MEMOIR VI.] HYPOTHESES OF VISION. 1Q3 



of an organ of vision. There being no optical apparatus 

 for the production of images, the luminous impression 

 must be felt as heat. For this the dark pigment is well 

 designed. It is an old physical experiment to lay upon 

 the snow on a sunshiny winter day pieces of differently 

 colored cloth. They will melt their way to a greater 

 depth in proportion as their tint is deeper; the black, 

 becoming the warmest, sinks deepest; the white, reflect- 

 ing most of the heat, scarcely melts the snow at all. 

 Now an animal destitute of any visual organ can only 

 be affected by the impressions of light in a very doubt- 

 ful manner; but if there be upon its exterior a black 

 spot, not only is there a much higher sensitiveness be- 

 cause of the increased absorptive power for heat, but 

 the sphere of its consciousness is greatly extended, from 

 the possibility of acquiring a knowledge of directions in 

 space a knowledge that becomes more and more exact 

 with the increasing number and symmetrical arrange- 

 ment of these ocelli. 



If we apply these principles to a more perfect form of 

 eye, as that of man, we are led to a new interpretation 

 of the function of some of its parts. The black pigment 

 becomes the receiving surface for images of external 

 things, and rays falling upon it, in their diversity of col- 

 or, brightness, and shade, in the act of becoming extin- 

 guished, engender heat. As with the tip of the finger 

 passing over an object we can discover, even in the dark, 

 spaces that are warm and those that are cool, so the rods 

 and cones of Jacob's membrane, acting as tactile organs, 

 convey to the brain a knowledge of the momentary dis- 

 tribution of heat on the dark concave of the eye. The 

 pigment has therefore a far more important office to dis- 

 charge than that of merely extinguishing stray light and 

 darkening the inside of the globe. 



But this calorific hypothesis is not without great dif- 



