BUNSEN-BOSCOE LAW 93 



cal reaction which results in the formation of a certain 

 mass of a reaction product. This mass acts on the periph- 

 eral nerve endings and brings about an as yet unknown 

 change in the brain elements with which these nerve end- 

 ings are connected. This change in turn affects the tone 

 or tension of the muscles with which the brain elements 

 are connected. When the rate of photochemical reaction 

 is the same in both eyes or in the photosensitive elements 

 on both sides of the body, the change of tone in the sym- 

 metrical muscles of both sides of the body is the same 

 and no change in the position or direction of motion of the 

 organism should occur. If the rate of illumination is dif- 

 ferent in both eyes, differences in the relative tension of 

 the symmetrical muscles occur, which make the motion 

 to the source of light easy and in the opposite direction 

 more difficult when the animal is positively heliotropic. 

 For the negatively heliotropic animal the opposite effect 

 will be brought about. 



These experiments, therefore, show that the tropism 

 theory not only allows us to predict the nature of the 

 animal reactions but allows us to predict them quantita- 

 tively. Thus far the tropism theory is the only one which 

 satisfies this demand of exact science. 



The degree of directness with which a heliotropic ani- 

 mal goes to or from a source of light depends, aside from 

 the degree of perfection of its locomotor apparatus, upon 

 the intensity of the light and the relative sensitiveness 

 of the animal. Animals which in strong light will move 

 in approximately straight lines to or from the source of 

 light may in weak light reach their goal in a more or less 

 irregular zigzag line. This is easily understood. When 



