348 SEA-SIDE STUDIES. 



110 matter how faint they may be, but also discharges the 

 well-known duty of darkening the interior of the eye. Per- 

 fection of vision requires that the images should form on a 

 mathematical superficies, and not in the midst of a trans- 

 parent medium. The black pigment satisfies that condition, 

 the retina does not." * 



Now comes the difficulty. If the retina is insensible to 

 the light which passes through it, it will be equally insensible 

 to the light which, according to some physiologists, is re- 

 flected 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 sensation produced ? Professor 

 Draper shall again furnish us with an answer: — "The pri- 

 mary effect of rays of light upon the black pigment is to 

 raise its temperatm-e, aud this to a degree which is in rela- 

 tion to their intensity aud intrinsic colour ; light which is of 

 a yellow 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 coiwrnences ; 

 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 

 explanation usually given of the nature of vision is correct ; 



• DiiAi'EU: Unman I'hysiologij, p. 387. 



