MICROSCOPE AND ACCESSORIES 



[CH. I 



FIGS. 1-9, showing the Principal Optic Axis and the Optical Center of various 

 forms of Lenses. 



Axis. The Principal Optic Axis. c-c f . Centers of curvature of the two sur- 

 faces of the lens, c.l. Optical center of the lens, r-r' '. Radii of curvature of 

 the two lens surfaces, t-t' '. Tangents in Fig. 4. 



$ 3. Principal Optic Axis. In spherical lenses, i. e., lenses whose surfaces 

 are spherical, the Axis is a line joining the centers of curvature and indefinitely 

 extended. In the lens it is the unbroken part of the line c-c f in the figures. In 

 lenses with one plane surface (Figs. 3, 6, 7) the radius of the plane surface is any 

 line at right angles to it; but in determining the axis it must be the one which is 

 continuous with the radius of the curved surface, consequently the axis in such 

 lenses is on the radius of the curved surface which meets the plane surface at right 

 angles. 



\ 4. Optical Center. The optical center of a lens is the point through which 

 rays pass without angular deviation, that is, the emergent ray is parallel to the 



