242
OPTICAL PROJECTION
in the lantern of an ordinary slide or diagram. As these
tanks frequently require to be washed clean for other experi¬
ments, one of the best methods of constructing them is that
shown in fig. 128, where a piece of smooth vulcanised tubing
bent into a semi-circle, is squeezed between two glass plates
by screws through the corners, so as to make a water-tight
vessel easily taken to pieces for cleaning, and unaffected by
most chemicals. Or the plates may be mounted between
outer plates of metal or ebonite, which enables plain or un¬
perforated pieces of glass to he used. Pieces of thick solid
rubber cut to shape may also
be used between the plates, but
require more pressure and are
not so safe. If a little more
solidity is desired for the com¬
pressed tubing, the latter may
he filled with fine sand after
being screwed up to the proper
tension.
Such tanks may be of any
size and thickness of content,
and a sufficiency should be pro-
pig. 128.—Tank vided for the proposed series of
experiments. Or the glass plates
may sometimes be more conveniently introduced into some
such wooden frame as was shown in fig. Ill, p. 223 ; and the
rubber tube then squeezed into place rather tightly, will
readily give a water-tight tank, only the glass and rubber
coming into contact with the fluid.
Occasionally it will he needful to employ tanks made
entirely of glass, cemented together, according to the nature of
the solution. For aqueous solutions, those sold as zoophyte
troughs for microscopic use are convenient, and may be had
of almost any desired size ; but for spirituous solutions marine
glue must be replaced by other cements, such as are used in