II.--CAMERA STAND.--The best constructed stands are made
of maple or blackwallnut wood, having a cast iron socket
(a, fig. 12,) through which the sliding rod b passes, and into
which the legs c, c, with iron screw ferules are inserted.
The platform d is made of two pieces, hinged together,
as at e, and having a thumb screw for the purpose of elevating
or depressing the instrument.
[hipho_9.gif]

III. MERCURY BATH.--Fig. 13 gives a front view of the mercury bath
now in general use in this country for mercurializing and bringing
out the picture. It is quite an improvement on those first used.
To make it more portable it is in three pieces, a b and c;
having a groove e on one side to receive the thermometre tube and scale
by which the proper degree of heating the mercury is ascertained.
Into the top are nicely fitted two or three iron frames, with shoulders,
for the plate to rest in, suitable for the different sizes of plates.
The bath is heated by means of a spirit lamp placed under it.
From two to four ounces of highly purified mercury are put into
the bath at a time.

IV. PLATE BLOCKS AND VICES.--There are several kinds of this article in use;
I shall describe the two best only.

Fig. 10 gives an idea of the improvement on the English hand block.
The top a is perfectly flat
[hipho_10.gif]
and smooth--a little smaller than the plate, so as to permit the latter
to project a very little all around--having at opposite angles c c