RETINA INSENSIBLE TO LIGHT. 347 



"Would you know the peculiarity of that spot ? It is a mass 

 of cells, without a continuous surface-layer of fibres. After 

 proving that the fibres are insensible to light, and that no 

 image is formed where the fibres alone exist, we are called 

 upon to show that some apparatus exists for the reception 

 of these rays of light out of which the necessary images are 

 formed ; and to Professor Draper we must turn for the best 

 hypothesis to aid us. 



Franklin, he reminds us, placed variously-coloured pieces 

 of cloth in the sunlight on the snow. They were so arranged 

 that the rays should fall on them equally. After a certain 

 period he examined them, and found that the black cloth 

 had melted its way deeply into the snow, the yellow to a less 

 depth, and the white scarcely at all. The conclusion which 

 he drew has since been abundantly confirmed ; namely, that 

 surfaces become warm in exact proportion to the depth of their 

 tint, because the darker the surface the greater the amount of 

 rays absorbed. A black surface, absorbing all rays, becomes 

 the hottest. This principle Professor Draper invokes in his 

 examination of the eye. The pigment layer is, he maintains, 

 the real optical screen on which the images are formed : " The 

 arguments against the retina, both optical and anatomical, 

 are perfectly unanswerable. During life it is a transparent 

 medium, as incapable of receiving an image as a sheet of clear 

 glass, or the atmospheric air itself ; and, as will be presently 

 found, its sensory surface is its exterior one — that is, the 

 one nearest the choroid coat. But the black pigment, from 

 its perfect oijacity, not only completely absorbs the rays of 

 light, turning them, if such a phrase may be used, into heat, 



