Significance of Tropisms for Psychology 41 



We will now designate as positively heliotropic those animals 

 which are forced to turn their head or move toward the source 

 of light, and as negatively heliotropic those animals which are 

 oriented or compelled to move in the opposite direction.^ 



The aphids serve here only as an example. The same phe- 

 nomena of positive heliotropism may be demonstrated with 

 equal precision in a great many animals, vertebrates as well as 

 invertebrates. We cannot, of course, give here an account of 

 all these cases. The reader who is interested in them must look 

 into the voluminous literature upon this subject. Heliotropism 

 is unusually common among the larvae of marine animals and 

 insects, but also not lacking in sexually mature individuals. 



Heliotropic animals are therefore in reality photometric 

 machines. According to photometric laws the intensity of 

 light varies with the sine of the angle at which the light strikes 

 a surface element of the animal (or with the cosine of the angle 

 of incidence). The animal is oriented by the light in such a 

 way that symmetrical elements of its photosensitive surface 

 are struck at about the same angle. In the presence of only 

 one source of light this condition is fulfilled if the axis of sym- 

 metry of the animal moves in the direction of the rays of light. 

 In this case the velocity of photochemical reactions on both sides 

 of the animal is the same and there is no reason why it should 

 deviate from this direction in its progressive motions. 



Experiments on the heliotropism of plants as well as on the 

 perception of light by our retina have shown that the effect of 

 light equals the product of the intensity into the duration 

 of illumination. This law is identical with the general law of 

 Bunsen and Roscoe which states that the chemical effect 

 of light is within wide limits equal to this product. We do 

 not yet know whether or not Bunsen's law holds good for the 

 heliotropic animals. If it does, we shall have to substitute 5\(^^ 



1 Whether an animal is positively or negatively heliotropic depends upon the 

 fact whether the light causes an increase or a decrease in the tension of the muscles . 

 Why light should have these opposite effects is as yet imknown. 



