ANATOMY AND PHYSIOLOGY OF THE RETINA. 157 



On the other hand, a temperature of 100° C. destroys the 

 colour, and alcohol, glacial acetic acid, and strong solution 

 of sodium hydrate produce the same effect. 



Kiihne's next observations were directed to the discovery 

 of the influence of light of different colour upon the vision- 

 purple. It would appear that the more refrangible rays of 

 the spectrum have the greatest action, and that the red rays 

 are as inactive as the yellow. 



Kiihne now found the incorrectness of Boll's assertion that 

 the retina of the living eye exposed to ordinary daylight does 

 not exhibit the vision-purple, for on preparing the eyes of 

 animals which had just been exposed to light, as rapidly as 

 possible in the chamber illuminated by sodium light, he dis- 

 covered that the retina was of a beautiful purple. It was 

 only when eyes were exposed for a considerable time to the 

 direct action of the sun's rays that a fading of the purple 

 colour was perceived. 



A most suggestive experiment now threw some light upon 

 the circumstances which retard the decolorisation, and which 

 restore the vision-purple. The two recently extirpated eyes 

 of a frog were taken; from one the retina was removed, 

 whilst an equatorial section was made through the other eye, 

 so as to expose the retina and still leave it in sit%. Both 

 preparations were exposed to diffuse daylight, until the iso- 

 lated retina had lost its purple colour. On now taking the 

 other preparation into the yellow chamber and removing the 

 retina, it was found that its colour yet remained : it was dark 

 red, but was bleached when exposed in its naked condition 

 to daylight. 



This experiment was confirmed by others, in which the 

 effect of strong sunlight was substituted for that of diffuse 

 daylight. 



But the most curious results of Professor Kiihne's experi- 

 ments have reference to the restoration of the vision-purple. 

 If an equatorial section be made through a recently extirpated 

 eye, and a flap of retina be lifted up from the underlying 

 choroid and exposed to light, the purple colour of the flap 

 will be destroyed, whilst the colour of the rest of the retina 

 persists. If, however, the bleached portion of the flap be 

 carefully replaced, so that it is again in contact with the 

 inner surface of the choroid, complete restoration of the vision- 

 purple occurs. This restoration is a function of the living 

 choroid, probably of the living retinal epithelium {i. e. of 

 the hexagonal pigment-cells, which used formerly to be de- 

 scribed as a 2icirt of the choroid), and it appears to be inde- 

 pendent of the black pigment which the retinal epithelium 



