rom
the grass is the green light, and the grass looks green.

Anything white, like a piece of paper, reflects all the light that
strikes it; so if all the colors (white light) strike it, all are
reflected to your eyes and the object looks white.

You have looked at people under the mercury-vapor lights in
photo-postal studios, have you not? The lights are long, inclined
tubes which glow with a greenish-violet light. No matter how good the
color of a person is in ordinary light, in that light it is ghastly.

[Illustration: FIG. 94. A mercury-vapor lamp.]

Go into the kitchen tonight, light a burner of the gas stove, turn out
the light and sprinkle salt on the blue gas flame. The flame will leap
up, yellow. Look at your hands, at some one's lips, at a piece of red
cloth, in this light. Does anything look red?

The reason why nothing looks pink or red in these two kinds of light
is this: The light given by glowing salt vapor or mercury vapor has no
red in it; if you tried to make a "rainbow" from it with a prism, you
would find no red or orange color in it. A thing looks red when it
absorbs all the parts of the light that are not red and reflects the
red light to your eyes. If there is no red in the light to reflect,
obviously a thing cannot look red in that light.

When you look through a piece of colored glass, the case is somewhat
different. A piece of blue glass, for instance, acts as a sort of
strainer. The coloring matter in it lets the blue light through it,
but it holds back (absorbs) the other kinds of light. So if you look
through a piece of blue glass you see everything blue; that is, only
the blue part of the light from different objects can reach your eyes
through this kind of glass. Anything that is transparent and colored
acts in a similar way.

WHY THE SKY IS BLUE. And that is why the sky looks blue. Air holds
back all colors of light except blue; that is, it holds them back a
little. A room full of air holds the colors back hardly at all. A
few miles of air hold them back more; mountains in the distance look
bluish because only the blue light from them can reach you through
the air. The hundred or more miles of air above you hold back a
considerable amount of the other colors of light, letting through much
more of blue than of any other color. So the sky looks blue; that is,
when the air scatters the sunlight above you, it is chiefly the blue
parts of the sunlight that it allows to reach your eyes.