THE HISTORY OF ASTRONOMICAL SPECTROSCOPY I QUALITATIVE CHEMICAL ANALYSIS AND RADIAL VELOCITIES

Donald H. Menzel Harvard College Observatory and Smithsonian Astrophysical Observatory

Cambridge. Massachusetts 02138

I divide the history of spectroscopy, somewhat arbitrarily, into three stages. First of all, man had to learn what the spectrum was and what the band of color signified. Not until the mid-nineteenth century did man come to realize that each chemical substance possessed a characteristic and individual spectrum. Thus began what I call the second stage, qualitative spectrum analysis, which consisted of matching the spectra of known materials against the spectra of the sun or stars. Finally, and much later, came the third stage: quantitative chemical analysis, the precise determidation of the chemical composition of the universe. This paper will deal with the qualitative phase of the subject. I shall discuss quantitative astronomical spectroscopy in a second paper, which follows this one.

The earliest record I can find of a spectrum occurs in the scriptures, the Book of Genesis: the rainbow that spanned the sky, marking the end of the great Noachian flood. Numerous other references to the rainbow occur in the Bible, which commonly calls the spectacle the “Glory of the Lord.”

I find it difficult or impossible to separate the astronomical applications of spectroscopy from the general physical side, dealing with the nature of light, the meaning of color, and the interpretation of the spectrum. Many of those who contributed to the development of optical science were themselves astronomers.

The early Greeks investigated the laws of optics, but they made relatively few discoveries concerning the nature of light and the meaning of color. Even the phenomenon of vision worried them. About 440 B.c., Empedocles developed what might be called the tentacular theory of sight. He reasoned that the sensation of light was produced by something leaving the eye rather than something entering it. Sight, he believed, was akin to touch, A person saw an object by “feeling” it with invisible tentacles emanating from the eye. This theory remained more or less popular for several centuries. The Platonists and Euclid favored the concept as late as 300 B.C. Democritus and other philosophers of the Pythagorean school, about 400 B.c., attributed vision to something that emanated from the body and entered the eye.

In the first century A.D., Seneca reasoned that the colors of the rainbow were produced in some fashion similar to those appearing at the edges of a glass prism. One could say, therefore, that he was the first true spectroscopist. Shortly there- after, about 130 A.D., the Alexandrian astronomer Claudius Ptolemaeus, better known as Ptolemy, carried out some optical experiments and concluded that when light enters a denser medium, the angle of refraction is proportional to the angle of incidence. Today we know that this law, as Ptolemy formulated it, is true only for small angles. About 1620, the Dutch mathematician W. Snell enunciated the correct law, which now bears his name: the sine of the angle of refraction is pro- portional to the sine of the angle of incidence. Ptolemy had also detected and measured the refraction produced by the earth‘s atmosphere.

The Middle Ages apparently contributed practically nothing to the field of optics. About 1038, the Arab scientist A1 Hazen investigated refraction; he was also aware of the effects of the earth’s atmosphere.

The first really great discoveries were those made by Isaac Newton in 1664.

225

226 Annals New York Academy of Sciences

In his experiments with prisms he definitely proved that color was a property of white light itself, not just something introduced by the glass prism. It was Newton who gave the name “spectrum” to the strip of rainbow colors resulting from the passage of sunlight through a prism. He also proposed the theory that light was corpuscular in nature. Christian Huygens was apparently the first to take issue with Newton’s view. In 1678, at the French Academy of Science, in the presence of such greats as Romer and Cassini, Huygens presented the theory that light was a form of wave motion.

A young Scottish scientist, Thomas Melvill, in 1752, was apparently the first scientist to actually see a spectral line. He was studying the spectrum of flames and recorded the characteristic yellow light from sodium. His early death in the following year at the age of 27, ended the career of this brilliant scientist.

In the year 1800, the astronomer William Herschel deduced the presence of infrared radiation in the solar spectrum, from its heating effect on a thermometer placed in the region beyond the red range of the spectrum. The very next year, the German physicist J. W. Ritter discovered the ultraviolet range, through its specific action on salts of silver. The English chemist William Hyde Wollaston inde- pendently made the same discovery the following year. Wollaston, who used,a slit to admit sunlight to the prism, instead of a circular aperture as Newton had done, was clearly the first person to note the existence of dark lines. He misinterpreted their significance, however, concluding that they simply marked divisions between the major colors of the spectrum.

Between 1801 and 1804, the English physicist Thomas Young carried on a number of optical experiments and developed the wave theory of light. He demon- strated that thin plates owed their color to interference. He studied the effect of scratched and striated surfaces on glass, and in this way was the inventor of the diffraction grating. Young’s wave theory was not generally accepted. In fact, many scientists of the time openly laughed at him. It remained for the great French physicist and engineer, Augustin Jean Fresnel, to furnish definitive evidence for the wave theory of light. He also demonstrated that the waves were transverse, not longitudinal like sound waves, as had been generally supposed until then. Ignorant of Young’s pioneer work, Fresnel repeated many of the earlier experiments and greatly extended, by rigorous mathematics, the wave theory of light. In this respect he followed Huygens, who a century earlier had formulated the principle of secondary waves. The astronomer and physicist Dominique Fransois Jean Arago became the first major French convert to the undulatory theory of light. He drew Fresnel’s attention to the’ prior work of Young. Gradually opposition to the undulatory theory waned and the wave concept of light came to be generally accepted.

Joseph von Fraunhofer, who developed into a great experimental genius, made tremendous advances in spectroscopy. Born near Munich, Germany, in 1787, Fraunhofer had little formal education. At the age of 11 he was apprenticed to a Munich optician who made mirrors. Fraunhofer developed many techniques for improving glass and glass manufacture. As he encountered new phenomena his genius led him to explore them further. Thus in 1814, when only 27 years old, Fraunhofer made a detailed examination of the spectrum of sunlight. Where Newton had used a spectroscope consisting merely of a slit, a prism, and a lens that focused the image of the slit on a screen, Fraunhofer used a theodolite, which not only gave much higher magnification but also allowed him to measure the angle. Using a 60” glass prism, set 24 feet away from a slit, he viewed the spectrum of sunlight with the theodolite and found that the spectrum was crossed by “an

Menzel : Qualitative Astronomical Spectroscopy 227

almost countless number of strong and weak vertical lines.” He mapped about 700 of these, and assigned to eight of the more prominent ones the letters of the alphabet from A to H, by which names they are still generally known. Thus we refer to the red hydrogen line as C and the yellow lines of sodium as D1 and D2.

Fraunhofer became the first astronomical spectroscopist. He placed a glass prism of angle 37’40‘ in front of a 4-inch objective of the telescope, and with it he observed the spectra of the stars and planets. He noted that the spectrum of Sirius differed from that of the sun, whereas the spectra of the planets were of the solar type. Hence he concluded that the spectra were not the result of the passage of light through the earth’s atmosphere, He did not understand, of course, what caused the dark lines of the spectrum.

Fraunhofer made another fundamental contribution. Where Young had dis- covered the phenomenon of interference .by permitting light to pass through a pair of slits, Fraunhofer experimented by increasing the number of slits up to hundreds. Thus he produced the first effective diffraction grating. He took a pair of h e machine screws, set them some distance apart, and then wound upon them successive turns of fine wire, each turn separated from its predecessor by the pitch of the screw. The result was a transmission grating, with a spacing of only 192 lines per centimeter. He developed the theory of such gratings and used the device to measure the wavelengths of some of the spectral lines. He thus determined the wavelengths of the D lines, which he could not separate, as .0005887 mm, which can be compared with the modem value of .0005893 mm for the average position of the pair.

Fraunhofer next manufactured a ruling engine, which had a diamond point that could be used to scratch fine lines on the face of a silvered mirror. In this way he made gratings of 3,625 lines per centimeter, with which he obtained even better determinations of the wavelength of the D lines. Without question, his was the genius that founded the great science of astrophysics.

The younger Herschel, J. F. W. Herschel, made extensive studies of the spectra of flames impregnated with salts of various metals. In 1817 he wrote: “The colours thus communicated by the different bases to flames afford, in many cases, a ready and neat way of detecting extremely minute quantities of them.” This was a step forward, though apparently he still did not recognize the truly unique character of the spectra of individual substances.

The ubiquity of sodium tended to impede the progress of spectroscopy. Almost every material contains enough sodium to show its famous yellow lines, because of the sensitivity of the spectroscopic test. The answer was simple. Sodium was almost everywhere. I remember when I was taking qualitative chemical analysis many years ago in college, we used the spectroscopic test for sodium. And I always entered sodium in the list of the substances I had identified in an “unknown,” because certainly the professor could never mark that wrong.

William Henry Fox Talbot, a wealthy English scientist, also devoted attention to the spectra of flames and sparks. In 1825 he commented: ‘“The orange ray may be the effect of the strontia, since Mr. Herschel found in the flame of muriate of strontia a ray of that color. If this opinion should be correct and applicable to other rays, a glance at the prismatic spectrum of a flame may show it to contain substances which it would otherwise require a laborious chemical analysis to detect.” Fox Talbot had already experimented with photography, in which he had achieved some success.

Photography played an extremely important part in the history of the study of spectra. With it, the spectroscope became a spectrograph, which could make a

228 Annals New York Academy of Sciences

permanent record that could later be compared with other spectra. Daguerre’s invention was announced in 1839. In America, Dr. John William Draper, who had already experimented with Fox Talbot’s techniques, turned his chemical knowledge to improving the sensitivity of Daguerre’s photo surfaces. By March, 1840, he had obtained the first photograph of the moon. He mapped the solar spectrum photographically, including the ultraviolet range. He pushed forward the study of the spectra of various flames and contributed significantly to the science of spectroscopy.

In 1847 Draper published an important paper, The Radiations of Ignired Bodies. As a result of his studies he announced in this paper that “an ignited solid will give a continuous spectrum, or one devoid of fixed lines; an ignited gas will give a discontinuous spectrum, one broken up by lines or bands or spaces.” He further concluded that “as the temperature of an incandescent body rises, it emits rays of light of an increasing refrangibility.” In other words, “the frequency of those vibrations increases with the temperature.” This is a clear statement of what is now called the displacement law, which Wilhelm Wien later proposed on theoreti- cal grounds, about 1893. With reference to dark-line spectra, Draper was not entirely clear. In a paper published in 1857, he does say unequivocally that “the occurrence of lines, whether bright or dark, is hence connected with the chemical nature of the substance producing the flame. For this reason these lines merit a much more critical examination than has yet been given to them, for by their aid we may be able to ascertain points of great interest in other departments of science. Thus if we are ever to acquire certain knowledge respecting the physical state of the sun and other stars, it will be by an examination of the light they emit. Even at present, by the aid of a few facts before us, we can see our way pretty clearly to certain conclusions respecting the sun.”

His comments on the possibility of determining the nature of the sun and stars through examination of their spectra may have been made in reaction to the statement made some years before by the French philosopher Isidore Auguste Marie Francois Comte: “There are some things of which the human race must remain forever in ignorance, for example, the chemical constitution of the heavenly bodies.” Comte was something of a mystic, though he did sometimes lecture on the subject of astronomy. He was not, however, at all conversant with experiment or the role of experiment, which is perhaps why he was never highly regarded as a philosopher. He was said to have contemplated a career in the United States, but was dissuaded by a friend’s report of the materialistic spirit existing there. He tried to develop a “plan of scientific works necessary to re- organize society.” He is perhaps best known for having coined the term “soci- ology.”

Starting about 1859, the brilliant team of Gustav Robert Kirchhoff and Robert Wilhelm Bdnsen made a number of startling discoveries that paved the way for the qualitative chemical analysis of the heavenly bodies. Kirchhoff reiterated the basic laws that had previously been stated by Draper, and to them he added a clarifying law concerning the nature of dark-line spectra. They were, he noted, merely. absorption produced by a relatively cool gas of the light from a con- tinuous source. The spectra of a number of chemical elements were measured and clarified. Kirchhoff and Bunsen themselves isolated two new elements in the alkali group containing sodium and potassium. (The new metals were rubidium and cesium.) Kirchhoff identified the existence of sodium, iron, and about a half a dozen ofher elements in the spectrum of the sun.

Now the era of qualitative spectrum analysis began in earnest. The English

Menzel : Qualitative Astronomical Spectroscopy 229

astronomer William Huggins was the pioneer in this work. By 1864 he had made visual observations of the spectra of several bright stars. He had attempted photog- raphy but was unsuccessful, since the spectra showed no visible structure. The first to obtain a photograph of a stellar spectrum was Henry Draper, son of William Draper, who obtained a plate of the spectrum of Vega in August 1872, using the 28-inch reflector that was later given to the Harvard Observatory. The spectrum showed only four dark lines, all of the element hydrogen. During the next few years he improved his techniques and photographed the spectra of more than 100 stars. Huggins then renewed his own efforts and was finally successful. Draper’s premature death in 1882, at the age of 45, brought his promising work to a close. Funds provided by Draper’s widow enabled Edward C. Pickering, by then Director of the Harvard College Observatory, to initiate and carry forward an enormous project known as the Henry Draper Catalogue, which eventually led to the classification of the spectra of more than 222,000 stars, down to about the ninth magnitude, in both the Northern and Southern Hemispheres. Later extensions of this work brought the total number of stars classified to more than 400,000.

The success of the Henry Draper Catalogue must be attributed largely to the efforts of two people, Miss Antonia C. Maury and Miss Annie Jump Cannon, both of the Harvard College Observatory. Miss Maury, starting with Secchi’s four spectral types, found that she could recognize gradations forming a regular se- quence from the hot blue stars down to the cool red ones. Her original classifica- tion, however, which like Secchi’s used many Roman numerals, proved to be extremely cumbersome. Miss Cannon substituted letters, which now form the well-known spectral sequence: 0, B, A, F, G, M, K, R, N, S. Following the letter, numerals from 0 to 9 indicated the decimal division between the successive main classes. Thus we might have AO, A2, F5, G8, and so on, It was Miss Cannon who accepted the primary responsibility of carrying through the laborious project of cataloguing the spectral types.

I should like to refer to some astronomical spectroscopy done during the total solar eclipse of 1868, when a number of astronomers observed for the first time what came to be known as the flash spectrum. The French astronomer Pierre Jules CCsar Janssen was particularly struck by the beauty and brilliance of the spectral lines coming from the brighter prominences. The next day he again turned his spectroscope on the edge of the sun, and successfully saw the lines without benefit of eclipse.

The English astronomer Sir Norman Lockyer had had a similar inspiration a couple of years earlier, but was not able to do anything about it until he obtained a spectroscope, about two months after the eclipse of 1868 had taken place. He then independently made the discovery that one can observe the bright lines without an eclipse. The following year, Sir William Huggins found that by open- ing the slit he could observe the actual form of the prominence. By the early seventies this technique was in general use in many observatories around the world, but it was Charles A. Young, of Princeton University, who in 1870 first photographed the image of a prominence by this method. Some twenty years later, George Ellery Hale of the Yerkes Observatory developed the technique into the instrument known as the spectroheliograph, which was also independently in- vented by the French astrophysicist Henry Alexandre Deslandres.

At this point the history of astronomical spectroscopy becomes somewhat con- fused. So many astronomers were rushing in to take advantage of the new tech- nique that it is somewhat difficult to single out the work of just a few.

230 Annals New York Academy of Sciences

Huggins made a notable contribution by recognizing that certain nebulae exhibited spectra consisting of bright lines. He correctly inferred that such objects consisted of rarefied gas. During the mid-1880s Huggins and Pickering main- tained a vigorous correspondence concerning the differences in the spectra of various stars.

In the early 186Os, the American astrophysicist Lewis Rutherfurd of Columbia, Father Angelo Secchi of Rome, and Hermann Carl Vogel, a German astronomer, were attempting to understand the differences between various stellar spectra. Secchi recognized four main types: Class I showed hydrogen spectra and consisted of white stars such as Sirius; Class I1 included yellow stars such as the sun and Capella; Class I11 referred to deep red stars such as Antares or Betelgeuse; and stars of Class IV contained bright lines as well as dark.

Astronomers realized that the differences in the spectra of stars represented some important physical and perhaps chemical differences in the structure of their atmospheres. For almost a century thereafter, they maintained a continuous struggle to understand the spectra and build them into a reasonable evolutionary picture.

I have already referred to Fraunhofer's introduction of the diffraction grating. Over the last century, a number of scientists have contributed to the development and improvement of such gratings, which have been increasingly used in astro- physical observation. Henry Augustus Rowland made the first major step, by im- proving ruling engines and inventing the concave grating. Robert W. Wood, of Johns Hopkins University, introduced other improvements into the ruling engine and was among the first to develop methods of making replica gratings. Albert Abraham Michelson, more famous for his work in interferometry, also made dif- fraction gratings. Harold D. Babcock, of the Mount Wilson Observatory, con- tinued this work for a time. But in recent years the finest, largest, and most versatile of all gratings are those that have been produced and are still being pro- duced by the MIT physicist George Russell Harrison.

Then there were the great physicists who determined standard wavelengths; the first of these was Angstrom, in 1868, followed by Rowland, about 1887, and Michelson in 1893. Next there was the German physicist Heinrich Gustav Johannes Kayser, who produced an enormous handbook of spectroscopy. I must also mention the two great spectroscopists of the National Bureau of Standards, Carl C. Kiess and William F. Meggers, who painstakingly observed, measured, and interpreted the spectra of many rare elements. I should here mention again the name of George Harrison, whose work led to improvements in the measure- ment of wavelengths and increased knowledge of the structure of many atoms. There are dozens, perhaps hundreds, of other spectroscopists whose names I cannot record here because of lack of time and space.

I must briefly discuss, however, the extraordinarily brilliant work of the English astrophysicist Sir J. Norman Lockyer. He concerned himself with the differences in the spectra of the same element under different conditions of excitation, for example in the flame, in the arc, and in the spark. Reasoning that the higher energy of the electric spark should break down an atom more than the relatively cool flame, he attributed the presence of certain lines that were strong in the spark but weak or absent in the flame to the actual breaking up of an atom into fragments. He thought this phenomenon was truly some form of transmutation, and in a sense it was. (We now know that the lines he found "enhanced" in the spark are lines showing higher states of ionization of the atom, from atoms that have lost one or more of their normal complement of outer electrons.) Arguing

Menzel : Qualitative Astronomical Spectroscopy 23 1

in this fashion, he built up a theory of stellar evolution that for a time was widely accepted. I shall refer again to this important work in the following paper.

During the latter part of the nineteenth century and the early part of the twentieth, any astronomer who used a spectroscope for any purpose whatsoever proudly proclaimed himself an astrophysicist. In 1842, Christian Doppler, of Prague, noting that the pitch of sound varies with the relative motion of an object toward or away from the observer, reasoned that light should exhibit a similar effect. He suggested that this phenomenon could explain the observed colors of stars: that the blue stars were moving toward us and the redder stars away from us. As far as the difference in the colors of stars was concerned, this conclusion of Dopplet’s was early recognized to be wrong. The changes in a continuous spec- trum would be negligible, because in the spectrum of a receding star, let us say, the radiation from the ultraviolet range would be shifted into the visible range, to replace the fraction lost at the red end by the Doppler shift. This fact was pointed out in 1845, by Christoph Heinrich Dietrich Buys-Ballot, the rector of the Royal Meteorological Institute at Utrecht, The Netherlands.

In 1848, the French astronomer Hippolyte Louis Fizeau, a colleague of Fou- cault, was the first to point out that the Doppler effect should cause a shifting of the spectral lines. Hence the iron lines in the spectrum of a star receding from the observer should be shifted to longer wavelengths than those in the iron spectrum observed in the laboratory. Many writers on astronomy, therefore, very pioperly refer to this phenomenon as the Doppler-Fizeau effect, rather than just as the Doppler effect.

The relative smallness of the spectral shift, the crudeness of the existing spectro- graphs, and the difficulty of observing with the naked eye long delayed the study of stellar radial velocities. Not until 1868, twenty years after Fizeau’s announce- ment, did Sir William Huggins obtain the first tangible results in this study, pre- senting measurements of the radial velocities of a number of bright stars. The probable errors of the determinations were high, and the first results were mainly qualitative. They did tend to stimulate work in the field, however. As for the stars, part of the difficulty lay in their faintness. One could use

spectroscopes with only relatively low dispersion to observe them. The brilliant sun, on the other hand, provided enough light for the use of high-dispersion spectroscopes. In 1871, H. C. Vogel was able to measure quantitatively the spectral displacement caused by solar rotation. Light from the east limb of the sun was indeed displaced toward the violet and that from the west limb toward the red, in conformity with the rotation velocities determined by the observation of sunspots. Professor A. C. Young, of Princeton, continued this work, and by 1876 had demonstrated that the “telluric lines” produced by absorption within the earth’s atmosphere showed no such displacement.

Observing with a spectrograph of advanced design at Uppsala, Sweden, M. Dun& appreciably extended our knowledge of solar rotation. Observers of sun- spots had long before clearly established the so-called equatorial acceleration, whereby the rotation periods of sunspots near the equator are less than those at higher latitudes. The fact that sunspots rarely appeared at latitudes greater than -+35O, however, hampered the determination of the law of rotation for higher latitudes. DunCr’s spectroscopic observations clearly showed that the rotation period continues to increase all the way to the polar regions.

Sir Norman Lockyer continued to observe solar prominences, and he and many other observers were surprised to find highly distorted spectral lines from the prominences at the solar limbs, a distortion clearly caused by velocities of an

232 Annals New York Academy of Sciences

explosive character. While the rotational velocity of the sun was of the order of 3 km per second, the displacement of the prominence lines indicated that some prominences were moving at speeds of up to nearly 1,000 km per second. These were exciting results indeed, in that they disclosed the presence of enormous “gales” in the atmosphere of the sun.

I have already referred to the difficulties that the early astronomers encountered in measuring the radial velocities of stars by visual techniques. The invention of photography greatly improved the situation, as the spectrograph replaced the now outmoded spectroscope. The catalogs of radial velocities began to grow, slowly at first, and then at an accelerated rate. As early as 1871, William Henry Fox Talbot had suggested that the spectroscope might reveal the presence of double stars, which could be recognized by the periodic displacement of spectral lines as the two stars revolved about one another. That such “spectroscopic binaries” could exist no one doubted, since visual doubles with relative orbital motion had long been recognized. Their existence was confirmed in 1889, when sharp-eyed Antonia C. Maury, at the Harvard College Observatory, noted that the K line of the spectrum of Mizar, Zeta Ursae Majoris, was sometimes single and sometimes double. From observations taken over the years 1887-89, she determined that the doubling was periodic, at intervals of 52 days. Almost immediately she came up with a second prize, the binary star Beta Aurigae, whose four-day period was revealed by the fact that the spectral lines were single and double on alternate nights.

Miss Maury’s interest in spectroscopic binaries never waned. I came to know her well during my graduate student days, when she would invite me to look at some particularly interesting structure she had noted in the spectrum of Beta Lyrae, which was one of her favorite stars. This binary consists of two egg- shaped objects, the longer axes of which were pointed toward one another. John Goodricke, a British amateur, had discovered the variability of this star in 1874 and had concluded that the regularity of its period, which was about 13 days long, indicated that the star was an eclipsing binary. He had invoked this explanation two years earlier to explain the variability of the second-magnitude star Algol (Beta Persei) . The variability arose from two effects, the changing projection of the revolving elliptical bodies and also the effect of eclipses. It became evident that the objects themselves were almost in contact, and that their ovate shapes were the result of tidal distortion.

Miss Maury clearly recognized that the binary’s peculiar spectroscopic varia- tions were not completely accounted for by the simple model of two egg-shaped stars in orbital revolution around one another, For one thing, certain of the spectral lines displayed several components. Moreover, she showed that spectra taken at the same phase of the orbital revolution often displayed distinctly dif- ferent fine structure. She argued, quite correctly, that the stars must be revolving in an irregular, striated atmosphere surrounding both stars. Such a view is not now uncommon. A number of years later, in an analysis of the spectroscopic features of an eclipse of the star Zeta Aurigae, I demonstrated that the limb of the star was not sharp, like the edge of the sun, but extended and hazy. Moreover, it seemed to possess condensed regions above the surface, somewhat analogous to solar prominences.

Astronomers recognized that spectroscopic binaries might exist in which one of the components would be too faint to contribute a spectrum. The result, therefore, would be that the single lines would oscillate back and forth across their mean position. Vogel discovered that Spica (Alpha Virginis) was such a star, with a

Menzel: Qualitative Astronomical Spectroscopy 233

period of about four days. In 1896 Belopolsky identified Castor (Alpha Geminorum) as another. In England, Campbell and Newall found that Capella is a binary.

When William Wallace Campbell went to the Lick Observatory in 1891, he started to build bigger, more powerful, and more accurate spectrographs. To minimize the possibility of instrumental distortion, he encased the spectrograph in a thermal enclosure, whose temperature was held constant throughout the night. The tens of thousands of spectra taken by Campbell, his colleagues, his asso- ciates, and his assistants represent one of the great contributions to the study of stellar radial velocities. His ambitious program included the establishment of a station in Santiago, Chile, for observation of the southern stars. The final catalog contained radial velocity data for 2,600 stars, almost all the stars in the sky brighter than magnitude 5.51. Dr. Joseph Haines Moore was Dr. Campbell’s principal colleague in this enormous program, the results of which were published in 1928. One result of the catalog was the determination of the solar motion from radial velocities.

I do not want to give the impression that Lick Observatory has been the sole contributor to this field. The other principal contributing observatories included Mount Wilson, Pulkovo, Bonn, Cape, Victoria, and Yerkes. The investigations included studies of such diverse objects as Wolf-Rayet stars, planetary nebulae, novae, such comets as appeared during that period, and some of the planets.

With reference to the planets, James Keeler, who preceded Campbell as Direc- tor of the Lick Observatory, demonstrated spectroscopically that the rings of Saturn do not move like a solid body. He also observed the spectrum of Uranus, but it remained for V. M. Slipher of the Lowell Observatory to show the very definite increase in the intensities of absorption bands along the sequence Jupiter, Saturn, Uranus, and Neptune. In 1923, while still a graduate student, I used this observation as an argument to support the idea that these planets were indeed very cold, since the intensity of these bands was obviously dependent on the planets’ distance from the sun.

The earliest determination of the rotation period of the planet Uranus was made spectrographically by Percival Lowell and V. M. Slipher at Flagstaff. The period of rotation was derived from the measured inclination of the spectral lines, presumably caused by the rotation of the object. Over the years from 1927 to 1930, Dr. Joseph H. Moore and I, at the Lick Observatory, obtained a number of spectra of Uranus and found values for the rotation period generally con- sistent with those previously determined by Lowell and Slipher. These observa- tions were some of the most exacting I have ever attempted. We worked only on the nights that had the best seeing. Dr. Moore and I took turns, guiding the telescope for about 15 minutes apiece, since the guiding needed to be extremely accurate.

Challenged by a statement made by some astronomer whose name I have now forgotten, who said that one probably could not determine the rotation period of Neptune by this kind of technique, because of its smaller apparent diameter, Dr. Moore and I took a number of plates during the opposition of 1928; these led us to conclude that its rotation period was about 15 hours and that its direction of rotation, unlike that of the satellites, was direct.

Space does not permit the detailed discussion of a large number of very im- portant studies carried on by means of the spectrograph during the early 1900s. In passing, I might refer to the basic studies of Wolf-Rayet spectra made by John S. Plaskett and his son Harry H. Plaskett at the Dominion Astrophysical Observa-

234 Annals New York Academy of Sciences

tory in Canada. Significant contributions were made to spectrum analysis by Dr. Walter S. Adams and his associates, of the Mount Wilson Observatory. I must also mention the fundamental contributions of Dr. William W. Morgan, of the Yerkes Observatory. All of these people contributed to an enormous improvement in our understanding of stellar spectra, particularly in the field of spectroscopic paral- laxes.

In this paper I have concerned myself mainly with qualitative spectrum analysis and the problems of radial velocity. My next paper leads into the quantitative phases. The comparison of stellaF spectra led to the determination of the dif- ferences in size between giant stars and dwarf stars. Their use in spectroscopic analysis is in itself a highly quantitative activity. Moreover, these later phases more or less paralleled the early developments of quantitative chemical analysis in astronomical spectroscopy. I shall make this the subject of the following paper.