Epicurean

sense.

They

do

not

consist of

pieces

of the

object;

they

are not

replicas. Rather they are powers representative of the object, capable of

producing

effects

in a

recipient.

However,

noting

Alhazen's

explanation

of

reflection on

the

analogy

of mechanical

rebound,

Ronchi

writes,

"The

idea

that the

rays

of lumen are

the

trajectories

of minute material

corpuscles

is

already

expressed

in

his

work."

66

But

this

is to misunderstand

Alhazen. The

analogy

of

mechanical rebound is meant

to elucidate the

equal angles

of

reflection,

not

the nature of the

reflected

entity.

Schramm,

basing

his

argu-

ment

on a

study

of both

the

Latin and

Arabic

texts

of the

Kitab

al-manazir,

has concluded

that

Alhazen's

form is not

a

three-dimensional

body,

but rather

that which is

impressed

on a

body through qualitative change.67

Form for

Alhazen is thus

very

close

to

Aristotelian

form.

The

concept

of

species,

as

applied

to

optics,

had

its

origin

in

the

Neopla-

tonic

doctrine

of emanation. Plotinus

(d.

270

A.D.)

maintained

that causation

64

Ibid., X,

49,

Vol.

II,

p.

579.

For a

good

p.

112.

Alhazen also elucidates

refraction

by

account

of the

Epicurean theory

of

vision,

see

means of mechanical

analogies

(ibid.,

VII,

Sec.

Cyril Bailey,

The

Greek

Atomists

and

Epicurus

8,

p. 241).

(Oxford:

Clarendon

Press,

1928),

pp.

406-413;

67

Schramm,

Weg

zur

Physik, p.

216.

A.

I.

cf.

Haas, "Lichttheorien,"

pp.

362-370.

Sabra

writes,

".

.

he

[Alhazen]

denies

that

65

Opt.

thes.,

I,

Sec.

18,

p.

9.

Alhazen also

light

is a

body

.

.

."

("Explanation

of

Optical

speaks

of

a

single

form for

the

whole

object

Reflection

and

Refraction:

Ibn-al-Haytham,

on occasion. Descartes, Newton," Actes du dixieme congres,

66

Optics,

Science

of

Vision,

p.

30.

Cf.

Ron-

Vol.

I,

p. 551).

Cf.

J.

Baarman,

"Abhandlung

chi,

Histoire,

p.

42,

and

Ronchi,

"The Evolu- iiber

das

Licht

von

Ibn

al-Haitham,"

Zeit-

tion

of the

Meaning

of

'Light'

in

Natural

Phi-

schrift

der Deutschen

Morgenldndischen

Ge-

losophy,"

in Actes

du

dixieme

congres

interna-

sellschaft,

1882, 36:197-199;

this is the

Arabic

tional d'histoire

des

sciences

(Ithaca, 1962),

Vol.

text and German translation

of

Alhazen's

short

II,

p.

725.

Cf.

Alhazen,

Opt.

thes., IV,

Sec.

18,

treatise

On

Light.

335






DAVID

C.

LINDBERG

may

occur

through

a

process

of emanation.

Indeed,

all

things

tend to emanate

their

power

outside

themselves.

In

The Enneads

Plotinus

writes,

All

existences,

as

long

as

they

retain their

character, produce-about

them-

selves,

from their

essence,

in

virtue

of

the

power

which

must be

in

them-some

necessary,

outward-facing

hypostasis

continuously

attached

to

them and

rep-

resenting

in

image

the

engendering archetypes:

thus

fire

gives

out

its

heat;

snow is

cold

not

merely

to

itself;

fragrant

substances

are

a notable

instance;

for,

as

long

as

they

last,

something

is diffused

from them

and

perceived

where-

ever

they

are

present.68

Light

emanating

from

a

luminous

body

is another

example

of

the same

effect.

In

the

same

vein,

Avicebron

(d.

c.

1058),

whose Fons

vitae was available to

Grosseteste

and other

thirteenth-century

writers,

argues

that

powers

and

rays

emanate from all simple substances on the analogy of the emanation of light

from

the sun: "The

essences of

simple

substances

do

not issue

forth;

it is

rather their

powers

and

rays

that

flow forth

and

spread

abroad. . . .

Just

as

light

flows

from

the sun into

the

air,

. . .

so

every

simple

substance extends

its

ray

and its

light

and

diffuses them into

that which

is

inferior.

".

.

69

These

ideas were

developed

by

Grosseteste into

the

doctrine of the multi-

plication

of

species.

As he

expresses

this

doctrine

in

his

De

lineis,

angulis

et

figuris,

A natural

agent

propagates

its

power

from

itself

to

the

recipient,

whether it

acts on the senses or on matter. This power is sometimes called species, some-

times a

similitude,

and is

the

same whatever it

may

be

called;

and it

will send

the same

power

into

the

senses

and into

matter,

or

into its

contrary,

as

heat

sends the same

thing

into

the

sense of

touch

and

into

a cold

body.

For

it

does

not act

by

deliberation

and

choice,

and

therefore it

acts

in

one

way,

whatever

it

may

meet,

whether

something

with sense

perception

or

some-

thing

without

it,

whether

something

animate or

inanimate. But the

effects

are

diversified

according

to

the

diversity

of

the

recipient.70

Every

natural

agent

propagates

its

power

from

itself

to

surrounding

bodies,

and this

power

is called

"species"

because the

effects bear

a

resemblance to

the

agent.

Hot bodies, for

example,

emanate

species

that

produce

heat in the

recipient,

and

bright

bodies

emanate

species

that

produce brightness

in

the

observer's

eye.71

Roger

Bacon

and

John

Pecham

adopted

Grosseteste's doctrine

of

the multi-

68

Plotinus,

The

Enneads, V, 1, 6,

trans.

71

As

Grosseteste

points

out

in

the

passage

Stephen

MacKenna

(2nd

ed.,

London:

Faber

quoted,

the effects do

not

always

resemble the

and

Faber,

1956),

p.

374.

agent;

but this is

an

insight

not

possessed

by

69

I

have

translated

this from the Latin text

all

writers on the

subject,

and

even

in

Grosse-

in Avencebrolis

(Ibn Gebirol),

Fons

vitae, III,

teste the

effects

are never

totally

unlike the

52,

ed. Clemens

Baeumker

(Beitrdge

zur Ge-

agent.

Bacon insists that

"haec

species

sit similis

schichte der Philosophie des Mittelalters, 1895, agenti" (De multiplicatio specierum, ed. J. H.

1), p.

196. Similar ideas

on the

multiplication

Bridges,

bound with the

Opus

majus,

Vol.

II,

of

power

had

been

expressed by

al-Kindi,

and

p.

410).

Cf.

Avicebron,

Fons

vitae,

III, 53;

these

may

have been

known to

Grosseteste as

Charles K.

McKeon,

A

Study

of

the Summa

well;

cf.

Vescovini, Studi,

Ch.

3.

philosophiae

of

the

Pseudo-Grosseteste

(New

70

Quoted

by

Crombie, Grosseteste,

p.

110.

York: Columbia Univ.

Press,

1948),

pp.

87,

94.

336






ALHAZEN'S THEORY

OF

VISION

plication

of

species

and

incorporated

it

in

their theories

of vision drawn from

Alhazen.

Bacon,

the more

vocal

on the

subject

of

species, explains,

For

every

efficient

cause

acts

through

its own

power,

which

it

exercises

on

the

adjacent

matter,

as

the

light

[lux]

of

the

sun

exercises

its

power

on the

air

(which

power

is

light

[lumen]

diffused

through

the

whole world from

the

solar

light

[lux]).

And

this

power

is called

"likeness,"

"image,"

and

"species"

and is

designated

by many

other

names,

and it is

produced

by

substance

as

well

as

accident.

. .

.

This

species

produces

every

action

in

this

world,

for

it

acts

on

sense,

on the

intellect,

and

on

all

the matter

of

the world

for

the

generation

of

things.72

Pecham's view is similar:

Every

natural

body,

visible or

invisible,

diffuses

its

power

radiantly

into

other

bodies. The proof of this is by a natural cause, for a natural body acts outside

itself

through

the

multiplication

of its

form.

Therefore the nobler

it

is,

the

more

strongly

it acts. And

since

action

in

a

straight

line

is easier

and

stronger

for

nature,

every

natural

body,

whether visible

or

not,

must

multiply

its

species

in

a

continuous

straight

line;

and this is

to

radiate.73

Now

the central

question

of this

essay

is

whether or

not

this

concept

of

species

was

"a

new

edition

of

the

ancient

eidola." The answer

is

clear-no

As

these

quotations

from

Bacon and Pecham

indicate,

species

are not material

replicas,

films

of

matter

peeled

from

the outer surface

of

the

object

and

pro-

pelled through space, but powers or forms diffused from one point to another

through

the matter

already

there.

The actual mode of

multiplication

or

propagation

of

species

is

very

clearly

described

by

Bacon:

But a

species

is

not

body,

nor is it

moved as a whole

from one

place

to an-

other;

but that which

is

produced

[by

an

object]

in the first

part

of

the

air

is not

separated

from that

part,

since

form cannot

be

separated

from the

matter

in which

it is

unless

it

should

be

soul; rather,

it

produces

a likeness

to itself

in the

second

part

of the

air,

and

so

on.

Therefore

there

is no

change

of

place,

but a

generation multiplied

through

the different

parts

of

the me-

dium;

nor

is

it

body

which

is

generated

there,

but

a

corporeal

form;

. . . and

it is not produced by a flow from the luminous body, but by a drawing forth

out

of the

potentiality

of

the

matter

of

the

air.74

The

multiplication

of

species

is more like the

propagation

of

waves

than

like the

motion

of

projectiles.

72

Opus

majus,

IV,

Dist.

2,

Ch.

1,

Bridges,

Huius

probatio

est

per

causam

naturalem,

Vol.

I,

p. 111;

cf.

De mult.

spec., Bridges,

Vol.

quoniam

corpus

naturale

agit per

formam suam

II,

pp.

407-418.

For a careful discussion

of se extra se

multiplicantem.

Ergo

quanto

nobi-

Bacon's

concept

of

species,

see

Vescovini, Studi,

lior

tanto est fortior

in

agendo.

Et

quia

actio

pp.

57-60. Bacon

distinguishes

between

lux

in

directum

est facilior

et fortior

nature,

necesse

and

lumen

in the

passage quoted,

but admits

est ut omne

corpus

naturale seu

visibile

seu

in De mult. spec., "Sed tamen usualiter lucem non visibile suam speciem multiplicet in con-

accipimus

pro

lumine

et e

contrario"

(Bridges,

tinuum et

directum,

et hoc

est

radiare"

(I,

Vol.

II,

p.

409).

Prop.

27

of Pecham's revised version

of Pers.

corn.).

Cf.

II,

Prop.

5.

73

"Omne

corpus

naturale visibile seu non

74

Opus

majus,

V,

1,

Dist.

9,

Ch.

4,

Bridges,

visibile

radiose virtutem suam in

alia

porrigere.

Vol.

II,

pp.

71-72.

337






DAVID

C.

LINDBERG

But

this does not

get

to the nub of

Ronchi's

argument.

His claim

is

not

so

much that

species

resemble

eidola

in

materiality

as that

species,

like

eidola,

are

coherent

wholes.

Alhazen's

principal

contribution

to

the-theory

of vision

had been to substitute a

point-by-point

analysis

of the visual field

(with

forms

issuing

from

every

point)

for

the

coherent eidolon

of

Epicurean

phi-

losophy,

and

Ronchi's view is that

the

concept

of

species

was

a return to

coherence.

But

this is

surely

not

true

of the

concept

in its

thirteenth-century

(and

most

influential)

form.

Bacon himself felt that the

difference

between

species

and

Alhazen's forms

was

merely

terminological,75

and he was

sub-

stantially

correct:

in

fact,

the

optical

properties

of his "visible

species"

and

Alhazen's

"forms of

light

and

color"

are identical.

Now

on the

question

of

coherence,

Alhazen, Bacon,

and

Pecham

are all

imprecise

in

their

termi-

nology.

Alhazen

frequently speaks

of the

singular

form of

a whole

object,

but this

form

has

the

crucial

property

of

susceptibility

to a

point-by-point

analysis.

Moreover,

when

explaining

the

process

of

sight

or

locating

images

formed

by

reflection or

refraction,

Alhazen

considers the

object

a

point

at

a

time and

appeals

to

individual

rays

instead of

the

unitary

forms. This is

not

unlike

our

practice

of

conceiving

of a

continuous

body

of

radiation

as

a

composite

of

discrete

rays,

emanating

from

individual

points

on the lumi-

nous

body.

On the

question

of

coherence,

Bacon and

Pecham

followed Alhazen's lead

in

every

detail.

Pecham often refers to

the

single

species

of a

whole

object,

explaining, for example, that the "species [singular] produced by a visible

object

has the

essential

property

of

manifesting

the

object

of which it

is

the

likeness,"76

but this

species (like

Alhazen's

form)

is

susceptible

to

puncti-

form

analysis.

On other

occasions,

Pecham

speaks

of

the

species

of

a

point,

remarking,

for

example,

that

"any

point

of an

object

seen

in a

mirror

fills

the

whole surface

of the

mirror

with its

species."77

The

very

foundation

of the

geometrical

and

physiological optics

of

Bacon

and

Pecham is this

ability

of the

visual

field to

be

analyzed

into

points

for

individual

treatment;

their

optics,

like

Alhazen's,

is

based on

rays

(representable by

geometrical

lines)

rather than

a

coherent

species.78

It is

evident, then,

that

Bacon

and

Pecham fully and successfully incorporated in their optics both Alhazen's

75

In De mult.

spec.,

Bacon

says explicitly

that

Alhazen

used the term "form" to denote

species:

"Forma

quidem

vocatur

in

usu

Alha-

zen,

auctoris

Perspectivae vulgatae"

(Bridges,

Vol.

II,

p.

410).

In

his

Questiones

supra

librum

de

causis,

Bacon

attributes

the

concept

of

species

to

Alhazen,

saying,

"Item

secundo

Per-

spective

[i.e.,

Alhazen's]

dicitur

quod

lux

et

colores

multiplicant

suas

species

usque

ad

sensum .

. ."

(ed.

Robert

Steele

and

F.

M.

De-

lorme

in

Opera

hactenus inedita

Rogeri

Baconi,

fasc. XII

[Oxford:

Clarendon

Press,

1935], p.

52).

76".

. .

species

genita

a

re

visibili

essen-

tiali habet rem

ostendere cuius est simili-

tudo

. . "

(Pers.

corn.,

II,

Prop.

5).

77

"

.

.

quilibet

punctus

rei

vise

in

speculo

replet specie

sua

totam

superficiem

speculi"

(Pers.

cor., I,

Prop.

3 of the

revised

version).

In De

mult.

spec.,

Bacon discusses both the

species

(singular) representing

the whole ob-

ject

and the

species (plural)

of

the

individual

parts

in

the

same

argument:

"Hoc

enim

non

est

quia

magnitudo

faciat

suam

speciem,

sed

quia

a tota rei

magnitudine

venit

species

coloris

et

lucis,

et

a tota

superficie.

Et

tunc

species

coloris

venientes a

singulis partibus

rei

visae

non confuduntur in una

parte

pupillae

. . .

(Bridges,

Vol.

II,

p.

429).

78

Bacon

asserts

unequivocally

that

"an

in-

finite

number of

rays

issues

from

every point

of

the

agent"

(Opus

majus,

IV,

Dist.

3,

Ch.

1,

Bridges,

Vol.

I,

p. 122).

338






ALHAZEN'S THEORY

OF

VISION

punctiform

approach

and

Grosseteste's

concept

of

species.

They

achieved

this

simply by endowing

species

with

all the

optical properties

of Alhazen's

forms.

At

only

one

point

in their theories of vision did Bacon and Pecham find

that

species

were

significantly

at odds with Alhazen's

"forms of

light

and

color."

Species,

according

to

Grosseteste,

issue

from all

natural bodies-from

eyes

as

well as from

perceived

objects;

and

he

argues

that

the

species

issuing

from

the observer's

eye,

as well

as

those

emanating

from

the

object,

play

a

role

in

sight:

Nor

is it

to be

thought

that

the

emission

of

visual

rays

is

only

imagined

and

without

reality,

as

those

think

who

consider

the

part

and

not

the whole.

But

it

should be

understood that

the visible

species

[issuing

from

the

eye]

is

a

substance, shining

and

radiating

like

the

sun,

the

radiation

of

which,

when

joined

with the

radiation

of

the

exterior

shining body,

entirely

com-

pletes

vision.

Wherefore natural

philosophers,

treating

that

which

is natural to

vision

(and

passive),

assert that vision

is

produced by

intromission.

However,

mathe-

maticians

and

physicists,

whose concern is

with those

things

that

are

above

nature,

treating

that which is

above

the

nature

of vision

(and

active),

main-

tain that vision is

produced

by

extramission. Aristotle

clearly

expresses

this

part

of vision

that

occurs

by

extramission

in

the

last

book of

De

animalibus,

saying,

"A

deep

eye

sees from a

distance:

for its

motion

is

neither

divided

nor

destroyed,

but

a visual

power

leaves it and

goes straight

to the

objects

seen." . . . Therefore true perspective is concerned with rays emitted [by

the

eye].79

In

his

Commentary

on

the Posterior

Analytics,

Grosseteste

says

further,

"Vi-

sion is not

completed

solely

in

the

reception

of

the sensible form

without

matter,

but

is

completed

in

the

reception just

mentioned

and

in

the radiant

energy

going

forth from

the

eye."80

Philosophers

of the thirteenth

century

were faced with

the task of

recon-

ciling

Alhazen's denial and

Grosseteste's affirmation

of

the

existence of

visual

rays.

The

situation was

complicated,

of

course,

by

the

presence

of other

treatises

pronouncing

on the

same

subject-the

works

of

al-Kindi,

Euclid,

Ptolemy,

Aristotle,

Avicenna,

and others.

Bacon and

Pecham

felt

they

could

do

justice

to all schools of

thought

by

acknowledging

with Grosseteste that

visual

rays

exist

and, moreover,

are

required

for

sight,

while

ignoring

the

vis-

ual

rays

in the

further

development

of

their

theories

of

sight.81

After

at-

tempting

a rather subtle

reconciliation

of

Aristotle,

Ptolemy,

al-Kindi,

Euclid,

Tideus,

Augustine,

Alhazen, Avicenna,

and

Averroes,

Bacon

concludes,

.

. . the

species

of the

things

of

the world

are not

immediately

suited

of

themselves to

bring

to

completion

an action on

the

eye

because of the

nobility

79

I

have translated this

from

Ludwig

Baur's

double-emanation

theory

of

sight.Latin text in Die

philosophischen

Werke des 80

Quoted

by

Crombie,

Grosseteste,

p.

114.

Robert Grosseteste

(Beitrige

zur

Geschichte

der

81

Relative

to

optics,

Bacon

says,

"I

have

Philosophie

des

Mittelalters, 1912,

9,

pp. 72-73).

determined

not to

imitate one

author;

rather,

The exact reference to

Aristotle is

De

genera-

I

have selected the most

excellent

opinions

tione

animalium,

V, 1,

781a1-10;

this

passage

is

from

each"

(Un

fragment

de

l'Opus

tertium,

obviously

one

of the

sources

of

Grosseteste's ed.

Duhem,

p. 75).

339






DAVID C.

LINDBERG

of the

latter.

Therefore these

species

must be assisted and

excited

by

the

species

of

the

eye,

which

proceeds

through

the

space

occupied

by

the

visual

pyramid, altering

and

ennobling

the medium and

rendering

it

commensu-

rate with sight. Thus the species of the eye prepares for the approach of the

species

of

the

visible

object

and, moreover,

ennobles the

species

of the

object

so

that

it is

wholly

conformable

to and

commensurate

with the

nobility

of

the

animate

body (i.e.

the

eye).82

Surprisingly, John

Pecham

begins by

apparently

attacking

the

emission

theory.

Following

Alhazen

in

every

detail,

he

goes

so

far

as to

argue,

By

assuming

that

sight

occurs

through

rays

issuing

from the

eye,

mathema-

ticians exert

themselves

unnecessarily.

For the

manner

in

which

vision

occurs

is

adequately

described above

[in

terms

of

intromission],

by

which all

the

phenomena

of

vision

can be saved. Therefore

it is

superfluous

to

posit

such

[visual]

rays.83

But then

Pecham finds that he must make the

appropriate

concessions

to

the

emission

theory

required

by

the

concept

of

species

and

adds,-

with

more hesi-

tation than

Bacon,

The natural

light

of the

eye

contributes to

vision

by

its

radiance. For as

Aristotle

says,

the

eye

is

not

merely

the

recipient

of

action,

but acts

itself

just

as

shining

bodies

do.

Therefore

the

eye

must

have

a

natural

light

in

order

to alter visible

species

and

make

them

commensurate with the

visual

power,

for the

species

are

emitted

by

the

light

of

the

sun and

moderated

with

respect

to the

eye

by

mixing

with the natural

light

of the

eye.

. . . Since vision is of

the same

kind in all

animals,

and certain animals

are

able to

bestow

the

mul-

tiplicative

power

on colors

by

the

light

of their

eyes

so as to

see

them

at

night,

it follows

that

the

light

of the

eye

has

some effect

on

[external]

light.

Whether

it

goes beyond

this,

I do not

determine,

save

only by following

in

the

foot-

steps

of

the

Author

[i.e.,

Alhazen],

as I have

said before.84

Two conclusions

emerge

from

this

analysis

of

Epicurean

eidola,

Alhazen's

forms,

and

Western

species.

First,

neither

Alhazen's forms

nor Western

species

bear

any

but

the most

superficial

resemblance

to

Epicurean

eidola.

82

Opus

majus,

V,

1,

Dist.

7,

Ch.

4,

Bridges,

Vol.

II,

p.

52.

83"Mathematicos

ponentes

visum fieri

per

radios

ab

oculo

micantes

superflue

conari.

Visus

enim sufficienter

fit

per

modum

prescrip-

tum,

per

quem

salvari

possunt

omnia

circa

visum

apparentia.

Ergo

superfluum

est

ponere

sic radios"

(Pers.

corn.,

I,

Prop. 44).

84

"Lumen oculi naturale radiositate sua

visui conferre. Oculus

enim,

ut

dicit

Aristoteles,

non solum

patitur,

sed

agit

quemadmodum

splendida.

Lumen

igitur

naturale necessarium

est oculo ad alterandum

species

visibiles et ef-

ficiendum

proportionatas

virtuti

visive,

quo-

niam

ex

luce

solari diffunduntur sed

ex lumine

oculi

connaturali oculo

contemperantur....

quoniam

visus in

omnibus

animalibus est

unius

rationis cum

igitur

quedam

animalia

per

lu-

men oculorum

suorum sufficiant

coloribus vir-

tutem

multiplicativam

dare

ut

ab eis nocte

videri

possint,

sequitur

ut

lumen oculi

aliquid

in

lumine

operetur.

Et

an

aliquid

ulterius fa-

ciat

non

diffinio

nisi huius

Auctoris,

ut dictum

est,

vestigia

sequendo"

(ibid.,

I,

Prop.

46).

Ba-

con's and

Pecham's admission

that

rays

issue

from

as well

as

enter the

observer's

eye

cannot,

of

course,

be

viewed

solely

as a

compromise

between Grosseteste and

Alhazen.

Bacon and

Pecham were

influenced

also

by

Aristotle and

by

the

Galenic and Platonic

traditions and

should be

considered heirs of these

teachings

as well. As Pecham

points

out in the

passage

quoted,

Aristotle

speaks

of

combined

emission

and

intromission;

and

Pecham's

argument

about

the

visual

power

making

the

visible

species

commensurate with

sight

is

based on

Aristotle's

remarks

in

De

generatione

anima-

lium,

V, 1,

780a5-25.

340






ALHAZEN'S

THEORY OF

VISION

Forms and

species

are

powers,

not

corpuscles

or coherent collections

of

cor-

puscles,

and

they

are

susceptible

to

punctiform

analysis.

Second

and more

important,

Alhazen's forms and Western

species

have

identical

optical prop-

erties and

perform

identical

optical

functions-with one

exception.

Against

Alhazen's

denial

of

visual

rays,

Bacon

and

Pecham

admitted

that

species

emanate from the

observer's

eye

and

play

a role

in

sight.

However,

this ad-

mission of

visual

rays

did not

otherwise

alter the

theory

of

vision

gained

from

Alhazen,85

and

Bacon and

Pecham almost

completely

ignored

the

species

emanating

from

the

observer's

eye

in the further

development

of their

theo-

ries. In their

theory,

as

in

Alhazen's,

powers

emanating

from

natural

bodies

produce

effects

in

a

recipient,

and both theories

permit point-by-point

analy-

sis

of

the

visual

field and offer

identical

descriptions

of

the act

of

vision. Thus

the admission of visual rays by Bacon and Pecham was of small consequence

for the

subsequent

history

of

optics.

It

did not

interfere

with

the

transmis-

sion,

through

their

works,

of

the broad

outlines

and

most

of the

details of

Alhazen's

theory

of vision.

Witelo,

whom

I

have

ignored

because of his

superior

status in

Ronchi's

scheme

of

things

and

his

failure

to use the term

"species"

or

discuss

rays

emanating

from the

eye,

shared similar

views

on the

properties

of the

rays

and the

nature of

sight.

After a

careful

study

of

Witelo's

Perspectiva,

Alek-

sander

Birkenmajer

concludes

that

"sous

la

plume

de Witelo le

mot 'forme'

doit etre

identifie

avec

la

species

baconienne . .

.

dans

toute

la

vaste

eten-

due de ce term chez Bacon."86

It is

evident

that

apart

from the

question

of

visual

rays,

Alhazen's

theory

of

vision

and views

on the

nature

of

light

were transmitted

intact to the

West.

I do

not,

of

course,

claim that the

concept

of

species

is

in

all

respects

identical

to Alhazen's

concept

of

form;

the

former,

for

example,

has meta-

physical implications

missing

from the

latter,

and

Vescovini has called

atten-

tion

to subtle

differences

in the

associated

psychologies

of

perception.87 My

point

is

simply

that the

two

concepts

function

identically

as

a

basis for

a

theory

of vision

and

that the

theories

of

vision built

upon

them

by

Bacon,

Pecham,

and Witelo

(on

the

one

hand)

and

Alhazen

(on

the

other)

are

substantially

the

same. Bacon, Pecham, and Witelo initiated a Western

optical

tradition that

faithfully

transmitted

the

essence of

Alhazen's achievement

in

optics

to

Kep-

ler and his

seventeenth-century

contemporaries.

85

Furthermore,

the

species issuing

from the

Bridges,

Vol.

II,

pp.

50-51.

observer's

eye perform

none of the

functions

86

"ltudes sur

Witelo,

II,"

Bulletin inter-

that

Alhazen

denied to

visual

rays

in

his

refu-

national de

l'Academie

polonaise

des

sciences

tation

of the

emission

theory,

as Bacon himself et

des

lettres,

1920,

p.

356.

notes

in the

Opus

majus,

V, 1,

Dist.

7,

Ch.

3,

87

Studi,

Chs.

4,

7.

341

lindberg - al hazen’s theory of vision and its reception in the west (1967)

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