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PRACTICAL PHOTO-MICROGRAPHY. 
Copper nitrate ... ... ... 160 parts. 
Chromic acid ... ... ... ••• 14 >> 
Water to ... ... ... ••• ••• 250 ,, 
A thickness of this of about one centimetre passes 
only yellow-green rays. In practice it will be found 
more convenient to use pieces of glass of suitable 
colour; a selection may be made from the odd bits of 
any glass maker, as, for example, Messrs. Cookson 
and White, London. Solutions of aurantia, turmeric, 
chromic acid, tropoeolin, etc., have been used and may 
be tried. For blue screens solutions of ammonia- 
sulphate of copper may be used; but again, for all 
practical purposes, glass will be found equally good ; a 
few samples of cobalt blue and “ signal-green ” glasses 
of various depths may be obtained. 
The more actinic our light is the more pronounced 
must be our screens. Thus, if we use an oil lamp, we 
do not require so strong a yellow screen as if we were 
using lime-light; sunlight would require very strong 
screens, whether blue, green, or yellow. 
It is impossible to formulate instructions likely to be 
useful for all circumstances that meet the photo-micro- 
grapher ; the plan is to master the principles governing 
the sensitiveness of plates, to learn to appreciate the 
colours of objects, and then, by experiment and practice, 
to arrive at the proper methods of procedure for the 
various cases that present themselves. Properly-cut 
and stained preparations will be found to present no 
real difficulties; for this reason we shall devote attention 
to the subjects of cutting and staining in a later chapter. 
