458 TROPISMS—THE SLAVES OF THE LAMP 



outlined at first. The relative intensities should be the governing 

 factor. These three predictions have been amply proved experi- 

 mentally. The following results (Table LXXVII.) from Patten's 

 investigations illustrate the nature of the findings. 



TABLE LXXVII 



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 line of the original path. 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. 235). 



(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 

 cells. The heliotropic machine consists of a rectangular box 

 about 3 ft. long, 1 J ft. wide and 1 ft. high mounted on three wheels, 

 two of which are geared to a driving motor. The third wheel, 

 mounted at the rear-end, controls the steering. It may be turned 

 right or left by the differential action of two solenoid electro- 

 magnets. The selenium cells are in circuit both with the driving 

 motors and with the steering magnets. In the former case, the 

 selenium cells control a series of very sensitive relays (cf. nervous 

 system) in such a way that the amount of energy sent through the 



