Lawrence democrat. (Lawrenceburg, Tenn.) 1884-1925, 1891-05-29
SPECTACLE GLASSES.
Valuable Information Concerning the Ma
tcrial Used i'm Their Manufacture.

Valuable Information Concerning the Ma
tcrial Used in Their Manufacture.
Although many opticians are in the
habit of recommending various descrip-
tions 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 spectacles for twenty-five
years on account of short-sightedness
has had no such experience. Acting
upon the advice of frst-rate oculist,
whom he consulted, he purchased peb-
BILE spectacles, and he used them for
years Lately, however, requiring a
pair of spectacles of a 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 it is
the object of this short paper to point
out, it is probable that it will ultimate
ly entirely supersede rock crystal. 1t
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 a polished peb-
bic lens of convex form of focus of
four inches, and diameter of one and
a 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
| erdeavored To i'm prove 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 We're
warmly recommended by Sir David
Brewster QSl9) and made chiefly by
Prichard, (1S24).
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 crys
tails having two axes, they suffer double
refraction, and it is this drawbacks
which has to be taken into considera
ton when dealing with rock crystal
This double refraction must greatly af-
feet it's optical application, and it car
only be rectitied to some extent by cut
ting the pebbles out of the raw materia
at right angles to the principal crystol
lographic axis.
Opera-glasses with eye lenses of rock
crystal have for some time past beer
made in Paris for which great distinct
ness of image is claimed. Closer ex
animation, however, have proved tina
such eye lenses are not only not bette
in that respect than those of ordinar,
glass, but mostly much worse, especial
ly it they have not received the prope
s.tting to the crystal axis. End if il
opera-glass lInses the condition of cut
ting them ont of the rock crystal a
right angle is neglected, this is mucl
more so the case with pebblca for spSC
tacleA Chamber JQl rrsl