^jZraL.SrS^ Object-glasses for the Microsco^pe. 93 
YI. — On the Construction of Object-glasses for the Microscojpe. 
By F. H. Wenham. 
(Continued fro7n page 347, No. VL) 
On the Production of Sjoherical Surfaces in Glass. 
As the radii required in the construction of microscopic object- 
glasses are seldom very long, the templates for all sizes above 
ith of an inch in diameter are usually made of steel, such as thin 
saw, spring, or busk-steel, not softened, but turned hard, as obtained. 
A hole is punched through the middle of a square plate with a 
centre punch, the hole is then rounded out with a taper rimer. 
The piece of steel is next broken round as near as possible to the 
size of the circle required, by clamping it in the vice and driving 
off the surplus metal round the edge with a chisel held close to the 
jaws. This steel plate is driven on to a mandril so as to turn true 
without any wabble. The lathe is run at a slow speed, and the 
T-rest placed rather high near the top of the work, which is turned 
true with the common square graver held over-hand. The cham- 
fered edge of the templates may form an angle of 90^. Every 
convex template should have its counterpart or concave ; the steel 
plate to form this is clamped flat on to a face-chuck by a ring with 
two opposite screws tapped into the plate. The inner circle is 
turned out with a side tool, consisting of an old saw-file ground to 
a point on the three faces. The turning is continued till the disc 
or gauge just drops through; the inner edge is then chamfered 
from both sides. 
Gauges below ^^th of an inch in diameter are made from steel 
wire turned to the annexed form (Fig. 1). The ^ 
disc end is hardened by heating it with the lamp 
and blowpipe, and quenching it in oil, and the jjlll 
counter-gauges are most easily formed by a coun- 
ter-sink rose-bit run in the lathe. The plate of steel is chamfered 
out alternately from opposite sides, by forcing it up on the socket of 
the back centre, till the disc will pass through ; the hollow tem- 
plates are, of course, cut in half before they can be used. 
An instrument for measuring the diameter of the discs, &c., is 
indispensable. It consists of a pair of sliding steel jaws, with a 
vernier and nonius capable of being read off to thousandths of an 
inch, and is sold by the watch tool-makers. 
The moulds for grinding minute lenses are always of brass ; they 
are also used in pairs. The concave is turned out to the gauge, 
and the convex to the counter-gauge. For small radii the hard 
gauges are finally used for the last correction, as a turning, or 
