THE MECHANISTIC CONCEPTION 9 



worked out by Helmholtz during the middle of the 

 last century, and have required little amplification. 

 The resemblance of the mechanism of the human 

 eye to a photographic camera is so close that only a 

 dull mind fails to see it. The analogy pertains not 

 merely to the crystalline lens, but to the extremely 

 sensitive receiving mechanism of the retina. 



In 1840 Kiihne, the Heidelberg physiologist, dis- 

 covered the singularly sensitive and unstable sub- 

 stance known as the retinal red. This evasive sub- 

 stance, so easily destroyed by the ordinary methods 

 of chemistry, apparently forms an evanescent, 

 constantly renewed layer in the retina. This layer, 

 although not essential to sight in all animals, never- 

 theless seems to sensitize the retina to the rays of 

 light which fall on it after convergence there through 

 the crystalline lens. The retinal red is thus the 

 analogue of the light-sensitive film of a photographic 

 plate. It is clearly the basis of a process in which 

 chemical changes are prominent. And not merely 

 does this wonderful substance in some way aid vision, 

 but it also gives us a possible intimation a vague 

 one, but the most definite we have as to how 

 certain layers of nerve cells in the brain may be sen- 

 sitized to the reception of some sorts of sensory 

 stimuli through the agency of chemically unstable 

 substances. Another instance of a living mechanism 

 which has its physical parallel outside the body is 

 furnished by the structures concerned with hearing. 

 The eardrum has a close analogue in the vibratory 



