SIGHT AND THE VISUAL ORGAN. 



4 6 3 



place can the irritation of each separate spot of the retina produce a 

 peculiar impression corresponding to the presence of the object-point; 

 in short, according to the optical expression for such a relation, a pict- 

 ure of the outside world must be painted on the retina. 



And this is indeed what takes place. As, on the one hand, the 

 retina stands as the terminal apparatus of the optic nerve; on the 

 other hand, it acts as a shade subservient to optical purposes ; a screen, 

 on which a perspective picture of the outside world is projected. If 

 you compare it with the dull glass on which the picture in the camera- 

 obscura falls, or the prepared plate in the photographer's camera, you 

 have a correct notion of what I mean. As, in the photographer's cam- 

 era, the picture falls on the sensitive plate, and is impressed on it by 

 means of chemical changes produced by light, so in the eye it falls on 

 the sensitive plate of the retina, whose irritations are telegraphed to 

 the brain in due form. 



"We henceforth have to consider this image painted on the retina 

 as the real object of the operations of the senses. 



But how does the picture imprint itself on the retina ? This is done 

 by an optical apparatus close behind the retina and in connection with 

 it ; and, in short, by means of that mechanism known to us as the eye. 



If we compare the retina with the sensitive plate in a camera-ob- 

 scura, we shall perceive that the eye has indeed an undeniable resem- 

 blance with this well-known optical instrument, the camera-obscura. 



Fig. 2. 



A 



/ 



/ 



3, Sclerotic; C, Cornea; L, Crystalline Lens; K, Aqueous Humor; K', Vitreous Humor; A, Choroid; 



N, Optic Nerve and Retina. 



This is essentially a box painted black in the inside, with an open- 

 ing fitted with a collective lens, turned on the objects of the outside 

 world, and which receives the images produced by this lens on the 

 wall behind. In order to show us the image, the one side of the box 



