Significance of Tropisms for Psychology 39 



the retina are also chemically unlike. The observations upon 

 visual purple, the differences in the color sensitiveness of the 

 fovea centralis, and the peripheral parts of the retina indicate 

 that the points of symmetry of the two retinae are chemically 

 alike, the non-symmetrical points chemically unlike. 



Now if an unequal amount of light falls upon the two 

 retinae the photochemical reactions in the one which receives 

 more light will also be more accelerated than those in the other. 

 The same naturally holds true for every other pair of sym- 

 metrical photosensitive surface elements. For it should be 

 mentioned that photochemical substances are not found in 

 the eyes only, but also in other places on the surface of many 

 animals. In planarians, as my experiments and those of 

 Parker have shown, not only the eyes, but also parts of the 

 skin, are photosensitive. But if more light falls upon one 

 retina than upon the other, the chemical reactions will also be 

 more accelerated in the one retina than in the other, and 

 accordingly more intense chemical changes will take place in 

 one optic nerve than in the other. S. S. Maxwell and C. D. 

 Snyder have demonstrated, independently of each other, that 

 the rate of the nerve impulse has a temperature coefficient of 

 the order of magnitude which is characteristic for chemical 

 reactions. From this we must conclude that when two retinae 

 (or other points of symmetry) are illuminated with unequal 

 intensity, chemical processes, also of unequal intensity, take 

 place in the two optic nerves (or in the sensory nerves of the 

 two illuminated points). This inequality of chemical processes 

 passes from the sensory to the motor nerves and eventually to 

 the muscles connected with them. We conclude from this 

 that with equal illumination of both retinae the symmetrical 

 groups of muscles of both halves of the body will receive equal 

 chemical stimuli and thus reach equal states of contraction, 

 while, when the rate of reaction is imequal, the sjonmetrical 

 muscles on one side of the body come into stronger action than' 



