BUNSEN-ROSCOE LAW 



357 



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

 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 perpendicular path outlined 

 at first. The relative intensities should be the governing factor. 

 These three predictions have been amply proved experimentally. 

 The following results (Table LXVI.) from Patten's investigations 

 illustrate the nature of the findings. 



TABLE LXVI. 



This table shows clearly that (col. 1) when the intensity of the 

 two lights was equal, the animal varied on the average only 0-09 

 degree from the perpendicular. It also demonstrates that when 

 the one light was reduced to three-quarters (col. 2) the intensity 

 of the other, the angle of deviation was about 8-86, and that when 

 a further reduction to a half was made, the angle of deviation was 

 more than doubled. Finally, the figures show that the angle of 

 deviation depends on the relative differences of light intensity and 

 is independent of absolute intensity (provided sufficient light is 

 present to overcome inertia (cf. stimulation, p. 193). 



(7) A model with a heliotropic mechanism has been constructed 

 by Hays Hammond, the inventor of the dirigible torpedo. The 

 principle on which the machine depends is the alteration in the 

 electrical resistance of metallic selenium when exposed to light. 

 The " eyes " are lenses separated from each other by a projecting 

 " nose " which permits the shading of one eye while the other is 

 illuminated. The lenses are each focused on separate selenium 



