202 OPTICAL PROJECTION
a slide f-inch diameter, is arranged to be used in the barrel of
the instrument ; the analyser is a square-ended Prazmowski
prism. It is needless to describe the arrangements in detail. In
the most complete form of the apparatus, the analyser is fitted
with a Klein’s quartz plate, and a quarter-wave plate ; and a
system of convergent lenses has also been arranged, somewhat
similar to that described on p. 348, which will exhibit the rings
in crystals, taking in both systems up to the angle of selenite.
The ordinary microscopic crystallisation slides make mag¬
nificent displays in this form of the instrument. The loss of
illumination being very great with polarised light, about the
utmost limit of the power of the instrument, with the 0. H.
light, will be to exhibit the rotating cross in one or two of
the larger starches, such as Tous-le-mois. The arc light will
of course go considerably farther.
101. Slides and Objects.—Organs and parts of insects
make excellent objects, if mounted flat ; those mounted ‘ with¬
out pressure ’ fail, being both opaque, and impossible to focus
in one plane. The essentials are flatness in the preparation,
and neither too much nor too little colour. Experience will
soon teach the proper amount of the latter, and a suitable
slide can always be found amongst any decent stock.
Transparent objects of all kinds are the most difficult, and
histological slides especially so, unless stained well and to the
proper degree. Such sections must also be thin, and it is not
easy to purchase such, stained so as to give the best results.
Where good ‘ differential ’ staining can be got in a thin section,
the object can generally be exhibited under the greatest
illumination, and the positive colour in the structure shows
distinctly ; but with a clear preparation the substage condenser
must either be racked back from the position that gives most
light, or, what is better, if an iris diaphragm is fitted to it,
this must be contracted so as to reduce the cone of light, and
thereby get more distinctness in the phantom-like detail. To
take an instance : a section of skin with the sudorific glands