564 MANUAL OF PHYSIOLOGY. 



places the screen, the light coming through the lens makes the 

 photographic picture. 



Just in the same way an inverted image of the things we look 

 at is thrown on the retina of the eye by the refracting media. 

 This may be seen in a dark room, if a caudle be placed at a suitable 

 distance in front of the cornea of a fresh eye taken from a recently- 

 killed white rabbit. When cleared of fat and other opaque tis- 

 sues, the sclerotic is transparent enough to act as a screen upon 

 which the inverted candle flame can be recognized. 



Though our organ of vision is commonly compared to a camera 

 obscura, the refractions of the light which occur in it are far more 

 complex than those taking place in that simple instrument. In 

 the latter we have only two media the glass lens and the air ; 

 in the eye, on the other hand, we have several, which are known 

 to have a distinct refractive influence on the rays which pass 

 through the pupil. 



Since the surfaces of the cornea, however, are practically paral- 

 lel, we may neglect the difference between it and the aqueous 

 humor, and look upon the two as one medium, having in front 

 the shape of the anterior surface of the cornea, and behind, the 

 anterior surface of the lens, so as to form a concavo-convex lens. 

 We thus have only three media to consider, viz. : (1) the aqueous 

 humor and cornea ; (2) the lens and its capsule ; and (3) the 

 vitreous humor. And only three refracting surfaces need be 

 enumerated viz. : (1) the anterior surface of the cornea; (2) the 

 anterior surface of the lens ; and (3) the posterior surface of the 

 lens. 



These refracting surfaces may all be looked upon as portions of 

 spheres whose centres lie in the same right line, and hence may be 

 said to have a common axis. And the eye may be regarded as an 

 optic system, centred around an axis which passes through the 

 middle point of the cornea in front, and the central depression 

 (fovea centralis) of the retina behind. This is commonly spoken 

 of as the optic axis of the eye. 



The rays of light entering the eye are most strongly refracted 

 at the surface of the cornea, because they have to pass from the 

 rare medium, the air, to the denser cornea and aqueous humor. 



