three
times the distance over nine times the surface. The varying amount
of light received by each planet is also shown in fractions above
each world, the amount received by the earth being 1.

[Illustration: Fig. 7.]

[Illustration: Fig. 8.--Measuring Intensities of Light.]

The intensity of light is easily measured. Let two lights of different
brightness, as in Fig. 8, cast shadows on the same screen. Arrange
them as to distance so that both shadows shall be equally dark.
Let them fall side by side, and study them carefully. Measure the
respective distances. Suppose one is twenty inches, the other forty.
Light varies as the square [Page 38] of the distance: the square of
20 is 400, of 40 is 1600. Divide 1600 by 400, and the result is that
one light is four times as bright as the other.

[Illustration: Fig. 9.--Reflection and Diffusion of Light.]

Light can be handled, directed, and bent, as well as iron bars.
Darken a room and admit a beam of sunlight through a shutter, or
a ray of lamp-light through the key-hole. If there is dust in the
room it will be observed that light goes in straight lines. Because
of this men are able to arrange houses and trees in rows, the hunter
aims his rifle correctly, and the astronomer projects straight
lines to infinity. Take a hand-mirror, or better, a piece of glass
coated on one side with black varnish, and you can send your ray
anywhere. By using two mirrors, or having an assistant and using
several, you can cause a ray of light to turn as many corners as you
please. I once saw Mr. Tyndall send a ray into a glass jar filled
with smoke (Fig. 9). Admitting a slender ray through a small hole in
a card over the mouth, one ray appeared; removing the cover, the
whole jar was luminous; as the smoke disappeared in spots cavities
of darkness appeared. Turn the same ray into a tumbler of water,
[Page 39]
it becomes faintly visible; stir into it a teaspoonful of milk, then
turn in the ray of sunlight, and it glows like a lamp, illuminating
the whole room. These experiments show how the straight rays of
the sun are diffused in every direction over the earth.

Set a small light near one edge of a mirror; then, by putting the
eye near the opposite edge, you see almost as many flames as you
please from the multiplied reflections. How can this be accounted
for?

Into your beam of sunlight, admitted through a half-inch hole,
put the mirror at an oblique angle; you can arrange it so as to