THE SENSES. 535 



The most striking character of the retinal red is that it is destroyed 

 by the action of light. On this account its existence remained long 

 unknown. When the retina is extracted from the eye of an animal 

 in the ordinary way, its exposure during the necessary manipulations 

 is usually sufficient to bleach its color and reduce it to the condition 

 of a grayish or opalescent membrane. In order to obtain it with its 

 normal hue, the animal should be kept in the dark for a short time 

 previous to death ; and the eyeball taken out and the retina extracted 

 by the light of a sodium flame, which has comparatively little effect 

 upon its color. If such a retina be exposed to bright daylight its pur- 

 ple-red tint, at first distinctly visible, is destroyed, according to Ku'hne, 

 in about half a minute. Under a dim daylight it lasts longer, and by 

 ordinary gaslight may continue visible for 20 or 30 minutes ; while in 

 a chamber lighted by the sodium flame, or in the dark, it remains for 

 24 or 48 hours, even after the tissues have lost their freshness and con- 

 sistency. 



By this means the existence of the retinal red has been demonstrated 

 in the rabbit, dog, ox, ape, and badger, in the owl and falcon, in the frog, 

 triton, toad, and salamander, and in several species of fish. In three 

 instances Kiihne found it in the human eye, extirpated in the dark or 

 in a sodium-lighted chamber, from subjects who had been protected 

 from the light for a certain time before death. 



The retinal red is also destroyed by the action of light during life. 

 This is not usually observable in an eye extracted with the above- 

 mentioned precautions, for the reason that during life the color is 

 regenerated nearly as fast as it is destroyed. Thus a living eye, under 

 moderate illumination, maintains the normal hue of its retina by the 

 constant reproduction of its coloring matter. But if long exposed to 

 light of considerable intensity it may become completely bleached, 

 though its color will be restored by repose in a darkened place. Kiihne 

 found that in frogs exposed to daylight, in a glass vessel with a white 

 bottom, the retina becomes bleached after several hours ; and that in 

 direct sunshine 15 minutes are sufficient to produce the same effect. But 

 if the animals be then kept in the dark, and examined at various inter- 

 vals, the color of the retina again begins to be perceptible in about 30 

 minutes, and is completely restored at the end of an hour and a half. 



The source from which the color is thus reproduced is the choroidal 

 epithelium, with which the retina lies in contact. If separated from 

 its attachment and exposed to daylight, its color disappears, as already 

 shown, in from 30 seconds to several minutes. But if allowed to 

 remain in the eyeball under a similar exposure, and then extracted 

 under the light of a sodium flame, on bringing it into ordinary day- 

 light it is at first of a deep red. In Kiihne 's experiments, a portion 

 of a frog's retina, separated from the choroid in daylight until quite 

 bleached, then replaced and allowed to remain in position in the dark 

 for a short time, exhibited, when finally removed, its normal red color. 

 It is accordingly evident that the regeneration of the color does not 



