674 THE SENSES 



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 



FIG. 453. Diagram of a Simple Optical System. (Foster.) The curved surface, bd, 

 is supposed to separate a less refractive medium toward the left from a more refractive 

 medium toward the right. 



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 \vhich 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 

 refracted in a line parallel with the axis, df"; such a point, F 2 , is called the 

 anterior principal focus. 



In any given system the principal foci can be found by erecting verticals 

 at the nodal and principal points of the optical axis and laying off lengths 

 on each, a and b, proportional to the refractive indices of the media. A line 



