PRECIOUS STONES. 229
myopia
by means of concave lenses. It is also said by Pliny that Nero watched
the gladiators fight, aiding his sight by an eye-glass of Emerald.
It
was much used by the Romans for ear drops, and was sometimes engraved
by them, though rarely, as they held it in too high an esteem to be
used .often as material for intagli. The Indians, too, pierced the
commoner varieties, and used them as elongated beads, while the more
precious kinds were mounted in gold in their natural state, and worn as
pendants in very much the same way as crystals have been used quite
recently.
Beryl is found in a variety of colours, and different terms are applied to the gem according to the colour.
Emerald
includes the bright green shade which is so well known that this
particular colour is constantly spoken of as emerald-green. Aquamarine
covers the pale blue, bluish-green, and greenish-blue shades of Beryl ;
the yellow-green shade is Aquamarine-Chrysolite, and the bright yellow
is Golden Beryl ; these three together are known as Precious Beryl.
Other colour shades include : a pale yellow-green, which may have been
the Chrysoprasius and Chrysoberyllus of Pliny, in part, at least; the
apple green, pale rose red, rarely a sapphire blue, and a pale violet.
Sometimes Beryl is found colourless.
The
mineral occurs from transparent to subtranslucent, but the modern gem
varieties are confined to the transparent kinds. The lustre is
characteristically vitreous. Double refraction is only feeble, the
index for the ordinary ray being 1.584, and for the extraordinary
1.578; and the dispersion is small, hence there is but little play of
colour due to refraction. The mineral is, however, distinctly dichroic,
