THE YOUNG SCIENTIST. 
69 
treme end of the glass is then turned off 
flat, till it equals the thickness of the in- 
tended 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 Fig. 9 ; 
Fig. 9. 
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 maybe 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 polished 
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 ob- 
servable 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 con- 
vex, to match the radius of the concave 
surface of the disc of glass that it is to re- 
ceive, this having been previously ground 
out and polished independently in the 
cell or chuck (Fig. 10), and after it is pol- 
ished as before directed, the chuck is 
again screwed into the mandril, and 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 i^recautions have to be ob- 
served in smoothing lenses as directed for 
prism-work; the finest emery is used, 
and the requisite moisture applied as re- 
quired 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 and concave 
are turned to their 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 oc- 
casionally be worked together, as this 
greatly tends to insure the accuracy of 
figure of the lens ; and after this is com- 
pletely smoothed, the moulds should 
again be matched, so as to leave them 
with a polished surface, for a reason to be 
hereafter explained. 
Fig. 10. 
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 
Training Plants. 
THOSE who have visited European gar- 
dens and pleasure grounds, must have 
been struck with the beauty of the trained 
plants, forming hedges and bowers, which 
form a prominent feature in the old home- 
steads. The wonderful skill shown in the 
culture and training of these plant mar- 
vels is surprising to those who are fa- 
miliar with the vain attempts made in 
this country to grow live hedges. All 
along our country roads we see attempts 
to produce these live hedges, but in almost 
every case the result is a miserable fail- 
ure. Although horticulture is not a sub- 
ject to which we propose to devote much 
attention in these pages, yet since a 
nicely-trained hedge or arbor forms a 
pleasant work of art for our young 
