The Manitowoc pilot. [volume] (Manitowoc, Wis.) 1859-1932, 1891-05-28
PEBBLE SPEcTACLES.
Not Much Betier than Those Made OF
Art fetal Glass.

Although many opticians are in the
nahit OF recommending various de-
scriptions Of glass for spectacles there
are in reality only two kinds-native
glass or rock or mountain crystal.
Usually called pebble. and artificial
rlass. saws Chambers journal. They
also advise users of spectacles to have
them of pebbles. rs more beneficial to
the eve sight. artificial glass being de-
cried rs heating and wearying the
eve. The writer, who has now been
habitually using spectacles for twenty
five years on account of short-sighted-
ness has had no such experience. Act
ing upon the advice of a first-rate
oculist, whom he consulted, he pur-
chased pebble spectacles, and he used
them for years. Lately. however, re-
curing pair of spectacies of a partic-
.lar focus. he has taken to spectacles
of artificial glass, aud he finds that
there is not the slightest difference,
with the exception that the latter are
only about half the price of pebble
spectaclcs. This is consideration i'll
case of persons of limited means.
The perfection 10 which glassnlaking
has now attained has rendered the use
Of artificial glass for spectacles practi-
cable, and. seeing the advantages at
tached 10 its employment, which it Is
the object of this short paper to point
out. it is probable that it will ulti-
mately entirely supercede rock crystal.
1t was different, however. before the
art of glassmaking had attained its
present perfection. The use of peb-
blcs for lenses extends far back into
the remote past. Sir Henry Layard
found among the ruins of old Ninereh
Il polished pebble lens of convex
form of n focus OF four inches and a
diameter of one and half inches.
1t may also be assumed that the
maonlfying power Of such lenses was
we'll known to and utilized by the old
cameo cutters in their difficult and del
icate work. The price-liSts of opti-
clans OF former centuries contain the
prices of pebble eyeglasses, and they
are known TO have endcayorcd to i'm
prove the microscope by the use of
crystals of precious stones Besides
rock crystal, they made microscopic
lenses Of sapphire. ruby garnet. bcryl.
topaz. and even of diamond. Diamond
lenses for microscopes were warmly
recommended by Sir David Brewster
OSLO, and made chiefly by Pritchard
QS2D.
All these early efforts. after the mar
velous development of the manufact-
ure of glass. now belong to history.
The greatest objection to the aDplica-
ton of precious stones to optical pur-
pose is their ssructure-the fact that,
rs crystals having two axes. they suffer
double refraction, and it is this draw
back which has to be taken Into con-
sideration when dealing with rock
crystal. This double refrsction must
greatly affect its optical application.
and it can only be rcctifi,d to some
extent BY cutting the pebbles out OF
the raw material nj right angles to the
principal crystallographic axis.
Opera glasses with eye lenses of rock
crystal have fer some time past been
made in Paris for which great distinct
ness Of image is claimed. Closer ex-
amination, however. has proved that
such eve lenses are not only not better
in that respect than those of ordinary
glass. but mostly much worse. espe-
cial, if They lave not received the
proper setting TO the crystal axis. And
if III opera glass lenses the condition
Of cutting them out of the rock crystal
ut right angles is neglected this is
much more so the case with pebbles
for spectaclcs.