TALBOT S LAW 



457 



the elements sensitive to light. Rodless mice are not photo- 

 tropically sensitive (Keiler). 



(5) If this is true, it follows that the animal will olx-y the 

 Bunsen-Roseoe Law. This is rather troublesome to prove for 

 free-moving animals. The following table shows the applicability 

 of the law to regenerating polyps of Eiidcndrium. The intensity of 

 the light was altered by varying the distance between the source of 

 light and the polyps. 



TABLE LXXVI 



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 



itP = constant. 



For silver bromide gelatine plates, the value of the exponent }) 

 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 light from two sources ? One might predict that, if 

 Loeb's hypothesis is correct, the organism will be orientated so that 

 it comes to rest in a position where it is symmetrically stimulated. 

 («) If the two sources of light are of equal intensity and duration 

 and are set at an equal distance from the organism it should be 

 orientated with its plane of symmetry at right angles to the line 

 joining the sources of light, {b) If the lights are of unequal 

 intensity, the animal should move so that its photosensitive 

 elements are in a position to absorb equal amounts of light energy. 

 Further, the absolute intensities of light should have no effect on the 

 deviation of the path of the organism from the straight path 



