special methods may be considered in detail. I will begin
with an account of the construction of an achromatic object glass for
a telescope, not because a student in a physical laboratory will often
require to make one, but because it illustrates the usual processes
very well; and requires to be well and accurately made.

A knowledge of the ordinary principles of optics on the part of the
reader is assumed, for there are plenty of books on the theory of
lenses, and, in any case, it is my intention to treat of the art
rather than of the science of the subject. By far the best short
statement of the principles involved which I have seen is Lord
Rayleigh's article on Optics in the Encyclopaedia Britannica, and this
is amply sufficient.

The first question that crops up is, of course, the subject of the
choice of glass. It is obvious that the glass must be uniform in
refractive index throughout, and that it must be free from air bubbles
or bits of opaque matter. [Footnote: The complete testing of glass
for uniformity of refractive index can only be arrived at by grinding
and polishing a sufficient portion of the surfaces to enable an
examination to be made of every part. In the case of a small disc it
is sufficient to polish two or three facets on the edge, and to
examine the glass in a field of uniform illumination through the
windows thus formed. Very slight irregularities will cause a "mirage"
easily recognised.]

The simplest procedure is to obtain glass of the desired quality from
Messrs. Chance of Birmingham, according to the following abbreviated
list of names and refractive indices, which may be relied upon:-

Density. Refractive Index.

C D F G

Hard crown

2.85 1.5146 1.5172 1.5232 1.5280

Soft crown

2.55 1.5119 1.5146 1.5210 1.5263

Light flint

3.21 1.5700 1.5740 1.5839 1.5922

Dense flint

3.66 1 6175 1.6224 1.6348 1.6453

Extra dense flint

3.85 1.6450 1.6504 1.6643 1.6761

Double extra dense flint

4.45 1.7036 1.7103 1.7273 ...

The above glasses may be had in sheets from 0.25 to 1 inch thick, and
6 to 12 inches square, at a cost of, say, 7s. 6d. per pound.

Discs can also be obtained of any reasonable size. Discs 2 inches in
diameter cost about L1 per dozen, discs 3 inches in diameter about
10s. each. The price of discs increases enormously with the size. A
16-inch disc will cost about L100.

For special purposes, where the desired quality o