INORGANIC RESPONSE 149 



the destructive breaking-down of the tissue, followed 

 by its renovation. Some support was at first given 

 to this chemical theory by the bleaching action of light 

 on the visual purple present in the retina, but it has 

 been found that the presence or absence of visual 

 purple could not be essential to vision, and that its 

 function, when present, is of only secondary importance. 

 For it is well known that in the most sensitive portion 

 of the human retina, the fovea centralis, the visual purple 

 is wanting ; it is also found to be completely absent 

 from the retinas of many animals possessing keen 

 sight. 



(2)1 Electrical theory. The second, or electrical, theory 

 supposes that the visual impulse is the concomitant of 

 an electrical impulse ; that an electrical current is 

 generated in the retina under the incidence of light, 

 and that this is transmitted to the brain by the optic 

 nerve. There is' much to be said in favour of this view, 

 for it is an undoubted fact, that light gives rise to 

 retinal currents, and that, conversely, an electrical 

 current suitably applied causes the sensation of light. 



Retinal currents. Holmgren, Dewar, McKendrick, 

 Kuhne, Steiner, and others have shown that illumination 

 produces electric variation in a freshly excised eye. 

 About this general fact of the electrical response there 

 is a widespread agreement, but there is some differ- 

 ence of opinion as regards the sign of this response im- 

 mediately on the application, cessation, and during the 

 continuance of light. These slight discrepancies may 

 be partly due to the unsatisfactory nomenclature as 

 regards use of terms positive and negative hitherto in 



