VISION DEPENDENT ON TEMPEKATURE. 367 



between the retina and the recently-discovered tactile 

 corpuscles is very close.* Professor Draper further 

 insists on the fact that all photographic effects result 

 from high temperature : " 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 neighbouring particles. 

 This conducted heat, by reason of its indefinitely lower 

 intensity, ceases to have any chemical effect, and hence 

 photographic images are perfectly sharp on their edges. 

 It may be demonstrated 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 sur- 

 face being a mathematical superficies, acting under the 

 preceding condition. All objects will therefore be 

 definite and sharply defined upon it, nor can there be 

 anything like lateral spreading. If vision took place in 

 the retina as a receiving medium, aU objects would be 

 nebulous on the edges.'' 



To explain the process by which the change of tem- 

 perature in the pigment becomes a luminous sensation 

 will not be difficult, if, remembering that the luminous 

 sensation is one not depending on the specific stimulus 

 of light, but on the specific nature of the optic centre, 

 we follow this change in its passage from the pigment 

 to the rods and cones of Jacob's membrane, which it 

 first affects ; these are in direct connection with the 



* See Leydig, Histologie ; and Funke, Physiologie, where diagi-ams 

 are given. 



