CHAP, in.] SIGHT. 923 



Visual purple is found as we have said exclusively in the 

 outer limbs of the rods; it has never yet been found in the 

 cones, and it is accordingly absent from (or exceedingly scanty 

 in) the retinas (such as those of snakes) which are composed of 

 cones only (or contain very few rods), and from the greater 

 part of the macula lutea and the whole of the fovea centralis of 

 the retinas of man and the ape. The intensity of the coloration 

 varies in different animals, and the retinas even of some animals 

 possessing rods (bat, dove, hen) seem to be wholly devoid of the 

 visual purple ; it is generally well marked in retinas in which 

 the outer limbs of the rods are well developed. Its absence or 

 presence is not dependent on nocturnal habits, since the intense 

 colour of the retina of the owl is in strong contrast to the 

 absence of colour in the bat. It has been found in the retina 

 of the embryo. 



The visual purple is bleached not only by white but also by 

 monochromatic light. Of the various prismatic rays the most 

 active are the greenish-yellow rays, those to the blue side of 

 these coming next, the least active being the red. Now it is 

 precisely the greenish-yellow rays which are most readily ab- 

 sorbed by the colour itself. A natural colour retina or a solu- 

 tion of visual purple gives a diffuse spectrum without any 

 defined absorption bands, and according to the amount of colour- 

 ing material through which the light passes, absorption is seen 

 either to be limited to the greenish-yellow part of the spectrum 

 or to spread thence towards the blue and, to a much less extent, 

 towards the red. Thus the various prismatic rays produce a 

 photochemical effect on the visual purple in proportion as they 

 are absorbed by it. Under the action of light the visual purple, 

 whether in solution, or in its natural condition in the rods, passes 

 through a purplish orange to a yellow, and finally becomes colour- 

 less ; and we appear to be justified in speaking of a "visual yel- 

 low" and "visual white" as products of the photochemical 

 changes undergone by the visual purple. 



For the restoration of the visual purple, after it has been 

 destroyed by light, the maintenance of the circulation of the 

 blood through the tissues of the eye is not essential. The reti- 

 nal epithelium has by itself, provided that it still retains its tis- 

 sue life, the power of regenerating the purple. If a portion of 

 the retina of an excised eye be raised from its epithelial bed, 

 bleached, and then carefully restored to its natural position, the 

 purple will return if the eye be kept in the dark. 



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

 dow 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 all 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 



