296 
On the Construeiion 
TMonthly Aficrosconlcal 
L Journal, May 1, 1869. 
Triblet tubing is not snfl&ciently accurate for the outer shell of 
the highest powers ; it is better, therefore, to make this of one 
casting, and bore it out of the solid, from its own chuck, and finish 
to the size with a fluted rimer. I have always made the inner 
tube, containing the back lenses, to traverse to and fro, in prefer- 
ence to the front lens, as the object is not thereby lost sight of 
during the adjustment, which is performed in one-third of a revo- 
lution of the outer ring, which has an inclined groove cut in it, 
acting on a screwed pin connected with the inner tube. This plan 
is more simple in construction, and less liable to derangement than 
the one commonly employed. 
On Reducing and Dividing Masses of Glass for Optical Purposes. 
For this, the lapidary sheer and diamond dust are generally 
employed. Discs of glass are split into slices by the working 
lapidaries at such a trifling cost, that it is scarcely worth while for 
the amateur to attempt it. Should, however, a small and rare 
sample be immediately required for experiment, it may be readily 
sliced with a circular disc of soft iron, running in the foot-lathe, 
and fed with flour emery, and water ; the edge of the slicer must 
be frequently notched with the sharp angle of an old file. The 
sample of glass or mineral is cemented to the end of a staff, and 
held preferably in the slide-rest. If the screw of the rest is taken 
out and the slide made slack, the work can be thrust up to the 
slicer with the pressure of the fingers, and there is less risk of 
fracture from undue violence. The sliced glass is cut into squares, 
a little exceeding the diameter of the intended lenses, by means of 
a glazier's diamond, and the corners rounded off with a pair 
of optician's " shanks " or nibblers, which are a species of pliers, 
made, in preference, of soft iron, as this grips the glass without 
slipping, as hard steel would do. This instrument, of a larger size, 
is capable of removing slivers of glass from the edges of a plate 
upwards of one inch in thickness. 
All glass is much softer than hardened steel ; but if this is set 
to cut in a dry state, the heat generated at the working or abrading 
point softens the cutting edge, and speedily destroys its action ; but 
if some turpentine is applied, this quite prevents the softening of 
the tool. In the lathe, or with a common Archimedian drill, holes 
may be drilled through thick plate-glass with surprising rapidity, 
if kept well bathed in turpentine. Masses of glass may also be 
turned in the lathe with a steel tool, if plentifully supplied with 
turps, and run at a moderate speed. 
The first experimental parabolic condensers were made from 
plate-glass l-|-inch thick ; pieces of this, nibbled rudely to form, 
were cemented on to a chuck. The J -rest was next placed nearly 
