VISION DEPENDENT ON TEMPERATURE. 349 



and, therefore, as when we pass the tip of the finger over the 

 surfaces of bodies, and recognise cold and warm spaces there- 

 upon, the same process occurs with infinitely more delicacy 

 in the eye. The club-shaped particles of Jacob's membrane 

 are truly tactile organs, which communicate to the sensory 

 surface of the retina the condition of temperature of the 

 black pigment." 



It is worth remarking that the analogy in structure between 

 the retina and the recently-discovered organs of touch 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 inten- 

 sity, ceases to have any chemical effect, and hence photo- 

 graphic 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 surface being a mathema- 

 tical 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, all objects 

 would be nebulous on the edges." 



To explain the process by which the change of temperature 



* See Leydig, Ilistologie ; and FuNKE, Physiologie, where diagrams are 

 given. 



