s the following simple experiment will show. Let a single candle
(Fig. 57) serve as our light, and at a distance of one foot from the
candle place a photograph. In this position the photograph receives a
definite amount of light from the candle and has a certain brightness.

If now we place a similar photograph directly behind the first
photograph and at a distance of two feet from the candle, the second
photograph receives no light because the first one cuts off all the
light. If, however, the first photograph is removed, the light which
fell on it passes outward and spreads itself over a larger area, until
at the distance of the second photograph the light spreads itself over
four times as large an area as formerly. At this distance, then, the
illumination on the second photograph is only one fourth as strong as
it was on a similar photograph held at a distance of one foot from the
candle.

The photograph or object placed at a distance of one foot from a light
is well illuminated; if it is placed at a distance of two feet, the
illumination is only one fourth as strong, and if the object is placed
three feet away, the illumination is only one ninth as strong. This
fact should make us have thought and care in the use of our eyes. We
think we are sixteen times as well off with our incandescent lights as
our ancestors were with simple candles, but we must reflect that our
ancestors kept the candle near them, "at their elbow," so to speak,
while we sit at some distance from the light and unconcernedly read
and sew.

As an object recedes from a light the illumination which it receives
diminishes rapidly, for the strength of the illumination is inversely
proportional to the square of distance of the object from the light.
Our ancestors with a candle at a distance of one foot from a book were
as well off as we are with an incandescent light four feet away.

101. Money Value of Light. Light is bought and sold almost as
readily as are the products of farm and dairy; many factories,
churches, and apartments pay a definite sum for electric light of a
standard strength, and naturally full value is desired. An instrument
for measuring the strength of a light is called a photometer, and
there are many different varieties, just as there are varieties of
scales which measure household articles. One light-measuring scale
depends upon the law that the intensity of illumination decreases with
the square of the distance of the object