LENSES.

Optical glasses have engaged the attention and investigation of

scientific men for centuries. We read of the wonderful exploits of the

burning lens of Archimedes, and find the remains of lenses thousands of

years old in the ruins of Nineveh, Babylon, and Pompeii. They are of

the utmost importance in the science of astronomy. The slow progress

made in perfecting them shows the inherent difficulties that exist in

obtaining glass of the required purity. One of these is the different

specific gravities of the material used. Hence the lower part of a pot

of melted glass is of greater specific gravity than the top, causing a

tendency to cords or threads, an evil which science has yet to learn to

overcome. Not even the large bounty offered by the English Government

and the Board of Longitude has been successful in effecting any

important improvement in this branch of manufacture. Munich enjoys the

reputation of producing the best lenses, and consequently the finest

telescopes. Sir Isaac Newton, Gregory, Dolland, Keir, and others

adopted lenses made from flint- and from crown-glass, it being

necessary to use both in the construction of achromatic telescopes, one

possessing as small and the other as great dispersive powers relative

to the mean refractive powers as can be procured. But the inherent

defect of the lenses still remained. M. Macquer remarks, "The

correction of this fault appears therefore to be very difficult." He

had tried in vain to remove it by very long fusion and fierce fire.

Others have found this by experience not to correct, but to augment the

evil. Mr. Keir is of opinion that some new composition must be

discovered, which, along with a sufficient refractive power, shall

possess a greater uniformity of texture.

Since then, it is certain some improvement has been made in the

composition for lenses. In an English paper we find the

following:--"One of the most remarkable optical lenses of modern

manufacture is that produced by Messrs. Chance, English manufacturers,

being an attempt by them to improve the manufacture of glass for

optical purposes. The diameter is twenty-nine inches, and it is two

inches and a quarter thick. It is really not a lens, but a plain disk

intended for a lens, should its quality be sufficiently fine. The

weight is about two hundred pounds. This piece of glass was inspected,

on its first public exhibition, by eminent scientific judges. It was

by them examined edgewise, transversely, and obliquely; it was viewed

by daylight and by candle-light; it was tested by the polariscope and

by other means; and after having been thus subjected to a severe

ordeal, it was pronounced to be the largest and finest known specimen

of the kind."

The promise held out by the foregoing we fear has failed, as in very

many previous cases, or the world ere this time would have heard of its

success. An achromatic object-glass for telescopes consists of at least

two lenses, the one made of flint-glass, and the other of crown-glass.

The former, possessing least power of dispersing the colored rays

relative to its mean refractive power, must be of greater value than

the latter. It is upon this principle that the achromatism of the

image is produced, the different colored rays being united into one

focus. Flint-glass, to be fit for this delicate purpose, must be

perfectly homogeneous, of uniform density throughout its substance, and

free from wavy veins or cords.

From the foregoing, the reader will see that, as has been said, the

chief difficulty which exists in making telescopic lenses arises from

want of pure glass. Every attempt to correct this evil has failed; it

is well known our best telescopes and like optical instruments have

always achromatic lenses, and for photographic purposes achromatic

lenses are indispensable. If philosophers and astronomers have with so

imperfect lenses attained so much, what may not the astronomer look for

when science gives him lenses made from pure glass? If the heavens, by

imperfect instruments, have so far been unveiled, to what extent may he

not then be able to penetrate the pure ether, and reveal planets and

heavenly bodies as yet unknown?

We close our reminiscences of Glass and its manufacture, by presenting

to our readers a view of an American model glass factory of the present

day.[4] By comparing this view with the sketches heretofore given of

the early Venetian and French factories, they will perceive the very

great improvement which is apparent over the ancient plans, an

improvement conducing alike to the health and comfort of the workmen.

Thirty years have passed in its development, during which many

difficulties arose from the conflicting opinions of the English and

German glass-makers; and, in fact, it was not until the proprietors

boldly separated themselves from the current and influence of old, and

almost fixed opinions, that any decided progress was shown in the

development of manufacturing efficiency, or any plan contributing to

the health and comfort of the workmen employed.

[4] See drawing No. 4, at end of book.

It is to be borne in mind that the first glass works in this country

were established by the Germans, who used no other fuel than wood, the

furnaces for window-glass constructed under their directions being for

that fuel only; on the other hand, the English workmen who introduced

the making of flint-glass had made use of no other fuel than coal, and

the English were therefore obliged to adopt (for the want of coal) the

German plan for furnaces, and adapt the same to the making of

flint-glass. The house was like the furnace, half English and half

German, and from the year 1812, for thirty years, little or no

improvement was made in this particular. Year after year the old plan

was followed, until necessity paved the way for new plans in the effort

to secure a less expensive mode of melting glass.

The result has been highly favorable. More than one half has been saved

in the melt, annealing leers, and working places, yielding the workmen

greater space and facilities in performing their work, and no longer

exposing them to the discomfort of extra heat, smoke, and unhealthy

gases. These improvements have enabled the American manufacturer to

sustain his business in the severe and trying competition with foreign

manufacturers, who forced their glass into this country through their

agents a few years since, in such quantities, and at such reduced

prices, as seriously to affect the prosperity of our artisans; yet,

aided as they have been by a tariff directly promoting foreign

interest, and by the very low rates of wages paid on the Continent,

they have been successfully contended with, and now a home competition

has sprung up, reducing prices below a fair standard,--a competition,

the result of enterprise, which will, erelong, regulate itself, for we

fully hold to the maxim, that competition, honest and well sustained,

is the soul and life of business:--

"No horse so swift that he needs not another

To keep up his speed."

There is no mechanical employment in this country yielding so good

returns to the industrious as a good worker in glass, of the present

day, can secure in the exercise of his skill. And we may still further

say that there is no mechanical branch of industry offering such

advantages for the full manifestation of a workman's real skill and

industry, if the conventional usages which restrict the work could but

be abrogated,--usages tending to a limited amount of work, and

consequently making the workman to realize but a limited amount per

week. Such workmen, of all others, should be allowed the inherent and

inalienable right to work as long, and at such times, as the individual

may deem for his comfort and interest.

We have expressed the opinion that the manufacture of glass is as yet

but in its infancy. The experience of every day confirms the assertion,

and illustrates the maxim that "life is short, art is long."

The time is not far distant when this country will become, we think,

the largest exporter of glass, and the manufacture compose a most

important item in every assorted export cargo. In this connection a

hint to ship-owners may not be amiss. It is well known that in England,

when a ship is put up for a foreign port, it is the custom to rate the

freight according to the value of the merchandise,--dry goods paying

the highest freight, hardware the next highest, earthen and glass ware

the lowest. If our merchants would adopt this plan, very many of our

bulky manufactures would find a market abroad; when, however, the same

rate is required for a cask of glass ware as for a case of silks or

prints, it taxes the latter a small percentage, but practically vetoes

the export of the glass.

Our task is now ended; our object has been to give a simple and

succinct outline of the characteristics and progress of the Glass

Manufacture, to suggest such hints as might bear upon the further

advance of the art, and the preservation of those practically

identified with the manufacture, and, if possible, to attract the

attention of those hitherto unacquainted with its nature and history.

If we have neglected the maxim that "_those who live in glass houses_,"

&c., it has not been from the want of honest endeavors to remember it;

and if we have contributed either to the instruction or the pleasure of

any reader, (and this is our hope,) we shall not regret the hours spent

in the preparation of this little work.