QUANTITATIVE DETERMINATION OF LIGHT REACTIONS 269' 



Recently Ewald ('13) has described experiments which prove 

 that the Bunsen-Roscoe law holds for the response to light in 

 the eye of Daphina, as Loeb has maintained it should hold in 

 animal light reactions (Ewald '13, p. 236): 



This law states that in a light reaction the effect is proportional to 

 the simple product of intensity and time. It was first proved to ])e 

 true for the formation of hydrochloric acid from chlorine and hydrogen 

 and for the blackening of silver chloride utider the influence of light. 

 Later it was found to apply to the phototropic curvature (Froschel, 

 Blaauw) of plants, as well as the human eye, though within rather nar- 

 row limits (Bloch, Charpentier). 



If the law holds for the light reactions of photosensitive ani- 

 mals, intensity must of course operate as a constant stimulus. 



Ewald found that the eye of Daphnia assumes "a definite 

 normal position with regard to light" and that if while the animal 

 remains fixed, the eye is subjected to lights from two sources, it 

 takes up a definite axial position depending on the relative in- 

 tensity of the lights. "In order to test the energy law, it is 

 necessary to combine different light intensities with different 

 times of exposure. If the product of time and intensity .... 

 is the same, the eye will always give the same reaction" (p. 236). 

 This was proven experimentally by observing the position of the 

 eye when subjected to one constant light and one light that could 

 be varied at will. The variable light could be taken either from 

 a constant low intensity source or through the apertures of a rotat- 

 ing sector wheel. An instantaneous shift from a slow, steady light 

 to an intense intermittent light, delivering the same amount of 

 light energy per second, caused no change in the position of the 

 eye. If a sector wheel is used, giving too long or too short 

 exposures to equalize the light, a change in the axial position of 

 the eye appeared when the difference was greater than 10 per 

 cent.^" "These observations prove that for the eye movements of 

 Daphnia the energy law holds within the limits of accuracy 

 characteristic of the reaction" (Ewald '13, p. 237). 



'" In these experiments, the speed of the sector wheel was about 30 revolutions 

 per second. With the reduction of the speed to 10 revolutions per second, a re- 

 action was in some cases obtained when the change was made from steady to 

 intermittent light. 



THE JOURNAL OF EXPERIMENTAL Z05L0GY, VOL. 17, NO. 2 



