Iowa County democrat. [volume] (Mineral Point, Wis.) 1877-1938, 1891-04-24
GUARD WELL YOUR EYES.
A Chapter of Useful Information Abou
the Right Kind of Spectacles

Chapter of Useful Information About
the Right Kind of SpectacIes
Although many opticians are in the
habit of recommending various descrip-
tiOns of glasses or spectacleS, there are In
reality only two kinds-native glass or
rock of mountain crystal, usually called
pebble. and artificial glass. They also adj
wise users of spectacles to have them of
pebbles, as more beneficial to the eyesight,
artificial glass being described as heating
and wearing the eye. The writer who
has been habitually using spectacles for
the past twenty-five years on account of
ghtedness, has had no such ex
perlence. Acting upon the advice of
tirst-rate oculist whom he consulted, he
purchased peoble spectacles, anc hc used
them for years. Lately, however, requir-
ing pair of spectacles of particular focus,
ne has taken to spectacles of artificial ass
and he finds that there is not tn slightest
difference, with the exception that he lat
ter are only about half the price of the
pebble spectacles. This is a Consideration
IN case of persons of limited mea;l-.
The perfection to which glass making
has now attained, has rendered the use OF
artificial glass for spectacles Practicable
ployment, which it is the object of this
short paper to point out, it l. probably
that it will ultimately entirely superced Z
rock crystal. It was different, however,
before the art of glass making had at
tanned its present perfection Tne use of
pebbles for lenses extends far back into the
remote past. Sir Henry Layard found
among the ruins of old Nineveh polished
pebble lense of a convex form of a
focus of four inches and diameter Of one
and a half inches. 1t may also be as
sumed that the magnifying power of
such lenses was well known to and util-
lied by the old cameo cutters in their
difficult and delicate work. The price
lists of opticians of former centuries con-
tain the prices of pebble eyeglasses, and
they are known to have endt avored to i'm
prove the microscope by the use Of crys ais
or precious stones. Besides lock crystals.
they made microscopic lense of sapphire,
rubby, garnet, perl, topaz and even of
diamond. Diamond lenses for microscopes
were warmly recommended by Sir David
Brewster, isis. ana made chiefly by
Pritchard, 1924.
All these early efforts, after the mar
velous development of the manufacture
Of glass, now belong to history The
greatest objection to the application of
precious stones to optical purposes is Their
structure-the fact that, as crystals
having two axes, they suffer double re
fraction, and ii is the draw back which
has to be taken into consideration when
dealing with rcuk crystals. This double
refraetion must greatly affect It's optical
application, and it c.n only be..rectifed to
some extent by cutting the pebbles out Of
raw material at right angles to the prin-
cipal crystallogrophie axis.
Opera glasses with eye lenses Of rock
crystal have tor some time past been made
in Paris for which great distinctness
of image Il claimed. Closer examination,
however has proved that such eye lenses
are not on: better IN that respect than
those of ordinary glass, but mostly worse,
especially If they have not received the
proper setting 10 the crystal axis. And
if II opera glass lenses the condition Of
cutting them out of the rock crystal at
right angles is neglected. this Is much
more so the case with pebbles for specta-
cles.