m Ohject-glasses for the Microscope. ["fJ^^^i^jTsTS' 
when touclied witli the moistened finger, it hisses smartly; a 
morsel of tallow is then put on the rough-turned composition to 
prevent adhesion, and the hot mould worked and rotated over it in 
every direction till cold; when removed, the polisher will have 
taken the exact form of the heated mould, and have acquired a 
fine polish. For either convex or concave lenses the "pip" is 
taken out as usual, and it is advisable to make a few concentric 
scratches in the polisher if of large diameter. 
As the mixture of crocus and putty-powder, recommended for 
polishing, is apt to cling in these moulds if applied at once, I first 
use the putty-powder alone; this cleans the hard polish off the 
face, and the operation may then be continued with the mixture. 
One great advantage of this composition, for a polishing mould, 
is the decided way in which it maintains a true figure ; for, unlike 
any other of the kind, it undergoes a very shght degree of wear, so 
that the face is always kept clean ; and any number of lenses of 
similar form and radius may be polished in the same tool without 
having to alter or mend the figure, and perfect accuracy is the 
result. This composition is now generally known, but Mr. James 
Smith is the original discoverer of it. For the last degree of 
poHsh, I sometimes rub a thin layer of pure soft beeswax in the 
mould, and smooth it down to form with the now finished lens ; 
then a small quantity of the very finest-washed crocus is applied, 
and the lens worked therein for about one minute. The extra 
brilliancy of surface obtained this way is quite appreciable, and 
well worth the pains bestowed, as the operation is not continued 
long enough to run the risk of injuring the figure. 
I have only now to give some directions for cementing the 
lenses together. The surfeces having been carefully cleaned, the 
two lenses are laid on a hot plate, a drop of Canada balsam is 
placed in the concave, the group of bubbles thrown up by the heat 
removed by a brass point; with this the convex lens (which is 
equally hot with the other) is lowered slantways into the balsam 
so as to avoid bubbles, and the two lenses are pressed together ; 
they are now lifted off the plate with a pair of curved forceps 
held nearly horizontally, and shifted one-quarter round, and then 
dropped down again. This is repeated a number of times, and the 
two lenses being exactly of the same diameter, this operation must 
set them concentric as a matter of course. If the lens is a triple, 
the opposite surface of the concave must be cleaned and the balsam 
removed with strong alcohol (turpentine must not be used, as it 
percolates the balsam too easily, and is apt to cause bubbles to 
appear at the edges), and the same operation repeated as on the 
other side. When the lens is cleaned with alcohol, and examined 
edgeways with a magnifier, the three lenses will appear quite con- 
centric, and should just pass into the cell without requiring any 
