The Democratic advocate. [volume] (Westminster, Md.) 1865-1972, 1891-01-31
Lens Making.


deeper and deeper into the lathomless
depths of space, and to bring order (to his
sight, for his mind tells him that order
exists out of the apparent confusion, has
kept the glass makers and opticians busy,
with the result that the five-inch single
lenses of Huyghens that required focal
length of 10, 30 or 100 feet, were i'm
proved upon by the invention of the achro-
matte lens by Dolland, upon which subse-
queDt Improvement has been made only in
quality and power, and not in principle.
The four-inch lens of Dolland in 1900
developed into the nine-inch lens in 1880,
and from 1915 to 1861 the 15-inch tele-
scopes at Harvard observatory and at st.
Petersburg were the largest and most pow
erful instruments in the world. From
that period till today the size and power
have constantly increased first to Is inches,
then TO 23, &c, 80, 36, and now the Clarks,
of Cambridge, will soon be working upon
one 40 inches in diameter. At various
stages of the progress of telescope making
it was thought that the limit of useful
power had been reached but, after each
increase, it was found that, with a proper
selection of site, the density of the atmos-
phere did not interfere so much with the
observers as was anticipated.
The great progress in telescope making
and the consequent development of astron-
omy are due in great measure to the eff
forts of one house, that of the Clarks, of
Cambridge. In 1910 Alvan Clark, Sr.,
was making seven and eight-inch refractors
of great excellence. In 1861 the Clarks
made telescope with an lS-inch aperture
The 261nch relractor of the Naval 0bser-
vatory at Washington was the masterpiece
of its day, and was made by the Clarks in
1873. A duplicate of this was made for
the University of Virginia, 23-inch for
Princeton, and then one of 30 inches for
the St. Petersburg Observatory. Next,
the 3G-inch Lick telescope was made, and
finally they have reached the 40-inch glass
Most of these telescopes were at the time
the largest and most powerful in existence,
and the 30-inch, the 36-inch and the 40
inch had no compeers. No other opticians
in the world are prepared 10 undertake the
task of making lenses of the last named
size.
The work of shaping these great lenses
is of the most delicate description, the least
departure from entire accuracy being suffi-
cient to spoil the result of years of work
Some of the work of fashioning the lenses
is done by machinery, but the most of it is
done by hand, the finishing touches being
accomplished by the rubbing of the thumb
covered with rouge over the surface. In
fact, after the lens has been to the eye of
the layman finished the hardest work
comes. In all the arts there are those who
have skill and those who have genius.
The skillful optician should go to work and
mathematically figure the curves of the
glasses, and if these calculations were cor
rect the lens would be good. The genius
fashions it out with but the incidental use
of measurements, depending upon his eye
and judgment, which are more unerring
than mathematics. The Clarks belong to
the order of genius.
In commenting on the skill of the opti-
clan, however, it should not be forgotten
that without good material to work upon
his skill would be useless. There is but
one firm in the world that can furnish such
glasses as that of the Lick telescope and the
forty-inch which is TO be shaped by the
Clarks, and that firm is Fell & Mantois, of
Paris. The production of disk perfect in
all respects for a lens of this size is a task
of great difficulty. The elements of which
it is made must have the perfect relation
or it will not have the necessary clearness,
and the slightest discoloration or irregu-
larity of density is fatal.
For this reason the glass has to be nodded
and remolded time and time again, until
\ the minutest examination shows no defect;
and even after this the optician may find
some little inequality necessitating its return
to the crucible. The utmost skill and pa
tience are necessary to produce the requisite
results. The mass from which the crown
disk of the Lick telescope was made weighed
primarily about one thousand and five hun
dred pounds, the flint glass disk about one
thousand eight hundred pounds, and that
for the 40-inch glass (crown and fin
lenses considerably more than two tons
The objective of telescope consists ii
reality of two lenses, the outer one being o
crown glass and the inner one of flint glass
The crown glass is bi-convex, or convex oil
Z both sides, and the flint glass plano-con
cave, or cOncave on one side and plane o
fiat on the other. The reason why tw,
kinds Of glass are used is because of chro
J matte alterations which occur when singh
one is used, through them the differen
rays of light, having different focal lengths
This was the most serious defect, thougl
there We're others; but it was obviated b,
combining two kinds of glass having differ
ent ratios of refraetive or dispersive powers
the plano-concave flint glass correcting tn
chromatic alterations of the bi-conve.
crown glass. 1t is not altogether the mag
. nifying powerof a telescope, but the amoun
of light that reaches the eye from the thin