i 4 8 RESPONSE IN THE LIVING AND NON-LIVING 



CHAPTEE XYII 



ON THE STIMULUS OF LIGHT AND RETINAL CURRENTS 



Visual impulse : (1) chemical theory ; (2) electrical theory Retinal 

 currents Normal response positive Inorganic response under stimulus 

 of light Typical experiment on the electrical effect induced by light. 



THE effect of the stimulus of light on the retina is 

 perceived in the brain as a visual sensation. The 

 process by which the ether-wave disturbance causes 

 this visual impulse is still very obscure. Two theories 

 may be advanced in explanation. 



(i) Chemical theory. According to the first, or 

 chemical, theory, it is supposed that certain visual sub- 

 stances in the retina are affected by light, and that 

 vision originates from the metabolic changes produced 

 in these visual substances. It is also supposed that the 

 metabolic changes consist of two phases, the upward, 

 constructive, or anabolic phase, and the downward, 

 destructive, or katabolic phase. Various visual sub- 

 stances by their anabolic or katabolic changes are 

 supposed to produce the variations of sensation of 

 light and colour. This theory, as will be seen, is very 

 complex, and there are certain obstacles in the way 

 of its acceptance. It is, for instance, difficult to see 

 how this very quick visual process could be due to 

 a comparatively slow chemical action, consisting of 



