Bauhaus-Universität Weimar

Titel:
Optical Projection: A Treatise of the Use of the Lantern in Exhibition and Scientific Demonstration
Person:
Wright, Lewis
PURL:
https://digitalesammlungen.uni-weimar.de/viewer/image/lit39439/212/
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
        

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