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(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,