from the light. Suppose we
wish to measure the strength of the electric light bulbs in our homes,
in order to see whether we are getting the specified illumination. In
front of a screen place a black rod (Fig. 58) which is illuminated by
two different lights; namely, a standard candle and an incandescent
bulb whose strength is to be measured. Two shadows of the rod will
fall on the screen, one caused by the candle and the other caused by
the incandescent light. The shadow due to the latter source is not so
dark as that due to the candle. Now let the incandescent light be
moved away from the screen until the two shadows are of equal
darkness. If the incandescent light is four times as far away from the
screen as the candle, and the shadows are equal, we know, by Section
100, that its strength is sixteen candle power. If the incandescent
light is four times as far away from the screen as the candle is, its
power must be sixteen times as great, and we know the company is
furnishing the standard amount of light for a sixteen candle power
electric bulb. If, however, the bulb must be moved nearer to the rod
in order that the two shadows may be similar then the light given by
the bulb is less than sixteen candle power, and less than that due the
consumer.

[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 th