THE YOUNG SCIENTIST. 
67 
use an extra quantity where the pots and 
pans join together. The bottom of the 14- 
inch pot was extended with plaster, so as 
to form the point of tiie acorn. The in- 
terior of the large pot was divided into two 
compartments, holes being cut into the 
sides of the pot for entrances. These 
holes are easily cut with an old knife 
made into a saw by " hacking " it. 
In ornamenting this bird house I took 
advantage of the subdued red color of the 
pot, which I slightly veined with plaster. 
The rough coating is applied with a 
broad-bladed table knife or ''pointing" 
trowel. After the plaster is dry, a coat of 
sombre green paint is applied. Where 
the stick passes through the top pot a 
cross is formed by nailing on a cross 
piece. The small inverted top pot is 
filled with earth, and planted with trades- 
cantia or German ivy. 
Under the eaves, formed by the pro- 
jecting pan, numbers of holes were bored 
through the pot and into the compart- 
ments, to secure sufficient ventilation 
inside. 
This bird house should be suspended 
from the branch of a tree by means of a 
-stout wire attached to a strong screw eye, 
firmly inserted into the rod which passes 
through the centre of the house. The end 
of the rod should be firmly bound with 
annealed wire to prevent its splitting, and 
the iron hook should be heated and dipped 
in melted grease before being screwed in. 
This will prevent its rusting and losing 
its hold. Eope or sash line should not be 
used for suspending the house, as they are 
apt to rot and break, and besides this they 
twist and untwist, and so cause the house 
to twirl round, making it unsteady. 
Home-Made Telescopes and Micro- 
scopes— VI. 
^ON THE PRODUCTION OF SPHERICAL SURFACES 
IN GLASS. 
A S the radii required in the construction 
of microscopic object-glasses are sel- 
dom very long, the templates for all sizes 
above l-5th of an inch in diameter are 
usually made of steel, such as thin saw, 
spring, or busk-steel, not softened, but 
turned hard, as obtained. A hole is 
punched through the middle of a square 
plate with a centre punch, the hole is 
then rounded out with a taper rimer. 
The piece of steel is next broken round as 
near as possible to the size of the circle 
required, by clamping it in the vise and 
driving off the surplus metal round the 
edge with a chisel held close to the jaws. 
This steel plate is driven on to a mandril 
so as to turn true v. ithout any wabble. 
The lathe is run at a low speed, and the 
T-rest placed rather high near the top of 
the work, which is turned true with the 
common square graver held over-hand. 
The chamfered edge of the templates may 
form an angle of 90^. Every convex tem- 
plate should have its counterpart or con- 
cave; the steel plate to form this is 
clamped flat on to a face-chuck by a ring 
with two opposite screws tapped into the 
plate. The inner circle is turned out with 
a side tool, consisting of an old saw-file 
ground to a point on the three faces. The 
turning is continued till the disc or gauge 
just drops through ; the inner edge is then 
chamfered from both sides. 
Gauges below l-5th of an inch in diame- 
ter are made from steel wire turned to the 
form shown in Fig. 7. The disc end is 
Tig. 7. 
hardened by heating it with the lamp and 
blowpipe, and quenching it in oil, and the 
counter-gauges are most easily formed by 
a counter-sink rose-bit run in the lathe. 
The plate of steel is chamfered out alter- 
nately from opposite sides, by forcing it 
up on the socket of the back centre, till 
the disc will pass through ; the hollow 
templates are, of course, cut in half before 
they can be used. 
An instrument for measuring the di- 
ameter of the discs, etc., is indispensable. 
It consists of a pair of sliding steel jaws, 
with a vernier and nonius capable of be- 
ing read off to thousandths of an inch, 
and is sold by the watch tool makers. 
The moulds for grinding minute lenses 
are always of brass ; they are also used in 
pairs. The concave is turned out to 
