924 PHOTOCHEMISTRY OF THE RETINA. [BOOK m. 



purple. In fact an " optogram " of external objects may thus 

 be obtained ; and if the retina be removed and treated with a 

 4 p.c. solution of potash alum before the retinal epithelium has 

 had time to obliterate the bleaching effects, the retina may 

 remain permanently in that condition : the photochemical effect 

 may, as the photographers say, be "fixed." 



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 con- 

 sciousness. But visual purple is absent from the cones, it is in 

 ourselves absent from the fovea cen trails, 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 possessing the pigment, continue to see very 

 well and even apparently to see colours when their visual pur- 

 ple 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 photochemi- 

 cal 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, colour- 

 less and therefore escaping observation, may exist, and by photo- 

 chemical changes be the means of exciting the optic nerves; 

 but if so we must suppose that these substances, though colour- 

 less, are capable of absorbing light, since otherwise they would 

 not be acted upon by it. Apart from their providing visual 

 purple the cells of the retinal epithelium, with their remarkable 

 amreboid pigment-carrying filamentous processes, have probably 

 in other ways to do with vision, though we cannot at present 

 state what their exact function in this respect is. Their impor- 

 tance in vision is indicated by their behaviour towards light. 



If an eye be fully exposed to light before removal and ex- 

 amination, the processes carrying pigment are found to stretch 

 a long way inwards between the outer limbs of the rods and 



