578 LENSES AND THEIR ACTION [Cn. XIV 



the ray before and after it passes the center of the lens extends in 

 parallel lines. As shown by the following diagrams the optic 

 center is found by drawing parallel radii from the two curved sur- 

 faces, or from the curved and plane surface, and joining the ends 

 of the radii. The center of the lens is the point where the line 

 joining the outer ends of the parallel radii cross the principal axis 

 (fig. 314). 



The reason why light rays traversing the optic center have no 

 angular deviation is as follows : The radii are perpendicular to the 

 surfaces of the lens; and the tangent plane perpendicular to the 

 radius, is tangent to the sphere at the end of the radius. As the 

 two tangents to parallel radii must themselves be parallel, it follows 

 that a ray of light passing from one tangential point to the other is 



FIG. 315. CONJUGATE Foci C,C 2 ON THE PRINCIPAL Axis. 



traversing a body with parallel surfaces at the point of entrance 

 and departure, and hence it will suffer no angular deviation 

 although the ray may be displaced, as in traversing any thick 

 transparent body with plane faces (fig. 313). With meniscus 

 lenses the crossing point (optic center) is on an extension of the line 

 joining the centers of curvature (fig. 314). 



805. Secondary axis. Every line traversing the optic center 

 of a lens, except the principal axis, is a secondary axis. It follows 

 therefore that every secondary axis must be more or less oblique 

 to the principal axis (fig. 317). 



806. Principal focal point. The principal focus or focal 

 point of a lens or of a lens system like a condenser or a projection 

 objective, is the point on the principal axis where rays of light 

 parallel with the principal axis before entering the lens or combina- 

 tion, cross the principal axis after leaving the lens or objective. 

 It is also sometimes called the burning point (fig. 319). 



