710 HANDBOOK OF PHYSIOLOGY. 



The arrangement of the optic nerve-fibres, and of the continuation 

 of these backward in the optic chiasma, and thence to special districts of 

 the brain, have already been discussed. 



The eye may be compared to a photographic camera, and the trans- 

 parent media corresponds to the photographic lens. In such a camera 

 images of external objects are thrown upon a ground-glass screen at the 

 back of a box, the interior of which is painted black. In the eye, the 

 camera proper is represented by the eyeball with its choroidal pigment, 

 the screen by the retina, and the lens by the refracting media. In the 

 case of the camera, the screen is enabled to receive clear images of objects 

 at different distances, by an apparatus for focussing. The corresponding 

 contrivance in the eye is the accommodation. 



The iris, which is capable of allowing more or less light to pass into 

 the eye, corresponds with the different sized diaphragms used in the 

 protographic apparatus. 



Refractive media and surfaces. At first sight it would seem as if the 

 refracting apparatus of the eye were very complicated, seeing that it 

 consists of so many parts. These parts are : the anterior surface of the 

 cornea itself, the posterior surface of the cornea, the aqueous humor, 

 the anterior surface of the lens, the substance of the lens itself (which is 

 also unequally refractive), the posterior surface of the lens, and the vit- 

 reous humor. Thus there are four surfaces, and at least including the 

 air, five media. For all practical purposes, however, these may be re- 

 solved into a somewhat simpler form, and the cornea may be considered 

 as one surface, the anterior, and one medium ; the aqueous and vitreous 

 humors as one medium ; the lens, as two surfaces and one medium. It 

 will be as well to consider the laws which govern the refraction of light 

 under such circumstances. 



In its simplest form, we may consider the refraction through a simple 

 transparent spherical surface, separating two media of different density. 



The rays of light which fall upon the surface exactly perpendicularly 

 do not suffer refraction, but pass through, cutting the optic axis (0 A, 

 fig. 420), a line which passes exactly through the centre of the surface, 

 at a certain point, the nodal point (fig. 420, N), or centre of curvature. 

 Any rays which do not so strike the curved surface are refracted toward 

 the optical axis. Rays which impinge upon the spherical surface paral- 

 lel to the optical axis, will meet at a point behind, upon the said axis 

 which is called the chief posterior focus (fig. 420, F,) ; and again there 

 is a point in the optical axis in front of the surface, rays of light from 

 which so strike the surface that they are refracted in a line parallel with 

 the axis df"\ such a point (fig. 420, F 2 ) is called the chief anterior 

 focus. The optic axis cuts the surface at what is called the principal 

 point. 



