MICROSCOPE AND ACCESSORIES. 



[CM. 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' . Centers of curvature of the tivo surfaces 

 of the lens. c. I. 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 the part of the line joining the centers of curvature and 

 traversing the lens ; it is the unbroken part of the line c-c' in all 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 con- 

 tinuous 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 incident 

 ray. It is determined geometrically by drawing parallel radii of the curved sur- 

 faces, r-r' in Figs. 4-9, and joining the peripheral ends of the radii. The optical 

 center is the point on the axis cut by the line joining the radii. In Figs. 4-5 it is 



