IMAGE FORMATION 725 



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. The curved surface, bd, is supposed 

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

 toward the right. N, the center of curvature or nodal point; OA, principal axis; P, 

 principal point; ab and OP, rays from an infinite distant point; bN and dN, secondary 

 axes; F and F 2 , posterior and anterior principal foci; PA and df", parallel rays that 

 meet in an infinite distance only. 



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

 curvature of a lens and thus pierces the nodal point N is called the optical 

 axis, and the point on the surface pierced by the optical axisP is the princi~ 

 pal point. In every optical system there are certain other cardinal facts 

 to be considered. All rays which do not strike vertical to the curved sur- 

 face 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 pos- 

 terior 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, 

 FtfLf"; such a point, F%, 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 



