82 
THE YOUNG SCIENTIST. 
into a nearly hemispherical cup by the 
appropriate steel gauge ; the "pip " is re- 
moved with a needle-point. 
The wax-polishing bed is turned out to 
the required radius, and finished by scrap- 
ing with the steel gauge ; but as the ma- 
terial is somewhat yielding, the lens soon 
plays to the mould and keeiJS 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 pol- 
ished ; the convex or concave mould, as 
required (which has been worked off at 
last near to a polish, as before explained), 
is then screwed on to a handle, and held 
in a flame till, when touched with 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 con- 
vex 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 composi- 
tion for a polishing mould is the decided 
way in which it maintains a true figure ; 
for, unlike any other of the kind, it under- 
goes a very slight 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 fig- 
ure, and perfect accuracy is the result. 
The composition is now generally known, 
but Mr. James Smith is the original dis- 
coverer of it. For the last degree of 
polish 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 sur- 
face obtained in this way is quite appre- 
ciable and Avell worth the pains bestowed^ 
as the operation is not continued long- 
enough to run the risk of injuring th& 
figure. 
I have now only to give some directions 
for cementing the lenses together. The 
surfaces liaAang 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 o'ff the plate with a 
pair of curved forceps held nearly horizon- 
tally, and shifted one-quarter round, and 
then dropped dowm again. This is re- 
peated 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 concavo 
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 ap- 
pear at the edges), a,nd the same operation 
repeated as on the other side. When the^ 
lens is cleaned with alcohol, and exam- 
ined edgeways with a magnifier, the three 
lenses will appear quite concentric, and 
should just pass into the cell without re- 
quiring any force ; and if the workman- 
ship has been correct — ^viz., all the cells 
turned true from one chucking, and the 
concaves of equal thickness and concen- 
tric with their respective convex lenses, 
no errors of centering can occur. The 
usual way of correcting this is by tilting 
the lenses in the cells in which they are 
