ssed the material in two or more directions, and the separation
and consequent doubling of image will be sure to result. For those who
wish to study double refraction more in detail, Chapter VI., pages
40-52, of G. F. Herbert-Smith's _Gem-Stones_ will serve admirably as a
text. As an alternative any text-book on physics will answer.

LESSON IV

ABSORPTION AND DICHROISM

CAUSE OF COLOR IN MINERALS. In Lesson III. we saw that many gem
materials cause light that enters them to divide and take two paths
within the material. Now all transparent materials _absorb_ light more
or less; that is, they stop part of it, perhaps converting it into heat,
and less light emerges than entered the stone. If light of all the
rainbow colors (red, orange, yellow, green, blue, violet) is equally
absorbed, so that there is the same relative amount of each in the light
that comes out as in the light that went into a stone, we say that the
stone is a _white_ stone; that is, it is not a _colored_ stone. If,
however, only blue light succeeds in getting through, the rest of the
white light that entered being absorbed within, we say that we have a
blue stone.

Similarly, the _color_ of any transparent material depends upon its
relative degree of absorption of each of the colors in white light. That
color which emerges most successfully gives its name to the color of the
stone. Thus a ruby is red because red light succeeds in passing through
the material much better than light of any other color.

UNEQUAL ABSORPTION CAUSES DICHROISM. All that has been said so far
applies equally well to both singly and doubly refracting materials, but
in the latter sort it is frequently the case, in those directions in
which light always divides, that the absorption is not equal in the two
beams of light (one is called the ordinary ray and the other the
extraordinary ray).

For example, in the case of a crystal of ruby, if white light starts to
_cross_ the crystal, it not only divides into an ordinary ray and an
extraordinary ray, but the absorption is different in the two cases,
and the two rays emerge of different shades of red. With most rubies one
ray emerges purplish red, the other yellowish red.

It will at once be seen that if the human eye could distinguish between
the two rays, we would have here a splendid method of determining many
precious stones. Unfortunately, the eye does not analyze light, but
rather blends the effect so that the unai