88 TROPISMS 



distances of the source of light, selected, as stated, on the 

 assumption that the Bunsen-Roscoe law holds. On that 

 assumption the ratio of percentage bent in any two or all 

 three dishes on any one day should equal 1.0. These ratios 

 for each pair of distances of the source of light are 

 given in the three other columns of the table. The per- 

 centage bending was only compared in dishes containing 

 material regenerated and exposed on any one day, since 

 only in this case was there any likelihood that the material 

 was in any way uniform, and since only in this case the 

 experiments Avere carried on at the same temperature and 

 the same conditions of regeneration. 



The result was that the observed ratios were as 

 1.02 : 0.99 : 1.02 (with a probable error of ± 0.01) while the 

 values calculated on the assumption of the validity of 

 the Bunsen-Roscoe law were as 1:1:1; i.e., the results 

 showed as great an approximation between observed and 

 calculated values as one could expect. 



There is a second method for testing the validity of 

 the Bunsen-Roscoe law, based on the use of two sources of 

 light of equal intensity. 



If it is true that the heliotropic efficiency of light is 

 determined by the product of intensity, i, into duration of 

 illumination, t, we can alter this product by varying t as 

 well as by varying i. 



Radl had shown that the position of the eye of the 

 fresh water crustacean, Daphnia, is determined by the 

 position of a. source of light, 447 and Ewald 145 found 

 that by exposing the eye to two different sources of light 

 simultaneously the eye is put into a position determined 

 by the relative intensity of the two lights. When one light 

 remained constant and the intensity of the other light 

 was lowered the position of the eye changed. He now 



