PHOTOGRAPHIC RELATIONS OF THE EYE. 393 



But this communication of a variation of temperature implies a varia- 

 tion in the waste and repair of the retina itself, for there can be no doubt 

 that all such changes are accelerated by an increase of heat, and dimin- 

 ished by its decrease. And though in this manner the origin of the ac- 

 tion which has been set up is calorific, and therefore physical, it imme- 

 diately becomes converted into a physiological equivalent in the meta- 

 morphosis and destruction of a nervous tissue. 



The eye can not perceive rays which come from a luminous source 

 the temperature of which is lower than 1000 F., for such rays can not 

 pass through a stratum of water or through the humors of the eye. 

 Natural philosophers, in making a distinction between light and heat, 

 have too often overlooked the fact that, though thermometers are sensi- 

 tive to rays of every sort, the eye is not. Its indications are complicated 

 by the necessary introduction of absorbent media, which stop all rays 

 the refrangibility of which is low. 



Many years ago, Count Rumford, from a limited examination of cases, 

 concluded that all photographic effects are the effects of Photographic 

 a high temperature. From an examination, continued for ff? t g ts f "tfh 

 many years, of numerous phenomena of the same class, which temperature. 

 have since been described, I have come to the same conclusion. The 

 impinging of a ray of light on a point raises the temperature of that 

 point to the same degree as that possessed by the source from which the 

 ray comes, but an immediate descent takes place through conduction to 

 the neighboring particles. This conducted heat, by reason of its indef- 

 initely lower intensity, ceases to have any chemical effect, and hence 

 photographic images are perfectly sharp on their edges. It may be dem- 

 onstrated that the same thing takes place in vision, and in this respect 

 it might almost be said that vision is a photographic effect, the receiving 

 surface being a mathematical superficies, acting under the preceding con- 

 dition. All objects will therefore be definite, and sharply defined upon 

 it, nor can there be any thing like a lateral spreading. If vision took 

 place in the retina as a receiving medium, all objects would be nebulous 

 on the edges. This sharpness and grading off are happily illustrated by 

 the metal daguerreotype and paper photograph respectively. 



Perhaps it might be thought that the sharpness of impressions upon 

 collodion or albumen stands in opposition to what is here Absorption nee- 

 said respecting the inefficiency of translucent media. Those 

 substances, however, would be totally inert unless there had tion 

 been purposely mingled with them some compound of easy decomposi- 

 bility, capable of absorbing the "blue rays, which are in these cases the 

 effective photographic ones. Such a compound must commonly be of a 

 yellow color. In these substances the absorption takes place with en- 

 ergy the moment the light has entered their surface. In the Philosoph- 



