other stones. In selecting properties for use in
distinguishing precious stones, such properties as can be determined by
quantity, and set down in numbers, are probably more trustworthy than
those that can be observed by mere inspection. Those also which have to
do with the behavior of light in passing through the stone are extremely
valuable.

IMPORTANCE OF NUMERICAL PROPERTIES. It is because gem dealers so often
rely upon the more obvious sort of property, such as color, that they so
frequently make mistakes. There may be several different types of stones
of a given color, but each will be found to have its own numerical
properties such as density, hardness, refractive power, dispersive
power, etc., and it is only by an accurate determination of two or three
of these that one can be sure what stone he has in hand. It must next be
our task to find exactly what is meant by each of these numerical
properties, and how one may determine each with ease and exactness.

LESSON II

REFRACTION

EXPLANATION OF REFRACTION. Perhaps the surest single method of
distinguishing precious stones is to find out the _refractive index_ of
the material. To one not acquainted with the science of physics this
calls for some explanation. The term _refraction_ is used to describe
the bending which light undergoes when it passes (at any angle but a
right angle) from one transparent medium to another. For example, when
light passes from air into water, its path is bent at the surface of the
water and it takes a new direction within the water. (See Fig. 1.)

[Illustration: FIG. 1.]

AB represents the path of light in the air and BC its path in the water.

While every gem stone refracts light which enters it from the air, _each
stone has its own definite ability to do this, and each differs from
every other in the amount of bending which it can bring about under
given conditions_. The accurate determination of the amount of bending
in a given case requires very finely constructed optical instruments and
also a knowledge of how to apply a certain amount of mathematics.
However, all this part of the work has already been done by competent
scientists, and tables have been prepared by them, in which the values
for each material are put down.

THE HERBERT-SMITH REFRACTOMETER. There is on the market an instrument
called the Herbert-Smith refractometer, by means of which anyone with a
little practice can read at once on the scale within