560 THE NERVOUS SYSTEM. 



Physiology of Vision. The eye has very aptly been compared 

 to a photographic camera, the transparent structure, through which 

 pass the rays of light, representing the lenses, and the retina 

 representing the sensitive plate on which the image is received, 

 while the pigmented choroid coat is the representative of the 

 lampblack with which the photographer darkens the interior of 

 the camera-box to prevent any reflected light striking the plate 

 and interfering with the sharpness of the picture. In t)ie camera, 

 in order to bring to a focus upon the plate the rays of light coming 

 from objects at different distances, the photographer uses a focus- 

 sing screw, by which the lens may be moved nearer to or farther 

 from the object he wishes to photograph ; and in order that clear 

 images may be obtained by the eye it is necessary to accomplish 

 the same result, for when the eye is focussed for near objects, those 

 at a distance are blurred, and vice versd. This fact may readily 

 be demonstrated by looking through a piece of mosquito-netting 

 at the windows of a house on the opposite side of a street. When 



FIG. 337. Principal focus of a lens. The parallel rays, a, 6, c, d, are refracted 

 by the lens so as to unite at the point F on the axis P; the ray P undergoes no 

 refraction. .Fis the principal focus. 



the threads of the net can be seen distinctly, the bars of the window 

 will be indistinct, and when the bars of the window are clear and 

 distinct, then the threads are blurred. In the optical apparatus 

 of the eye there is no provision for altering the position of. the 

 lenses, but there is one which answers the same purpose, and which 

 is called accommodation. In connection with every camera there 

 is an arrangement of openings or diaphragms by which a greater or 

 lesser amount of light may be admitted, according to circum- 

 stances. In the eye the iris serves a similar purpose. In many 

 cameras it is necessary to have a number of such diaphragms, each 

 having an opening of a different size, but some are provided with a 

 single one, the size of whose opening can be altered ; this is called 

 an " iris^ diaphragm," and is a rude contrivance compared with the 

 natural iris from which it derives its name, and which by means 

 of its muscular fibers can alter in a moment the size of the pupil. 

 Rays of light coming from an object, in order to produce a 

 distinct image of that object, must be brought to a focus upon the 

 retina (Fig. 337). If the media through which the light from an 



