rners the bacteria thrive, increasing steadily in number. For
this reason our houses should be kept light and airy; blinds should be
raised, even if carpets do fade; it is better that carpets and
furniture should fade than that disease-producing bacteria should find
a permanent abode within our dwellings. Kitchens and pantries in
particular should be thoroughly lighted. Bedclothes, rugs, and
clothing should be exposed to the sunlight as frequently as possible;
there is no better safeguard against bacterial disease than light. In
a sick room sunlight is especially valuable, because it not only kills
bacteria, but keeps the air dry, and new bacteria cannot get a start
in a dry atmosphere.

CHAPTER XIII

COLOR

126. The Rainbow. One of the most beautiful and well-known phenomena
in nature is the rainbow, and from time immemorial it has been
considered Jehovah's signal to mankind that the storm is over and that
the sunshine will remain. Practically everyone knows that a rainbow
can be seen only when the sun's rays shine upon a mist of tiny drops
of water. It is these tiny drops which by their refraction and their
scattering of light produce the rainbow in the heavens.

The exquisite tints of the rainbow can be seen if we look at an object
through a prism or chandelier crystal, and a very simple experiment
enables us to produce on the wall of a room the exact colors of the
rainbow in all their beauty.

[Illustration: FIG. 86.--White light is a mixture of lights of rainbow
colors.]

127. How to produce Rainbow Colors. _The Spectrum._ If a beam of
sunlight is admitted into a dark room through a narrow opening in the
shade, and is allowed to fall upon a prism, as shown in Figure 86, a
beautiful band of colors will appear on the opposite wall of the room.
The ray of light which entered the room as ordinary sunlight has not
only been refracted and bent from its straight path, but it has been
spread out into a band of colors similar to those of the rainbow.

Whenever light passes through a prism or lens, it is dispersed or
separated into all the colors which it contains, and a band of colors
produced in this way is called a spectrum. If we examine such a
spectrum we find the following colors in order, each color
imperceptibly fading into the next: violet, indigo, blue, green,
yellow, orange, red.

128. Sunlight or White Light. White light or sunlight can be
dispersed or separated into the primary colors or rainb