764 SIGHT. 



color changes to yellow and then fades away, leaving, however, the retina 

 not only white, but more opaque than it was before. Upon examination 

 with the microscope it is found that the purple color is confined exclusively 

 to the rods and to the outer limbs of the rods, the inner limbs being wholly 

 devoid of it. 



652. The color of the rods is due to the presence of a distinct pigment, 

 the " visual purple," diffused through the substance of the outer limbs ; and 

 this may be extracted from the rods by dissolving these in an aqueous solution 

 of bile-salts. A clear purple solution is thus obtained, which is capable of 

 being bleached by the action of light, and in its general features and be- 

 havior is similar to the pigment as it naturally exists in the retina. 



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

 cones only, and from the macula lutea and 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 color of 

 the retina of the owl is in strong contrast to the absence of color in the bat 

 It has been found in the retina of the embryo. 



653. The visual purple is bleached not only by white, but also b] 

 monochromatic, light. Of the various prismatic rays the most active are tin 

 greenish-yellow rays, those to the blue side of these coming next, the leas* 

 active being the red. Now it is precisely the greenish-yellow rays which ar< 

 most readily absorbed by the color itself. A natural colored retina or a solu 

 tion of visual purple gives a diffuse spectrum without any defined absorption 

 bands, and according to the amount of coloring material through which tin 

 light passes, absorption is seen either to be limited to the greenish-yellow 

 part of the spectrum or to spread thence toward the blue, and, to a mud. 

 less extent, toward the red. Thus the various prismatic rays produce *. 

 photo-chemical effect on the visual purple in proportion as they are absorber 

 by it. Under the action of light the visual purple, whether in solution o* 

 in its natural condition in the rods, passes through a purplish-orange to a 

 yellow, and finally becomes colorless ; and we appear to be justified ih 

 speaking of a " visual yellow " and " visual white " as products of tin 

 photo-chemical 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 choroidal 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. The choroidal epithelium may, in 

 fact, be spoken of as a " purpurogenous " membrane. 



654. 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 re- 

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

 obtained ; and if the retina be removed and treated with a four per cent, 

 solution of potash alum before the choroidal epithelium has had time to 

 obliterate the bleaching effects, the retina may remain permanently in that 



