River Falls journal. [volume] (River Falls, Pierce County, Wis.) 1872-2019, 1891-05-21
SPECTACLE GLASSES.
vair able Information Concerning the Ma
terial Used In Their Manufactures.

VARIABLE information Concerning the Ma
terial Used In Their Manufacture.
Although many opticians are in the
habit of recommending various descrip-
tons of glass for spectacles, there are
in reality only two kinds-native glass
or rock or mountain crystal, usually
called pebble, and artificial glass They
also advise users of spectacles to have
them Of pebbles, as more beneficial to
the eyesight, artificial glass being de-
cried as heating and wearying the eye
The writer, who has now been habitu-
ally using speetacles for twenty-five
years o. account of short-sightedness,
has had no such experience. Acting
upon the advice of first-rate oculist,
whom he consulted, he purchased peb-
BILE spectacles, and he used them for
years Lately, however, requiring q
pair of spectacles of particular focus,
he has taken to spectacles of artificial
glass, and he finds that there is not the
slightest difference, with the exception
that the latter are only about half the
price of pebble spectacles. This is con-
sideration in case of persons of limited
means
The perfection to which glassmaking
has now attained has rendered the use
of artificial glass for spectacles practi-
cable, and, seeing the advantages at
tached to its employment, which il is
the object of this short paper to point
out, 1t is probable that 1t will ultimate
ly entirely supersede rock crystal II
was different, however, before the art
of glassmaking had attained its present
perfection. The use of pebbles for
lenses extends far back into the remote
past. Sir Henry Layard found among
the ruins of old Nineveh polished peb-
bic lens of a convex form of focus Of
four inches, and diameter of one and
half inches. 1t may also be assumed
that the magnifying power of such
lenses was well known to and utilized
by the old cameo cutters in their diff
cult and delicate work. The price lists
of former opticians of former centur-
les contain the prices of pebble eye
glasses, and they are known to have
endeavored to improve the microscope
by the use Of crystals of precious stones
Besides rock crystal, they made micros-
copic lenses of sapphire, ruby, garnet
beryl, topaz and even of diamond. Dia-
mond lenses for microscopes were
warmly recommended by Sir David
Brewster, QS19) and made chiefly by
Priehard, (1s24).
All these early efforts, after the mar
velous development of the manufacture
of glass, now belong to history. Tie
greatest objection to the application Of
precious stones to optical purposes is
their structure-the fact that, as crys-
tails having two axes, they suffer double
refraction, and it is this drawback
which has to be taken into considera-
ton when dealing with rock crystal.
This double refraction must greatly af-
feet its optical application, and it can
only be rectified to some extent by cut
ting the pebbles out of the raw material
at right angles to the principal crystol-
lographic axis.
Opera-glasses with eye lenses Of rock
crystal have for some time past been
made in Paris for which great distinct
ness of image is claimed Closer ex-
animation, however, have proved that
such eye lenses are not only not better
in that respect than those of ordinary
glass, but mostly much worse, especial
ly i. they have not received the proper
setting to the crystal axis. And if in
opera-glass lenses the condition of cut
ting them out of the rock crystal at
right angle is neglected. this is much
more so the case with pebbles for spec
tacles-Chamber' Journal.