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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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