When a ray of light passes through a piece of plane glass, such as a
window pane (Fig. 67), it is refracted at the point _B_ toward the
perpendicular, and continues its course through the glass in the new
direction _BC_. On emerging from the glass, the light is refracted
away from the perpendicular and takes the direction _CD_, which is
clearly parallel to its original direction. Hence, when we view
objects through the window, we see them slightly displaced in
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_.