68 
THE YOUNG SCIENTIST. 
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 rather scraping tool, and 
finished by grinding the two moulds to- 
gether with the finest emery. 
There is some difference in practice be- 
tween the grinding of lenses for long and 
short radii. In the former, as for tele- 
scopes, the glasses are fixed, or have but 
a very slow rotary movement, and the 
concave tool is worked over them, either 
several at a time in blocks, or else, if a 
shallow curve is required, only on one 
single disc ; this is placed in the centre, 
and a number of smaller pieces of glass 
planted round the circumference to sup- 
port the figure, the whole being ground as 
one. But in the lenses to which this 
paper particularly refers, the concave tool 
is invariably caused to revolve rapidly, 
and the convex lens worked into it. 
The same rules for guarding the extreme 
edges of lenses should be observed, as de- 
scribed in prism-work, shown by the fol- 
lowing examples. Fig. 8 a, represents a 
plano-convex lens which has been made 
and finished upon a flat disc of glass, to 
which it has been attached with hard 
Canada balsam. The two discs are ce- 
mented to the stick with black sealing- 
wax; the lens and supporting disc are 
rough ground on the zinc plate till they 
nearly fit the concave gauge; they are 
then ground in the brass mould till the 
lens measures very nearly the diameter 
required, leaving a small allowance for 
smoothing and polishing. 
For double convex lenses, the disc of 
glass, cemented on a stick as usual, is 
first ground and polished on one side. A 
piece of glass tube of suitable size is se- 
lected for a handle, and the end of the 
bore ground out to a similar radius ; the 
polished side of the unfinished lens is then 
cemented Into this concavity, and the lens 
and tube ground and polished off together, 
as shown by Fig. 8 h, taking the same pre- 
cautions as before to work the lens up to 
the exact diameter required. The end 
lines show the rough disc as cemented 
down. By this method all the marginal 
errors are taken up by the glass tube- 
holder, of which an assortment of various 
sizes will be required, from a minute 
bugle up to half an inch in diameter. 
Before using the holders again for other 
lenses, the end must be ground out on 
each occasion, so as to increase the di- 
ameter of the cup. The lens, when taken 
out by being warmed, will have a knife- 
edge perfect in the extreme. 
In minute lenses, some difficulty will be 
experienced in obtaining the measure- 
ments by means of gauge instruments,, 
when near the right diameter. I there- 
fore, for small sizes, always use the mi- 
I croscope with micrometer eye-piece, hav- 
a 
ing previously taken the exact size from 
the diameter of the cell in which the lens 
is to go. This is very accurate and con- 
venient. After the finished lens is taken 
out of the holder, if it should be found too 
large to enter the cell, it may be slightly 
cemented to the end of a wire, and twisted 
into a piece of the finest emery paper, held 
in a hollow form, and the keen edge is 
taken off till it passes through. 
The single fronts, for the highest powers, 
from their form, do not admit of being 
ground in this way. A piece of brass or 
steel is screw^ed into the mandril, and the 
end turned of a size to enter the cell inta 
which the lens is to go ; the end is turned 
flat, or rather slightly hollow, and the 
centre taken out. A piece of crown-glass- 
is cemented by its polished side to the flat 
end, with the best orange shellac, 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 ex- 
6 
Fig. 8. 
