518 VISUAL SENSATIONS. [BOOK in. 



then carefully restored to its natural position, the purple will 

 return if the eye be kept in the dark. The choroidal epithelium 

 may in fact be spoken of as a 'purpurogenous' membrane. 



If the image of some bright object such as a lamp or a window 

 be thrown on to the retina, either of an eye in its natural position 

 or of one recently excised, care having been taken to keep the 

 retina for some time previous away from any rays of light, the 

 portion of the retina on which the rays have fallen will be found 

 to be bleached, the rest of the retina remaining purple. In fact 

 an " optogram " of external objects may thus be obtained ; and 

 if the retina be removed and treated with a 4p.c. solution of 

 potash alum before the choroidal epithelium has had time to 

 obliterate the bleaching effects, the retina may remain permanently 

 in that condition: the photochemical effect may, as the photo- 

 graphers 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 

 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. Unfortunately visual purple is absent 

 from the cones, and from the fovea centralis which, as we shall 

 see, is the region of 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 purple has been absolutely bleached, as it may be by 

 prolonged exposure of the eyes to strong light. We cannot there- 

 fore, at present at least, explain the origin of visual impulses by 

 the help of visual purple. 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. 

 And, as we shall see later on, one theory of colour vision is based on 

 the assumption that vision is carried on in some way or other by 

 changes in what may be called visual substances present in the 

 retina, these substances being used up and regenerated as vision is 

 going on. 



But even admitting as probable the existence of these sensitive 

 visual substances, the changes in which lead to stimulation of the 

 real endings of the retinal nervous mechanism, we cannot at 

 present state anything definite concerning those nerve endings or 

 the manner of their stimulation. It may be that even the outer 

 limbs of the rods and cones, in spite of the apparent break of 

 continuity between the outer and inner limbs, are really nervous 

 in nature. It may be on the other hand that the outer limbs 



