1256 PHOTOCHEMISTRY OF THE RETINA. [BOOK m. 



It seemed very tempting, especially upon the first discovery 

 of it, to suppose that this visual purple is directly concerned in 

 vision. If we suppose that visual purple itself is inert towards, 

 produces no effect on, the endings of the optic nerve, but that 

 either visual yellow or visual white, i.e. some product of the action 

 of light on visual purple, may act as a stimulus to those endings, 

 the way seems opened to understanding how rays of light can give 

 rise to sensory impulses in the optic nerve. And such a view 

 receives incidental support from the fact that the visual efficiency 

 of rays of different wave-lengths corresponds very closely to their 

 photochemical efficiency towards visual purple; the greenish- 

 yellow rays which are most active towards visual purple are 

 precisely those which seem to us the brightest, most luminous, 

 which produce the greatest effect on our consciousness. But 

 visual purple is absent from the cones, it is in ourselves absent 

 from the fovea centralis, the region of most distinct vision ; it 

 is further entirely wanting in some animals which undoubtedly 

 see very well ; and lastly animals such as frogs, naturally possess- 

 ing the pigment, continue to see very well and even apparently 

 to see colours when their visual purple has been absolutely 

 bleached, as it may be by prolonged exposure of the eyes to 

 strong light. We cannot therefore, at present at least, explain 

 the origin of visual impulses by the help of visual purple. It is 

 difficult to suppose that it plays no part in the origination of 

 visual impulses; but even in a photochemical theory of vision 

 we cannot allot to it more than a subsidiary function, possibly 

 something analogous to the "sensitizer" of the photographer. 

 At the same time its history suggests that some substances, 

 sensitive like it to light, but unlike it, colourless and therefore 

 escaping observation, may exist, and by photochemical changes be 

 the means of exciting the optic nerves ; but if so we must suppose 

 that these substances, though colourless, are capable of absorbing 

 light, since otherwise they would not be acted upon by it. 



774. If the eyeball be steadily compressed so as to arrest or 

 greatly interfere with the circulation, the anaemic retina becomes 

 insensitive to light, so that the eye becomes temporarily blind. 

 If while the pressure is being applied the eye is directed to a 

 white and black surface, so arranged that half the field of vision 

 is white and the other half black, that is to say half the retina 

 is stimulated while the other half is at rest, and if as soon as 

 the white half becomes invisible, the black half is made white 

 (by the withdrawal for instance of the black sheet which fur- 

 nished the black half) then for an instant that new white half 

 becomes visible, though the sensation soon fades away. This 

 result has been interpreted as shewing the existence in the 

 retina of a "visual substance" nourished by the previous blood 

 supply but ceasing to be replenished when the blood supply was 

 interfered with. We may suppose that over the half of the 



