566 MANUAL OF PHYSIOLOGY. 



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 par- 

 allel, 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. 

 So, also, more bending of the rays occurs between the aqueous 

 humor and the anterior surface of the lens than between the 

 posterior surface of the lens and the vitreous humor. 



The lens is not of the same density throughout, but denser in 

 the centre, and being made up of layers, the central part refracts 

 more than the outer layers. 



The manner in which the inversion of the image is produced 

 by a convex lens is shown in the accompanying figure, in which 

 the lines correspond to the rays passing from two points through 

 the lens. If the arrow a a' be taken for the object, from either 



