2 Photographs on the Retina. 



of different colours, and obtained interesting results helping 

 to explain some of the curious phenomena of colour-blindness, 

 to which reference will be made further on. His communi- 

 cations are to be found in the Monatshericht for November, 



1876, and January and February, 1877. 



The subject obtained considerable development in the 

 hands of Professor W. Kiihne, of Heidelberg, who has pub- 

 lished his results in a collected form in the Heidelberg TJnter- 

 suchungen, Vol. I., 1877, with which I am acquainted only 

 at second hand in Schnidfs Jahrhucher, No. 10, December, 



1877. He found that the colours ^een by Boll are not 

 merely owing to refraction, but that there is an actual 

 pigment which he has succeeded in isolating in the form of 

 a solution. His first efforts to obtain optograms failed alto- 

 gether ; but he has had more success subsequentl}'" by the 

 help of improved methods. One of his experiments was 

 conducted in the following way: — The head of a rabbit, 

 with the one eye fixed open, was held in front of an opening 

 in a window shutter, and after being covered for five minutes 

 with a black cloth, was exposed to the hght. The animal was 

 then quickly decapitated, the eye removed under the sodium 

 light, opened, and laid in 5 per cent, solution of alum. The other 

 eye was exposed to the light after decapitation. Both retinas 

 were examined next morning, and found of the usual milky 

 appearance, but close inspection showed on both a sharply 

 defined quadrangular figure of the same form as the opening 

 in the shutter. In the eye which had been acted on during 

 life there was still a reddish colour, but in the other the 

 figured spot was quite white. In another experiment Kiihne 

 succeeded in getting a complete picture of a window with 

 one round-topped and six square panes, white on a red 

 ground, the cross markings being also red. The method now 

 followed is, to place the head of an animal, or the extirpated 

 eye in a box, whose lid is formed of a plate of dim glass> on 

 which can be laid figures cut out of black paper. The retina, 

 on which the figure has become printed, is laid on a porcelain 

 plate, and dried over sulphuric acid, when the picture is 

 found to be more permanent. The eyes of other animals 

 than the rabbit have been used, and Kiihne has found that 

 of the ox to be sensitive for about an hour after death. The 

 presence of the pigment is not dependent on the retina being 

 in a state of freshness as regards its functional capacity. It 

 is bleached only by light; very quickly (in about thirty 

 seconds) by direct sunlight, and in twenty to thirty minutes 



