n
position, but otherwise unchanged. The deviation or displacement
caused by glass as thin as window panes is too slight to be noticed,
and we are not conscious that objects are out of position.

[Illustration: FIG. 67.--Objects looked at through a window pane seem
to be in their natural place.]

111. Chandelier Crystals and Prisms. When a ray of light passes
through plane glass, like a window pane, it is shifted somewhat, but
its direction does not change; that is, the emergent ray is parallel
to the incident ray. But when a beam of light passes through a
triangular glass prism, such as a chandelier crystal, its direction is
greatly changed, and an object viewed through a prism is seen quite
out of its true position.

Whenever light passes through a prism, it is bent toward the base of
the prism, or toward the thick portion of the prism, and emerges from
the prism in quite a different direction from that in which it entered
(Fig. 68). Hence, when an object is looked at through a prism, it is
seen quite out of place. In Figure 68, the candle seems to be at _S_,
while in reality it is at _A_.

[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