[Illustration: FIG. 58.--The two shadows are equally dark.]

102. How Light Travels. We never expect to see around a corner, and
if we wish to see through pinholes in three separate pieces of
cardboard, we place the cardboards so that the three holes are in a
straight line. When sunlight enters a dark room through a small
opening, the dust particles dancing in the sun show a straight ray. If
a hole is made in a card, and the card is held in front of a light,
the card casts a shadow, in the center of which is a bright spot. The
light, the hole, and the bright spot are all in the same straight
line. These simple observations lead us to think that light travels in
a straight line.

[Illustration: FIG. 59.--The candle cannot be seen unless the three
pinholes are in a strait line.]

We can always tell the direction from which light comes, either by the
shadow cast or by the bright spot formed when an opening occurs in the
opaque object casting the shadow. If the shadow of a tree falls
towards the west, we know the sun must be in the cast; if a bright
spot is on the floor, we can easily locate the light whose rays stream
through an opening and form the bright spot. We know that light
travels in a straight line, and following the path of the beam which
comes to our eyes, we are sure to locate the light.

103. Good and Bad Mirrors. As we walk along the street, we
frequently see ourselves reflected in the shop windows, in polished
metal signboards, in the metal trimmings of wagons and automobiles;
but in mirrors we get the best image of ourselves. We resent the image
given by a piece of tin, because the reflection is distorted and does