868 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
surface of a clear glass lens will have many advantages over a metal 
mirror. The front surface will give, perhaps, the finest definition, but by 
silvering the back part of a spherical glass film, or that of a ground lens, 
the brilliant portion will remain untarnished for an indefinite time, and 
the whitish bloom formed by slow volatilization of the incandescent lime 
is easily removed. This silver film adheres with remarkable tenacity, 
and it will bear a great deal of heat without blistering or becoming 
detached. I have had considerable success in constructing such mirrors 
from the large ornamental glass spheres blown in Germany, and silvered 
within by Liebig’s process, viz. with milk, sugar, and ammonio-nitrate 
of silver. 
A glass sphere of 10 or 11 in. in diameter may be easily cut into 
eight or nine mirrors by a red-hot iron, and this without disturbing the 
silvering, which will require only gentle friction with a pad of cotton 
impregnated with a trifle of rouge to brighten it. Thus, at the cost of 
a few shillings, eight or more mirrors can be made, and also provision 
be made against possible accidents of cracking by heat. 
The light-radiant is so placed that the secondary focus is intercepted 
by a plano-concave lens of dense glass, as has been happily proposed by 
Mr. L. Wright. The convergent rays from the speculum are thus made 
into a parallel beam, which must be deprived of its heat by an alum 
trough, for the light and heat at the substage condenser are very great. 
Convergence, I find, is usefully promoted by a plano-convex lens of 
about 8 in. focus placed two or three inches before the above-noted 
plano-concave lens. In all other respects the arrangements are like 
those of the usual modern projection Microscope. 
I have pretty constantly used the ether-oxygen saturator, and I 
consider it to be perfectly safe if ordinary precautions be taken. The 
oxygen, compressed in cylinders, is much recommended, as there can be 
no mixture of vapour, except at the right place. The U-shaped hori- 
zontal saturator, plugged with flannel, must be well charged with ether, 
or with the best gasoline, and care should be taken, before beginning or 
ending an exhibition, to shut off the oxygen-tap before closing the ether- 
tap. This will prevent the harmless ‘ snap 5 from the mixture in the 
small chamber at the joining of the gas-tubes. If a disc more than 8 ft. 
be required for the Microscope, it will be well to use hydrogen gas 
instead of ether, since the calibre of the jet cannot in the ether light 
very well exceed 1/14 in. As an extra security I pack the mixing 
chamber with asbestos fibre moistened with glycerin; but, as before 
urged, the oxygen must leave the saturator saturated. 
To ensure the coincidence of the foci of the reflector with the optical 
axis of the Microscope it will be well to place three adjusting screws 
in a triangle behind the mirror, and this last may have both a small 
vertical and horizontal movement. I claim for this catoptric arrangement 
a larger grasp of light than can be got from ordinary lenses, and this 
may be effected also at a small outlay. For the amateur constructor the 
plan will afford many advantages.” 
(4) Photomicrography. 
Processes of Photomicrography. — The following is the text of 
Dr. Piffard’s letter, read at the October Meeting: — “In the last issue 
