\_CH. I 



object may be seen. It always consists of one or more converging lenses or 

 lens-systems (Fig. 16), and the object must be placed within the principal 

 focus (g 12-14). The simple microscope may be held in the hand or it may be 

 mounted in some way to facilitate its use (Figs. 19-22). 



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 two 

 surfaces of the lens. c. I. Optical center of the lens. r-r f . Radii of curva- 

 tur t e of the two lens snrfaces. t-t' . Tangents in Fig. 4. 



$3. Principal Optic Axis. In spherical lenses, i. e., lenses which have 

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

 indefinitely extended. In the figures (1-9) this line (c-c') is broken except 

 where it traverses the lens. 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. 



