[Illustration: FIG. 68.--When looked at through the prism, _A_ seems
to be at _S_.]

112. Lenses. If two prisms are arranged as in Figure 69, and two
parallel rays of light fall upon the prisms, the beam _A_ will be bent
downward toward the thickened portion of the prism, and the beam _B_
will be bent upward toward the thick portion of the prism, and after
passing through the prism the two rays will intersect at some point
_F_, called a focus.

[Illustration: FIG. 69.--Rays of light are converged and focused at
_F_.]

If two prisms are arranged as in Figure 70, the ray _A_ will be
refracted upward toward the thick end, and the ray _B_ will be
refracted downward toward the thick end; the two rays, on emerging,
will therefore be widely separated and will not intersect.

[Illustration: FIG. 70.--Rays of light are diverged and do not come to
any real focus.]

Lenses are very similar to prisms; indeed, two prisms placed as in
Figure 69, and rounded off, would make a very good convex lens. A lens
is any transparent material, but usually glass, with one or both sides
curved. The various types of lenses are shown in Figure 71.

[Illustration: FIG. 71.--The different types of lenses.]

The first three types focus parallel rays at some common point _F_, as
in Figure 69. Such lenses are called convex or converging lenses. The