George W. Holley

(Reprinted from the Journal of the Franklin Institute, August 1884.)

Suggestions for Improvement for the Construction of Large Tele-

scopic lenses.

By George W. Holley.

[Read in section A (Physics), at the meeting of the American Association for the

Advancement of Science, held at Cincinnati, Ohio, August 1881.]

It is many years since Sir David Brewster very confidently expressed

the opinion that before the end of the present century the world would

possess a refracting telescope the mirror of which would be twenty feet in

diameter. And no individual of his time, by reason of his extensive scientific attachments and researches, and his exalted character as a man,

was more entitled to speak authoritatively on this subject. Having

read the glowing records of the ski with the patient zeal of the scholar

and the devout ardor of the Christian, he extended the boundaries of

our knowledge by his discoveries, and enriched our literature by his

writings. Undoubtedly it was his knowledge of the powers and capa-

bilities of optical instruments that led to the expression of the opinion

just quoted.

In reference to the refractor, Sir David's anticipations have been

more than realized, while in reflectors only two improvements have

been made: one in the manner of the mounting and manipulating the

speculum, the other in the process for silvering its face. Great improve-

ments have also been made in the mechanical processes for handling,

shaping and finishing all kinds of material substances, and of melting

in large masses all fusible matter. By reason of these improvements

it has become possible to construct a metallic speculum of the size

mentioned by Sir David. But there are so many reasons why refract-

ing instruments should be preferred, that it is not probable that any

important efforts will be made in this direction, until exhaustive experi-

ments have demonstrated the impossibility of vast improvement in

refractors.

The present inquiry, therefore, will be directed, as regards lenses,

to the consideration of methods by which telescopic object glasses may

be greatly increased in size and improved in efficacy.

The first requisite to success it he ability to manufacture pure,

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,

naturally suggests the introduction of the metal, in some form, in the

manufacture of the glass, as has long been done with lead and other

metals. Doubtless experiment would demonstrate that be using the

metallic silver properly comminuted, or some of its compounds, in

making the glass the refractive power of the latter and its optical

value would be very greatly increased. *

Having secured glass of the desired purity the next step is to make

the lens. From the ability to procure much larger masses of pure glass,

by the use of the new style of furnace, then have hitherto been attain-

able, results the possibility of making much larger solid lenses than

have ever been attempted. Whether the maximum of success can

be obtained in this direction experiment only can determine. But it

is believe that maximum can best be obtained by adopting an entirely

new method or methods of lens construction. Before describing them

a few preliminary observations will be useful. Many persons who

have had occasion to use opera glasses and spectacles have noticed that

a cleavage or crack in the lenses does not injure their power to produce

correct images provided the edges of the crack are not crap;d or rag-

ged. The writer used for some years a telescope, the object glass of

which had a crack entirely across it. But it was not perceptible to

the eye when directed to a distant object nor did it impair the image

or produce unusual diffraction of the solar rays. We may also note

the fact that the firm adhesion of different parallel surfaces of glass,

after they have been properly prepared, is secured by the use of trans-

parent cements which do not impair their refractive power. It may

be further noted that the sand blast, recently utilized, is used by the

Messrs. Clark in shaping unground lenses, which process they also find

to be greatly facilitated by the use of the chilled cast iron globules

introduced by Mr. B.C. Tilghman, of Philadelphia.

Such being the facts, it is proposed to make the pure silver-bearing

glass into bars, two- or three-inches square and ten to twenty inches

long, or such size as experiment shall prove to be best; and, after pro-

perly testing every bar, to select for use only those that prove to be

absolutely pure and homogeneous. But for this experiment, bars of

the very pure glass, made by the Messrs. Glance, could be used, and if

*It may be that M.M. Feil and Son of Paris, who have experimented

extensively and skillfully in glass-making have made experiments in this

direction: but if so, the writer has met no account of them.

The experiment proved a success -if a perfect lens of a given power

should be constructed -then further effort to secure solid lenses of the

same or greater power would be unnecessary.

President Bernard, of Columbia College, having acted as one of the

United States Commissioners to report upon the Paris Exposition in

1867, and the mechanical department and instruments of precision

having been assigned to him, states in his report that M. Steinheil, of

Munich, exhibited "hollow prisms' . . . that "were formed of

plates of plane glass' and "united without cement, being made water-

tight by the perfection and polish of their surfaces." a most important

fact bearing upon this inquiry, since this perfect finish, in addition to

the use of cement, would insure an adhesion of the surfaces of plates

or bars which it would be difficult to overcome.

Having secured a sufficient number of bars of the highest attainable

purity and finish, let the cement be applied, and then lay or pile them

together like cordwood, until a block of any required size is obtained.

Let these be bound firmly together with steel hoops, or otherwise, and

afterward shaped and finished as may be desired. It is not supposed

that the slight amount of polarized light which would be produced

around the circumference by the strongest pressure would effect the

function of the lens. Large and thin plates or the same kind of

glass could be prepared and cemented together in a similar manner,

the largest plate being place in the middle of the pile, with those on

the two sides of it demising somewhat in diameter, until the neces--

sary thickness should be obtained, after which they could be shaped

and finished. When finished their surfaces would present a series

of concentric rings on each side the middle plate. Lenses made

after either of these plans would not be in danger of destruction by

such an accident as occurred to Mr. Common's large speculum, before

noticed, since they could not burst from unequal expansion or con-

traction.

It will be observed that the two methods of construction her pro-

posed are suggested by that most beautiful piece of mechanism the

human eye. In proof of this, it is only necessary to note some ele-

mentary facts concerning the structure of eye. It globe, or ball,

is enclosed in a wall composed of three membranes--the sclerotic,

choroid and retina. Its outer lens--the cornea--consists of several

concentric layers of transparent, homogeneous matter. The choroid is

a thin membrane which adheres loosely to the sclerotic, except at a

Single melting by using the rotating gas furnace which will secure any

required degree of agitation of the liquid mass.

2. While the attempt to make large solid lenses may be successful,

still to make the effort to secure better results by adopting the new

methods of construction herein described.

That the difficulties connected with the undertaking are formidable

cannot be denied, but that they are insurmountable, who will venture

to assert, after recalling to mind the physical and mechanical triumphs

of the last half century? If success shall crown earnest and well-

directed effort, it will be worth to the world all possible cost.

Mar. 17, 1888

My dear sir:

By a letter recieved a few days since from [Mr. Alvan Black?] I [learn?] that [ Mr. O'neil?] of Paris, one of the two best and most sucessful manufactures of optical glass in the world, has made 19 unsuccessful attempts to cast a disk for the object glass of the great [Lick?] telescope, and the experiments one to be continue with success is attained. The practical method of making the unground lens is to prepapre a double [convex?] mold somewhat larger than the lens required and into this [to pour?] the mother glass. This rough