THE YOUNG SCIENTIST. 
42 
with wet plaster, and let it remain until 
perfectly hard. 
When the bust is quite drij, get some 
very fine sandpaper and rub off the line 
caused by the joining of the mould ; then 
give it a few finishing touches and it is 
done. 
To maiic anv number of the same bust, 
Si piece mould must be constructed. They 
are very complicated, ana difficult to 
make, and are never attemptea out oy 
professional casters. 
How to Grind Convex Lenses. 
BY E. M. BAILEY. 
TO many persons the process of shaping 
and polishing glass lenses seems 
beyond the powers of those who have not 
the acquired skill and the mechanical ap- 
pliances of the professional optician. But 
this is not the case. Any one of fair me- 
chanical ability, and with means that are 
to be readily obtained almost anywhere, 
can produce good magnifying glasses of 
any desired powers within certain limits. 
Perhaps it is not too much to say that 
first-class convex lenses can be made by 
any reader of The Young Scientist, after 
a little practice. We will show how this 
can be done. 
Any turning lathe having the end of the 
mandrel hollow, can be used as a grinding 
and polishing machine, and is all the 
machinery that is required. The Lester 
scroll saw, with lathe attachment, answers 
very well for grinding microscope lenses. 
Decide upon the focal length of the lens 
you wish to grind, and also whether it 
shall be plano-convex or double-convex. 
If plano-convex, set a pair of dividers to 
half the focal length required; if double 
convex, set them to the whole focal length. 
We will suppose that you wish to make a 
plano-convex lens of one-inch focal length, 
which will magnify ten diameters. Set 
the dividers to half an inch, and upon a 
piece of sheet brass, or zinc, strike a quar- 
ter or third of a circle. With shears or a 
narrow cold chisel, cut out the inner part 
of the circular segment, finishing, with a 
half round file, carefully and exactly to 
the. mark. You thus have a concave gauge 
like Fig. 1. 
Next procure a strip of thick saw blade 
or other flat steel, not over an inch in ^ 
width, and long enough to be conveniently ! 
handled. An old chisel will answer well. 
Grind one end to a convex shape that will 
exactly fit the concave gauge, giving it a 
sharp cutting edge of large angle, not less 
Fig. 1. 
than sixty degrees, rounding off the other 
angle, so that on holding it edgewise, the 
edge will appear like Eig. 2. Fig. 3 repre- 
sents the edge of this gouge, as we will 
call it, when completed. It must fit the 
concave gauge so exactly that on placing 
the edges together no light can be seen 
between them. The curve must be the 
Fig. 2. 
same at every point in both gauge and 
gouge, which may be proved by fitting 
them together at different angles. 
This gouge is to be used in making the 
next tool, which is a concave grinder or 
lap required in polishing the lens. Take 
a block of hard wood, plane one side 
smooth, and with an inch bit bore a hole 
Fig. 3. 
half an inch deep in the planed surface. 
Then in the centre of this hole bore an- 
other hole through the block, this last 
hole being a little larger than the hole in 
the mandrel of the lathe. These holes 
must be bored quite perpendicular to the 
planed surface of the block. Fig. 4 repre- 
