ded eye gives but a poor means
of telling whether or not a stone exhibits twin colors, or _dichroism_,
as it is called. (The term signifies two colors.) A well-trained eye
can, however, by viewing a stone in several different positions, note
the difference in shade of color caused by the differential absorption.

THE DICHROSCOPE. Now, thanks to the scientific workers, there has been
devised a relatively simple and comparatively inexpensive instrument
called the _dichroscope_, which enables one to tell almost at a glance
whether a stone is or is not dichroic. The construction is indicated in
the accompanying drawing and description.

[Illustration: THE DICHROSCOPE.]

[Illustration: FIG. 3.

A, simple lens; B, piece of Iceland spar with glass prisms on ends to
square them up; C, square hole.]

[Illustration: FIG. 4.]

If the observer looks through the lens (A) toward a bright light, as,
for example, the sky, he apparently sees two square holes, Fig. 4.

What has happened is that the light passing through the square hole (C
of Fig. 3) has divided in passing through the strongly doubly refracting
Iceland spar (B of Fig. 3) and two images of the square hole are thus
produced.

If now a stone that exhibits dichroism is held in front of the square
hole and viewed toward the light, two images of the stone are seen, one
due to its ordinary ray (which, as was said above, will have one color),
and the other due to its extraordinary ray (which will have a different
color or shade of color), thus the color of the two squares will be
different.

With a singly refracting mineral, or with glass, or with a doubly
refracting mineral when viewed in certain directions of the crystal
(which do not yield double refraction) the colors will be alike in the
two squares. Thus to determine whether a red stone is or is not a ruby
(it might be a garnet or glass or a doublet, all of which are singly
refracting and hence can show no dichroism), hold the stone before the
hole in the dichroscope and note whether or not it produces twin colors.
If there seems to be no difference of shade turn the stone about, as it
may have accidentally been placed so that it was viewed along its
direction of single refraction. If there is still no dichroism it is not
a ruby. (_Note._--Scientific rubies exhibit dichroism as well as natural
ones, so this test will not distinguish them.)

A dichroscope may be had for from seven to ten dollars, according to the