CHAP, in.] SIGHT. 1255 



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 de- 

 pendent 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 absorbed by the 

 colour itself. A natural coloured retina or a solution of visual 

 purple gives a diffuse spectrum without any defined absorption 

 bands, and according to the amount of colouring 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 colourless ; and we appear to be 

 justified in speaking of a "visual yellow" 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 retinal 

 epithelium has by itself, provided that it still retains its tissue 

 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 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 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 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 perma- 

 nently in that condition: the photochemical effect may, as the 

 photographers say, be "fixed." 



