RETINA INSENSIBLE TO LIGHT. 365 



After a certain period he exaniined 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 wliich he drew has since been 

 abundantly confirmed ; namely, that surfaces become 

 warm in exact proportion to the depth of theii^ tint, be- 

 cause the darker the surface the greater the amount of 

 rays absorbed. A black surface, absorbing all rays, be- 

 comes the hottest. This principle Professer Draper 

 invokes in his examination of the eye. The pigment 

 layer is, he maintains, the real optical screen on which 

 the imao'es are formed : " The aro;uments aoainst the 

 retina, both optical and anatomical, are perfectly un- 

 answerable. During life it is a transparent medium, 

 as incapable of receiving an image as a sheet of clear 

 glass, or the atmosj)heric air itself ; and, as will be pre- 

 sently found, its sensory surface is its exterior one — 

 that is, the one nearest the choroid coat. But the 

 black pigment, from its perfect opacity, not only com- 

 pletely absorbs the rays of light, turning them, if such 

 a phrase may be used, into heat, no matter how faint 

 they may be, but also discharges the well-knowm duty 

 of darkening the interior of the eye. Perfection of 

 vision requires that the images should form on a 

 mathematical superficies, and not in the midst of a 

 transparent medium. The black pigment satisfies that 

 condition, the retina does not." * 



Now comes the difficidty. If the retina is insensible 

 to the light which passes through it, it will be equally 



* Draper : Human Physiology, p. 387. 



