94 
On the Construction 
[Monthly Microscopical 
Journal, Aug. 1, 1869. 
rather scraping tool, and finished by grinding the two moulds 
together with the finest emery. 
There is some difierence in practice between the grinding of 
lenses for long and short radii. In the former, as for telescopes, 
the glasses are fixed, or have but a very slow rotary movement, 
and the concave tool is worked over them, either several at a time 
in blocks, or else, if a shallow curve is required, only on one single 
disc : this is placed in the centre, and a number of smaller pieces of 
glass planted round the circumference to support the figure, the 
whole being ground as one. But in the lenses to which this paper 
particularly refers, the concave tool is invariably caused to revolve 
rapidly, and the convex lens worked into it. For the longest 
radii and lowest powers the ordinary foot-lathe is suitable, but this 
is not so well adapted for grinding and polishing very minute 
lenses. A bow lathe, such as used by watchmakers for heading 
their screws and other purposes, is far preferable. This tool is 
represented half size by the annexed cut (Fig. 2), and scarcely needs 
Fig. 2. 
explanation ; it has a hollow screwed mandril and T-rest, and is 
held in the vice by the tongue at the bottom. The pulley has 
three speeds, the smallest of which is three-eighths of an inch in 
diameter ; it should also have a socket for carrying a fixed magnifier, 
under which the minutest lenses are turned. The best bow is an 
old fencing-foil ground down so as to be very thin and light. 
Catgut does not answer well for the string, as it soon gets frayed 
out over the small pulley. I have found the best packing-twin© 
preferable. During work this is kept sHghtly moist, and rubbed 
with a piece of soap ; in this way a length of it will outlast a day's 
work, especially if a little more twisted before it is attached to the 
wire hook at the top of the bow. A surplus stock of string may 
be wound about the guard, just above the handle, so that it can be 
drawn out as required. 
The same rules for guarding the extreme edges of lenses should 
