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homogeneous glass. The next is to make this into lenses that shall
Properly concentrate, transmit and sift, so to speak, the solar rays, the
most beautiful and exacting of all imponderables. In 1776, M.
Brisson in a report to the French Academy, on the results of experi-
ments made with he Trudiane lens, so-called, says: "We ought to
consider it impossible to make a perfect glass lens of large size.'" Sir
David Brewster in his treatise on "New Philosophical Instruments."
referring to the astronomical telescope, says: "The imperfection of this
instrument arises from two causes; from the partial correction of color
which is a consequence of an inequality in the colored spaces of the
spectra produced by crown and flint glass, and from the difficulty of
procuring flint glass free from veins or specks."
While the votaries of science in view of their triumphs during the
last fifty years will be reluctant to admit the word "impossible' into
their vocabulary of progress, still it must be admitted that M. Bris-
son's incredulity was quite justifiable since we stand to-day, more than
a century after his prophetic utterance, in almost hopeless contempla-
tion of the problem, how we are to obtain pure, homogeneous glass in
the desired masses if we are to depend upon the old methods of manu-
facture. A slight increase in diameter and thickness of the un-
ground lens greatly increases the difficulty of securing homogeneity in
the mass. It is said that the stewards of the magnificent bequest of
Mr. James Lick, with ample funds in hand to raise, in the pure atmos-
phere of some one of the Rocky Mountain summits, an instrument
far superior to any now in existence, are standing with folded hands
unwilling to go forward because they have no assurance that even
moderate success will reward their expenditure and satisfy the wish of
their generous patron. The largest lens that the world renowned opti-
cians, the Messrs. Clark and Sons, of Cambridge, were willing to
undertake for Prof. Struve in behalf of the Emperor of Russia, to be
placed in the famous observatory at Pulkova, is to be only 31 inches
in diameter, so the world waits for its great telescope. With genius,
gold and good-will to aid the grand scheme it would seem that success
should be assured. Are we to be fettered and foiled by the old methods
and practice whose maximum of utility seems to have been long
since exhausted? Let us consider some facts that make for the nega-
tive of this question. And first the process of glass making demands
attention. It can hardly be said to have been improved since the
time of Dolland, Frauenhofer and Guinaud, when England, Germany
and France were honorable and earnest competitors in the good work.
But the quality of the glass has been improved because it is made of
better material -the siliceous sand of Massachusetts, the purest bed of
which, known to the world, was discovered some years since in Berk-
shire county in that State. This is now sent to the manufactures of
the finest glass in England and on the Continent, and although they
cannot yet make it pure in masses large enough to satisfy the desires
of the most advanced opticians, still it is true that thin plates of glass
of great size and purity can be made. It is only necessary to look
into the magnificent mirrors which adorn the dwellings of some of our
wealthy citizens, to be convinced of this fact. It is also true that bars
of glass of great purity, from two to four inches square, and from ten
to twenty inches long can be made.
Mr. Charles Tomlinson in his "Cyclopedia of Useful Arts and
Manufactures" mentions that fact that "agitation of glass, while in a
liquid state, improves its quality." and it is believed that this discovery
was made by the Dolland's and is the secret of their great success in the art.
But the present arrangement of melting pots and ovens is such as to
render thorough agitation almost impossible and also restricts, within
narrow limits, the size of the masses of glass that can be produced.
A remedy for these hitherto insurmountable difficulties seems to be
offered by the use of a most important modern invention, the rotating
gas furnace which produces the highest available temperature-about
4,600 °F. --, and will supply the largest masses of metal and a the same
time secure any degree of agitation that may be desired. As a general
rule it is certain that the more thoroughly liquid a metal can be made
the more likely it is to be pure in quality and homogeneous in structure.
The benefit resulting from 'the improved process for silvering the sur-
speculum constructed for Mr. A. Ainsleee Common which is 37 1/2 inches
in diameter, is mounted at Ealing and has proved a decided success.
The first speculum made for him, after it was just ready for mounting
was lost by bursting "into a thousand pieces," a calamity that can only
happen. to those that are made of solid glass. By the methods of
construction hereinafter proposed such a misfortune would be impossible.
The silvering of the surface of glass, to improve its refractive power,
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