REFRACTIVE MEDIA AND SURFACES 639 



parts may be added, 3, an apparatus for focussing light from objects at differ- 

 ent distances from the eye; and 4, since both eyes are usually employed in 

 vision, an arrangement by means of which the eyes may be turned in the 

 same direction so that binocular vision is possible. The arrangement of 

 the optic nerve fibers, and of the continuation of these backward in the optic 

 chiasma, and thence to special districts of the brain have already been dis- 

 cussed. 



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

 refracting media correspond to the photographic lens. In a camera images 

 of external objects are thrown upon a screen, the sensitive plate, at the back 

 of the camera box. In the eye, the camera proper is represented by the eye- 

 ball with its choroidal pigment, the sensitive screen by the retina, and the 

 lens by the refracting media. In the case of the camera, the screen is adjusted 

 to receive clear images of objects at different distances by an apparatus for 

 focussing. The corresponding adjustment in the eye is accomplished by 

 the accommodating apparatus. 



Refractive Media and Surfaces. At first sight it would seem as if 

 the refracting apparatus of the eye were very complicated, since 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 unequally refractive), 

 the posterior surface of the lens, and the vitreous humor. Thus there are 

 four surfaces, and at least, including the air, five media. For all practical 

 purposes, however, we may leave out of consideration all but three refracting 

 surfaces and their adjacent media. These are: the anterior surface of the 

 cornea, separating the air and the corneal substance; the anterior surface 

 of the lens, separating the aqueous humor and the lens substance; and the 

 posterior surface of the lens, separating the lens surface from the vitreous 

 humor. 



Image Formation. In the refraction through a simple transparent 

 spherical surface there are certain cardinal points to be considered. The 

 rays of light which fall perpendicularly on such a surface pass through with- 

 out refraction. All such rays cut the center of the radius of curvature of the 

 lens, called the nodal point. A line that passes through the center of curva- 

 ture of a lens and thus pierces the nodal point is called the optical axis, and 

 the point on the surface pierced by the optical axis is the principal point. 

 In every optical system there are certain other cardinal facts to be considered. 

 All rays which do not strike vertical to the curved surface are refracted 

 toward the optical axis. Rays which impinge upon the spherical surface of 

 a lens parallel to the optical axis will meet at a point upon the axis called the 

 posterior principal focus, figure 453, F. The posterior principal focus is 

 outside of the nodal point. Again, there is a point in the optical axis in front 

 of the surface, rays of light from which strike the surface so that they are 



