356 



TROPISMS THE SLAVES OF THE LAMP 



through the source of light, provided only one source of light be 

 present. If, however, the light falls sideways on the animal the 

 rate of photochemical reaction will be unequal in both eyes and 

 the rate at which the symmetrical muscles on both sides of the 

 body work will no longer be equal ; as a consequence, the direction 

 in which the animal moves will change. This change will take 

 place in one of two ways, according as the animal is either posi- 

 tively or negatively heliotropic." 



(5) If this is true, it follows that the animal will obey the Bunsen- 

 Roscoe Law. This is rather troublesome to prove for free-moving 

 animals. The following table shows the applicability of the law 

 to regenerating polyps of Eudendrium. The intensity of the light 

 was altered by varying the distance between the source of light 

 and the polyps. 



TABLE LXV. 



The calculated values of t tend to be somewhat larger than the 

 observed results. Schwarzschild observed that when develop- 

 ment followed exposure to light the formula should be modified to 



it p = constant. 



For silver bromide gelatine plates, the value of the exponent p 

 varies between 0-8 and 1 according to the brand of the plate. 



Talbot's Law is the Bunsen-Roscoe Law modified to make it 

 applicable to intermittent light. Intermittent light is as effective 

 as constant light of the same intensity provided that the total 

 duration of the intermittent light is equal to that of the constant 

 light. 



(6) What is going to be the result when the organism is sub- 

 jected to two sources of light ? One might predict that, if Loeb's 

 hypothesis is correct, the organism will be oriented so that it comes 

 to rest in a position where it is symmetrically stimulated, (a) If 

 the two sources of light are of equal intensity and duration and 



