August 15, 1913] 



SCIENCE 



237 



reaction. To this end I proceeded in the fol- 

 lowing manner. The animal was fixed in a 

 definite position on the stage of a microscope, 

 illuminated from below by a weak electric 

 light of constant intensity. The microscope 

 stood in a blackened dark-room. Through a 

 hole in the wall of the room the light of an 

 80 candle-power Tungsten lamp fastened out- 

 side could enter. The light was made diiluse 

 by a sheet of oiled paper fixed across the open- 

 ing. The hole was 55 mm. in diameter and 

 ■was closed by a piece of cardboard containing 

 two diaphragms of varying sizes, side by side. 

 A shutter with a spring motion could alter- 

 nately close either the one or the other open- 

 ing. I could thus make an instantaneous 

 change from a stronger to a weaker light, and 

 -vice versa, by using diaphragms of difFerent 

 sizes and moving the shutter to and fro. One 

 ■diaphragm was maintained at constant size 

 (25 mm. diameter) and a sector wheel or 

 episcotister, driven by a small electromotor, 

 could be rotated before it. The light passing 

 this diaphragm had an intensity of 0.9 c.p. 

 The distance between the animal and the 

 diaphragm was about 60 cm. Obviously, if 

 two diaphragms were used whose areas were 

 as 1 : 10 and a sector wheel with 1/10 of the 

 periphery cut out were rotated before the 

 larger one, so as to let light pass during 1/10 

 of a revolution, then equal amounts of radiant 

 energy would reach the eye of the animal 

 through either diaphragm. 



The microscope was placed in such a posi- 

 tion that the light from the diaphragms could 

 fall on the stage from the side. If the 

 smaller diaphragm was opened, the eye of the 

 Daphnia took up a position, defined by the 

 ratio of intensities of the light coming from 

 the weak lamp below and from the diaphragm 

 above. Changing from the smaller to the 

 larger diaphragm would cause a change in the 

 position of the eye. By varying the sizes of 

 the diaphragms I found that a noticeable 

 reaction was obtained upon changing from 

 one diaphragm to the other, even when the 

 •difierence between their areas was as small as 

 10 per cent. Change between diaphragms of 

 equal size, however, did not produce a reaction. 



Using the diaphragm ratios 5 : 10, 2.5 : 10 

 and 1 : 10 I invariably found that upon using 

 a sector wheel cutting down the time of ex- 

 posure for the larger diaphragm so as to make 

 the amount of energy equal to the smaller one, 

 I obtained no reaction on change from one to 

 the other. If I used sector wheels giving too 

 long or too short exposures, a reaction was 

 noticed, where the error exceeded 10 per cent. 

 These ohservations prove that for the eye 

 movements of Daphnia the energy law holds 

 within the limits of accuracy characteristic of 

 the reaction. The speed of the sector wheel 

 in these experiments was about 1/30 of a 

 second for one revolution. If slower speeds 

 were used, marked deviations from the law 

 began to appear, the intermittent having a 

 weaker effect than the constant light. In 

 some cases I got a marked reaction of the 

 eye on change from constant to intermittent 

 light of equal energy when the speed of the 

 sector wheel was about 1/10 of a second per 

 revolution. The deviation becomes more 

 marked, the slower the speed. The explana- 

 tion for this phenomenon will be dwelt upon 

 in a subsequent paper. 



Strictly speaking, the law proved by my 

 experiments is not the Bunsen-Roscoe law, but 

 the law discovered more than twenty years 

 earlier (1834) by Talbot, which states that the 

 effect of intermittent light equals that of a 

 constant light, if it emits the same amount 

 of energy through a given period. In our 

 case it means practically the same as Bunsen- 

 Roscoe's law, each revolution of the sector 

 wheel constituting one period, in which there 

 is a given relation between intensity and 

 duration of the light flash and a definite time 

 for reaction. The variously arranged sector 

 wheels provide the possibility of testing dif- 

 ferent ratios. The constant light coming from 

 the smaller diaphragm is used in such a way as 

 to serve as a measure or standard of compari- 

 son and circumvent the necessity of determin- 

 ing a threshold of stimulation. 



Wolfgang F. Ewald 



The Rockefellek Institute, 

 Department op Biology, 

 July 14, 1913 



