Untitled Page 3

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