Photographs on the Retina. 3 



by gaslight; whilst in the dark or in the sodium light it does 

 not disappear in less than twenty-four to forty-eight hours. 

 During life, and even for some time after death, the colour is 

 continually renewed, and does not owe its existence there- 

 fore to the continuance of the circulation of blood in the eye, 

 but to the layer of epithelium which connects the outer por- 

 tion of the rods with the choroid. 



It was mentioned that Kllhne had obtained the red pig- 

 ment in solution. It is got by adding a clear watery solution 

 of crystallised ox gall to the fresh retina, on which it has a 

 remarkable effect, causing the plates composing the outer 

 section of the rods, to fly asunder like coins from a roll, and 

 then wholly disappear. The solution thus obtained is of a 

 rich carmine hue, and gradually bleaches in the light, passing 

 first into yellow. Monochromatic light also acts on it in the 

 same way, though more slowly, the most active being green 

 and yellowish green (in about fifteen minutes), then blue in 

 about an hour, violet still longer, and pure (spectral) red 

 having very little influence on it. 



The part played by retina red in the physiology of vision 

 can in the present state of our knowledge be Little more than 

 matter of speculation. That it is indispensable to mere 

 visual perception can scarcely be held, since it is absent, or 

 at least has not yet been found, in the retina of many 

 animals which certainly see — such as the pigeon, the hen, 

 the bat ; and further is not to be found in the yellow spot, 

 the seat of direct vision in man, which has no rods. Its 

 importance, however, can scarcely be doubted when we con- 

 sider that it has been discovered in almost aU animals, and 

 also in view of the remarkable influence exerted on it by 

 ordinary white light. Two cases reported by Dr. Adler, of 

 Vienna, also testify to its importance. In one of these an 

 eye which had been blind for several years had no trace of 

 the red colour. In the other case one eye was partially 

 blind, and the afiected half of the retina was colourless, the 

 other half showing a distinct rose tint, like that in the sound 

 eye. 



It may serve in some way for the perception of colours, 

 the varying effect on it of difierent kinds of coloured light 

 pointing in that direction. Boll noticed that the microscopic 

 appearance of the coloured rods was very much the same in 

 animals which had been kept for a time under red and green 

 glass, while it differed considerably when the cover had been 

 blue ; and he connected this with the well-known fact that 



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