92 



TROPISMS 



B, it was assumed that the ratio of effects would be the same as if, with 

 constant light, B had been placed at the double distance and the ratio 

 of intensities of the two lights had been 4 : 1. Going on such a calculation 

 we should expect the same values for a as in Table VII. 



As one sees from Table VIII, the observed values are slightly smaller 

 but practically identical with the values obtained when the two lights 

 are constant. The deviation is probably due to the well established 

 fact that the photochemical efficiency of an intermittent light is a trifle 

 less than that calculated on the basis of the Bunsen-Roscoe law. 



TABLE VIII 



Value of a when one light is inter- 

 mittent (90 sector) and the 

 other constant, and the efficiency 

 of the two lights is calculated on 

 the basis of the validity of the 

 Bunsen-Roscoe photochemical 

 law 



We carried out some experiments with a sector of 144. When the 

 efficiency of both lights was equal on the assumption of the validity 

 of the Bunsen-Roscoe law a was found to be 44.9 (instead of 45), and 

 for the ratio 2 : 1 a was found to be 38.8. The values are, within the 

 limits of error, identical with the values in Tables VII and VIII. 309 



Bradley M. Patten also showed that for the heliotropic 

 reactions of the negatively heliotropic larvas of the fly 

 the law of Bunsen and Eoscoe holds. 



Photochemical processes have a very small tempera- 

 ture coefficient and it agrees with this that lowering of 

 temperature within the limits compatible with the motility 

 of animals does not affect the heliotropic response ; on the 

 contrary, we shall see that in certain crustaceans (e.g., 

 Daphnia) lowering of the temperature may enhance posi- 

 tive heliotropism. 296 



We must, therefore, conclude that the light produces 

 in an eye or an element of the photosensitive skin a chemi- 



