536 THE NERVOUS SYSTEM. 



depend on the circulating blood, since it will take place in the extir- 

 pated eyeball ; but is affected by the aid of the choroidal epithelium. 



The coloring matter of the retina is soluble in purified bile, or in 

 watery solutions of the biliary salts, and has been extracted by Kiihne 

 in this way under the form of a transparent solution. The freshly- 

 extracted frog's retina is macerated from one to two hours in one 

 cubic centimetre of a five per cent, watery solution of the biliary salts. 

 It is then replaced, in the same solution, by another retina, also freshly 

 prepared ; and so on until 20 or 30 retinas have been employed for the 

 purpose. The mixture is then filtered and the filtrate allowed to stand 

 until the pigment granules mingled with it have subsided to the bot- 

 tom, after which the supernatant liquid is removed by a pipette. It 

 forms a clear solution of a carmine-red color. By concentration it 

 assumes a more violet tinge, and if diluted becomes rose-red or pale 

 lilac, according to the amount of dilution. 



Solutions of the retinal red are bleached by light, in the same man- 

 ner as the retina itself. Their color changes, under these circumstances, 

 first to a clear red, then becoming orange, then yellowish, and lastly 

 they are entirely decolorized. Similar changes are effected in the dark 

 by an elevated temperature ; beginning at 50 or 52 C., becoming 

 more rapid as the temperature rises, and taking place almost instan- 

 taneously from 10 to 14 C. 



The local bleaching of the retina under concentrated illumination 

 makes it possible to obtain retinal optograms, that is, colorless images 

 of brilliant objects which have been placed before the eye, surrounded 

 by the purple-red hue of the remaining retina. The first result of this 

 kind was obtained by Kiihne in the following manner : The fresh retina 

 of a rabbit, extracted under the light of a sodium flame, was spread out 

 on a glass plate and secured by a thin cover-glass on which were sev- 

 e"ral strips of tinfoil, each about one millimetre in width. In this con- 

 dition it was exposed to light until the bleaching process was complete ; 

 and on removing the cover-glass, bands of unchanged purple-red were 

 visible in the retina, wherever it had been protected by the tinfoil. As 

 the form of any luminous object in front of the eye is concentrated 

 upon a single part of the retina, this part will be bleached, if the 

 exposure be sufficient, while the remaining portions retain their color, 

 thus presenting a positive image of the luminous object. The method 

 adopted by Kiihne for obtaining optograms in the rabbit's or ox's eye 

 is as follows : The eyeball is taken out in a dark chamber with the aid 

 of the sodium flame, and fixed, with the cornea upward, in a blackened 

 box or cylinder, the cover of which is removable. The box containing 

 the eyeball is then placed upon a table directly beneath an illuminated 

 skylight of ground glass, at about four metres' vertical distance, and the 

 cover removed. After an exposure of from one to twenty minutes, 

 according to the intensity of the daylight, the opaque cover is replaced, 

 the eyeball opened in the dark chamber by an equatorial incision, its 

 posterior half freed from the vitreous humor, and placed for twenty- 



