366 SEA-SIDE STUDIES. 



insensible to the light which, according to some phy- 

 siologists, is reflected from the pigment layer. 



On the other hand, although the pigment layer is 

 capable of absorbing light, we cannot suppose it also 

 sensitive to light. How, then, is the luminous sensa- 

 tion produced ? Professor Draper shall agam furnish 

 us with an answer : — " The primary eff'ect of rays of 

 light upon the black pigment is to raise its tempera- 

 ture, and this to a degree which is in relation to their 

 intensity and intrinsic colour ; liglit which is of a yel- 

 low tint exerting, as has been said, the most energetic 

 action, and rays which correspond to the extreme red 

 and extreme violet, the feeblest. The varied images of 

 external objects which are thus painted upon the black 

 pigment, raise its temperature in becoming extinguished, 

 and that in the order of their brilliancy and colour. 

 . . . In this local disturbance of temperature the 

 act of vision commences ; this doctrine being in perfect 

 harmony with the anatomical structure of the retina, 

 the posterior surface of which is its sensory surface, 

 and not the anterior, as it ought to be, if the explana- 

 tion usually given of the nature of vision is correct ; 

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

 over the surfaces of bodies, and recognise cold and 

 warm, spaces thereupon, 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 



