ToutnS. aS"? ?gf ] of Object-glasses for the Microscope. 97 
and concave are turned to tlieir respective gauges, and then ground 
together. The diameter of the mould should always rather exceed 
that of the lens intended to be ground ; and the centre, or " pip," 
is taken out ; unless this is done, a prominence is left at this spot, 
which injures the work. During the smoothing, the two moulds 
should occasionally be worked together, as this greatly tends to 
ensure the accuracy of figure of the lens ; and after this is com- 
pletely smoothed, the moulds should be again matched, so as to 
leave them with a polished surface, for a reason to be hereafter 
explained. 
Having got our lens perfectly smoothed and figured, the next 
operation is the polishing. It is almost impracticable to perform 
this in the hard mould, and therefore various substances are em- 
ployed, of a less degree of hardness, in which the coarser particles 
of polishing-powder may become imbedded. For the larger sized 
lenses in microscope work, beeswax, hardened with some resin 
and finely- washed ochre, is very suitable; but for medium sizes 
this is too soft and yielding; a mixture of shell-lac and washed 
putty-powder is therefore employed, which is very enduring. These 
are melted together and stirred diligently; the shell-lac is added 
till the whole arrives at the consistence of thick paste ; and as the 
lac is apt to burn, to prevent this, a lump of beeswax should be 
thrown into the mass* This does not actually mix with the other 
ingredients, but lessens the risk of spoiling the composition by 
overheating: when cool enough this may be rolled into sticks 
between two greased boards. 
For the very smallest lenses, such as the fronts of a J^th and 
:^Vth, the last composition is still too soft and fragile to maintain a 
true figure. The polishing mould is therefore, for these, made 
in the end of a rod of pure tin, which is cut out into a nearly 
hemispherical cup by the appropriate steel gauge; the "pip" is 
removed with a needle-point. 
The wax-pohshing bed is turned out to the required radius, 
and finished by scraping with the steel gauge ; but as the material 
is somewhat yielding, the lens soon plys to the mould and keeps its 
figure during the polishing. 
The second composition is very hard and brittle, and does not 
yield at all, and as the body is composed of the hard oxide of tin, 
this would speedily injure the gauges if used as cutting tools. The 
method that I have adopted for forming the polishing moulds from 
this substance is as follows :— A lump of the material is fastened by 
heat into a ferrule, or hollow cup, running in the lathe ; the end is 
then turned either convex or concave, and of a diameter suitable for 
the lens to be polished ; the convex or concave mould, as required 
(which has been worked off at last near to a polish, as before ex- 
plained), is then screwed on to a handle, and held in a flame till, 
VOL. II. H 
