ive, and in the dark
places where the light never penetrates man sickens and dies. Both
heat and light are essential to life, and each has its own part to
play in the varied existence of man and plant and animal.

Light enables us to see the world around us, makes the beautiful
colors of the trees and flowers, enables us to read, is essential to
the taking of photographs, gives us our moving pictures and our magic
lanterns, produces the exquisite tints of stained-glass windows, and
brings us the joy of the rainbow. We do not always realize that light
is beneficial, because sometimes it fades our clothing and our
carpets, and burns our skin and makes it sore. But we shall see that
even these apparently harmful effects of light are in reality of great
value in man's constant battle against disease.

99. The Candle. Natural heat and light are furnished by the sun, but
the absence of the sun during the evening makes artificial light
necessary, and even during the day artificial light is needed in
buildings whose structure excludes the natural light of the sun.
Artificial light is furnished by electricity, by gas, by oil in lamps,
and in numerous other ways. Until modern times candles were the main
source of light, and indeed to-day the intensity, or power, of any
light is measured in candle power units, just as length is measured in
yards; for example, an average gas jet gives a 10 candle power light,
or is ten times as bright as a candle; an ordinary incandescent
electric light gives a 16 candle power light, or furnishes sixteen
times as much light as a candle. Very strong large oil lamps can at
times yield a light of 60 candle power, while the large arc lamps
which flash out on the street corners are said to furnish 1200 times
as much light as a single candle. Naturally all candles do not give
the same amount of light, nor are all candles alike in size. The
candles which decorate our tea tables are of wax, while those which
serve for general use are of paraffin and tallow.

[Illustration: FIG. 57.--A photograph at _a_ receives four times as
much light as when held at _b_.]

100. Fading Illumination. The farther we move from a light, the less
strong, or intense, is the illumination which reaches us; the light of
the street lamp on the corner fades and becomes dim before the middle
of the block is reached, so that we look eagerly for the next lamp.
The light diminishes in brightness much more rapidly than we realize,
a