148 RESPONSE IN THE LIVING AND NON-LIVING 
CHA PTME xOvat 
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 heht 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. 
(1) 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 
