Animal Instincts and Tropisms 267 



will be put at a less efficient angle and hence the rate 

 of photochemical reaction on this side will be diminished. 

 The deviation from the perpendicular in which the 

 animal will ultimately move will be such that as a 

 consequence, the rate of photochemical reaction in 

 symmetrical elements is again equal. The ultimate 

 direction of motion will, according to our theory always 

 be such that the mass of chemical products formed 

 'under the influence of light in symmetrical photo- 

 sensitive elements during the same time is equal. 



Patten also investigated the question whether the 

 same difference of percentage between two lights would 

 give the same deviation, regardless of the absolute 

 intensities of the lights used. The absolute intensity 

 was varied by using in turn from one to five glowers. 

 The relative intensity between the two lights varied 

 in succession by o, 8J, 16%, 25, 33^, and 50 per 

 cent. Yet the angular deflections were within the 

 limits of error identical for each relative difference of 

 intensity of the two lights no matter whether, I, 2, 3, 

 4, or 5 glowers were used. The following table shows 

 the result. 



