even hours at least are required for a good deposit; a
good plan is to leave the mirrors in the bath all night. On removal
from the bath the mirrors require to be well washed, and allowed to
dry thoroughly in sun heat for several hours before they are touched.

Care should be taken not to pull the mirrors out of shape when they
are mounted for the bath. A single drop of varnish or paint (a mere
speck) on the centre will suffice to hold them. The back of the
deposit requires to be varnished or painted as a rule to preserve the
silver. All paints and varnishes thus applied tend to spoil the
figure by expanding or contracting. On the whole, I think boiled
linseed oil and white or red lead--white or red paint in fact--is
less deleterious than other things I have tried. Shellac varnish is
the worst.

Of course, the best mirror can be easily spoiled by bad mounting. I
have tried a great number of methods and can recommend as fairly
successful the following:- A little pure white lead, i.e. bought as
pure as a chemical--not as a paint--is mixed with an equal quantity
of red lead and made into a paste with a little linseed oil. I say a
paste, not putty. A trace of this is then worked on to the back of
the mirror at the centre as nearly as may be, and to this is attached
the support. The only objection to this is that nearly a week is
required for the paste to set. If people must use shellac let it be
remembered that it will go on changing its shape for months after it
has cooled (whether it has been dissolved in alcohol or not).

Sec. 66. Preparation of Large Mirrors or Lenses for Telescopes.

So much has been written on this subject by astronomers, generally in
the English Mechanic and in the Philosophical Transactions for 1840,
that it might be thought nothing could be added. I will only say here
that the processes already described apply perfectly to this case; but
of course I only refer to silver on glass mirrors. For any size over
6 inches in diameter, the process of grinding and polishing by hand,
particularly the latter, will probably be found to involve too much
labour, and a machine will be required. A description of a
modification of Mr. Nasmyth's machine--as made by my assistant, Mr.
Cook--will be found below.

There is no difficulty in constructing or working such a machine, and
considered as an all round appliance, it possesses solid advantages
over the simple double pulley and crank arrangement