crystallized, and it has been well ascertained what stones display simple and what stones double refraction,
it sometimes becomes a matter of importance to know whether a given
stone is really doubly-refrangent, in order to distinguish it from one
which possesses simple refraction, but to which in other respects it is
quite similar in appearance. As precious stones are all small, a
special method of procedure is here necessary in order to obtain the
phenomenon of double refraction.
Take, for example, a small stone, cut in form of a brilliant, concerning whose nature there is some doubt.
Place
the stone at a level with the eye, holding it in one hand; in the other
hand take an object of small dimensions, a pin for instance, and move
it slowly on the other side of the stone until the eye is able to
perceive it. If the stone is doubly-refractive, the rays will bifurcate
on entering it, and accordingly two images of the pin will be seen, if
it is not held too near the stone. If it is held very near the stone,
the rays will not be far enough apart at the point where they emerge
into the air to allow their separation to be evident.
If
the experiment is made at night, instead of a pin a lighted candle may
be used, the candle being placed beyond the reach of currents of air,
so that its flame may be pure and regular. The pheno-
