96 
On the Construction 
t Monthly Microscopical 
Journal, Aug. ], 1869. 
orange shell-lac, and turned with the diamond point till it nearly 
enters the cell. The last finish may be given by fine emery-paper 
wrapped round a flat piece of hard wood. The extreme end of 
the glass is then turned off flat, till it equals the thickness of the 
intended lens, from the apex to the flat, as measured by the jaws of 
the gauge ; the lens is next turned off by the diamond to the curve 
required, as shown in the cut (Fig. 5) ; and, 
finally, the chuck is removed, and the lens 
ground and polished in the mould as usual. 
In all cases of cementing lenses on to chucks 
in this way, care must be taken that they are 
well pressed down, so that the layer of cement 
may be of the same thinness all round, otherwise the lens will be 
tilted and out of centering from unequal thickness. When taken 
off, the lac may be cleaned off with alcohol. 
A similar mode of chucking is employed for a plano-concave 
lens. The polished flat side of the flint glass is cemented to the 
chuck, made just to enter the cell ; but in order to appreciate the 
thickness in the centre, the circumference of the disc, after it is 
turned to fit the cell, is pohshed with a piece of hard wood and 
crocus. The concavity is then turned out a trifle deeper than the 
radius of the circular gauge, till a mere line of light only is observ- 
able by looking through the polished edges. The chuck is then 
removed from the mandril, and the lens thereon ground and finished 
on the convex tools. 
For a double concave lens, such as is used for a triple back, the 
end of the chuck, instead of being flat, must be convex, to match 
the radius of the concave surface of the disc of glass that it is to 
receive, this having been previously ground out and polished 
independently in the usual way of cementing it on to a stick ; but 
as the curves are shallow, it is best not to turn the disc down to 
the intended size at once, but leave it much 
larger than the cell or chuck, thus (Fig. 6) ; 
~7 and after it is polished as before directed, the 
! chuck is again screwed into the mandril, and 
V the lens turned down so as to fit the cell ; this 
is done in order to avoid the marginal errors 
which would arise from working a shallow curve of small diameter. 
The same precautions have to be observed in smoothing lenses 
as directed for prism-work; the finest emery is used, and the 
requisite moisture applied as required by breathing on the lens, 
taking care that the accumulation of powder is removed from time 
to time from where the centre of the mould has been dug out, 
otherwise this may contain some coarser particles that may cause 
scratches. 
As before remarked, the moulds are made in pairs ; the convex 
