748 CURRENT TOPICS. [J. F. I.

A n A t t e m p t to D e t e r m i n e W h e t h e r a M i n i m u m T i m e is N e c e s s a r y to Exci te the H u m a n Retina. J . H . J . POOLE. (Phil. Mag., February, I 922. )--The general method employed was this: Light, f rom a small incandescent lamp passed through a lens, fell upon a plane mirror mounted on top of the vertical shaft of a cen- tr ifuge and, af ter reflection, reached a slit behind which was the eye of the observer. The lens caused the image of the filament to fall upon the slit. When the centrifuge rotated 80o0 r.p.m, the eye received light for only a small part of each revolution, but there were many successive flashes in a second. To cut down this number a sectored wheel was interposed between the lamp and the lens. The experiments were conducted in a cellar in order to command large distances and to secure entire darkness.

The adaptation of the eye in the dark is an important factor in the experiments. I t takes at least 45 minutes in darkness to attain the greatest sensitiveness, but after 25 minutes the gain is very slow. The eye adapted in the dark is from 5ooo to lo,ooo times as sensitive as in ordinary daylight.

The duration of the flash received by the eye of the observer could be calculated f rom the dimensions of the apparatus and the speed of rotation. " I t was found that a flash of two ten-millionths seconds was still visible. A flash of duration eight hundred-millionths seconds was, however, found to be invisible. The energy in each of these flashes could be approximately calculated from the candle- power of the radiant and the various dimensions of the apparatus, and it was found that, neglecting any loss by reflexion, etc., the energy in the visible flash was about four ten-millionths erg, while that in the invisible flash was about eight hundred-millionths erg. As to whether the invisibility of the shorter flash is due to its smaller duration alone or simply to the decrease in the energy available, is uncertain. It seems, however, that the latter hypothesis is more probable as the intensity of the longer flash was exces- sively feeble." G . F . S .

Fer t i l izer f rom Wool - scour ing W a s t e s . - - F . ]7. VF.ITCH (Jvztr. Ind. Eng. Chem., I922, xiv, 434) states that, on the average, un- scoured wool contains 4 per cent. of water-soluble potash and o.6 per cent. of water-soluble nitrogen. Both the potash and the nitrogen are present in the wool as water-soluble compounds which are removed by scouring and pass into the sewage f rom the scouring plant. In this way water courses are polluted, and much valuable fertilizer is lost. In fact, potash worth $840,000 and nitrogenous compounds worth $280,000 as fertilizer are wasted annually. The liquor obtained by scouring may be concentrated and mixed with the wastes f rom other industries to produce a satisfactory fertilizer.

J. s. H.