392 NERVOUS ffliCHANISM OF THE EYE. 



Of the Function of the Nervous Mechanism of the Eye. 



The reasons have already been given for considering that it is the 

 The black pig- black pigment which acts as the receiving eft optical screen, 

 ment, and not and no t the retina. If no other argument was adduced for 

 the receiving departing from the opinion usually expressed, which attrib- 

 screen. u ^es this function to the retina, the thickness of that struc- 



ture would be sufficient ; images can only form with precision or sharp- 

 ness upon an abrupt surface. And since it is now indisputably ascer- 

 tained that both the chemical effect and the heating effect of the rays of 

 light depend upon their absorption, those effects being in direct propor- 

 tion to the completeness with which absorption is taking place, we are 

 justified in inferring that, since the eye is sensible to rays of so low a 

 degree of intensity, and to each of the colored ones equally, its screen of 

 reception must not only be a superficies, but likewise a black one. Such 

 a surface the black pigment is. In the case of albinos, and animals in 

 which the black pigment is imperfectly developed, the receiving surface 

 or screen is still the interior of the choroid. Under such circumstances, 

 vision must be indistinct. 



Recalling what has been said respecting the diffuse sensibility of the 

 Heating effect lower members of the animal series to light, and the struc- 

 on the pigment, ture of ocelli, it accords well therewith to consider that the 

 primary effect of the rays of light upon the black pigment is to raise its 

 temperature, and this to a degree which is in relation to their intensity 

 and intrinsic color ; 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 the 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 brill- 

 iancy and color ; the pigment thus discharging a double duty, as a sur- 

 face of extreme sensibility for calorific impressions, and also as darken- 

 ing the interior of the globe. 



In this local disturbance of temperature, in my opinion, the act of 

 Manner of er- v^ 011 commences, this doctrine being in perfect harmony 

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

 the surfaces of bodies, and recognize warm and cold spaces thereupon, 

 the same 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 tern' 

 peratura. of the black pigment. 



