THE SENSE OF SIGHT. 



669 



the rods and cones, but after exposure to light it becomes displaced and 

 extends over and between the rods almost to the external limiting mem- 

 brane. These conditions are represented in Fig. 329. 





FIG. 329. SECTION OF THE RETINA OF A FROG. A. In darkness. B. In light. 

 (After Van Genderen Start, from T scheming 1 s "Physiologic Optics."} 



The Eye a Living Camera. In its construction, in the arrangement 

 of its various parts, and in their mode of action the eye may be compared 

 to a camera obscura. Though the comparison may not be absolutely exact, 

 yet in a general way it is true that there are many striking points of simil- 

 arity between them; e.g., the sclera and chorioid may be compared to the 

 walls of the camera; the combined refracting media to the component glasses 

 of the lens, the action of which results in the focusing of the light rays; the 

 retina to the sensitive plate receiving the image formed 

 at the focal point; the iris to the diaphragm for the 

 regulation of the amount of light to be admitted, and 

 for the partial exclusion of those marginal rays which 

 give rise to spheric aberration; the ciliary muscle to the 

 adjusting screw, by means of which the image is brought 

 to a focus on the sensitive plate, notwithstanding the 

 varying distances of the object from the lens. The 

 presence of the visual purple in the rods of the retina 

 capable of being altered by light makes the compari- 

 son still more striking. 



Kuhne even succeeded in obtaining a fixed image Streak 

 or an optogram of an external object in a manner 

 similar to that by which an image is fixed on the sensitive plate of a 

 camera. An animal is kept in the dark for about ten minutes in 

 order to permit the retinal pigment to be completely regenerated. 

 The animal, with the eyes covered, is then brought into a room with 

 a single window. While the head is steadily directed to the window, 



FIG. 330. RETINA OF 

 A RABBIT. OPTOGRAM 

 OF A WINDOW FOUR 

 METERS DISTANT a. 

 Yellow spot, b, b. 

 of Medullated 

 nerve-fibers. (Kuhne.) 



