470 VISION WITHOUT VISUAL PURPLE. [BOOK I. 



While the epithelium at the back of the retina is the agent in the 

 restoration of the visual purple, it is ascertained that it may impart 

 something to the rods themselves, leading to " auto-regeneration" as 

 Kiihne terms it. Frequently when an isolated retina is bleached it 

 will on being removed from the light regain somewhat of its purple 

 colour ; and similarly bile solutions of the visual purple, if they contain 

 no ether, may also exhibit this " auto-regeneracy," especially if both 

 retina and epithelium have been employed in making the solution. 



Do the rods then contain a something out of which the visual 

 purple may be regenerated, and are the epithelial cells the agents of 

 this elaboration, withdrawing the supposed substance from the rods 

 and working it up into visual purple ? 



In concluding this account of the visual purple it is 

 Vision expedient to point out what bearing, if any, the facts 



Without i j "L j i_ i IT 



Visual Purple which nave been described nave upon our knowledge 

 of the physiology of vision. 



The most sensitive region of our eye that which we turn upon 

 any object which we wish to see with the utmost distinctness con- 

 tains cones only, and cones are just those elements of the bacillary 

 layer which are destitute of visual purple. Again, many animals which 

 are keen-sighted may be seen to have retinae which are quite free 

 from this colouring matter. 



Under these circumstances we cannot assert that these beautiful 

 discoveries relating to the visual purple have succeeded in solving the 

 tempting problem as to the mode in which light affects the retina. 

 They, however, open up a promising field for speculation and hold out 

 inducements to those who would pursue similar lines of enquiry. The 

 changes in the visual purple are perhaps little more than accidental 

 accompaniments of more important, though by our senses unseen, 

 chemical processes processes which may in reality be initiated by 

 the undulatory movements of the ether and of which the results may 

 be the real stimuli which normally throw the optic nerves into 

 activity. 



