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POPULAR SCIENCE REVIEW. 
agreeable with objects of equal lustre and a greater proportion 
of orange and reddish tints. 
Looking at slides of these kinds can, however, scarcely be 
termed making experiments, unless their appearance is varied 
by employing tinted lights for their illumination. Monochro- 
matic lights of course are inadmissible ; but when a good beam 
of white light is mingled with a little of some coloured light, 
the effect is analogous to putting a transparent wash or glaze 
over a picture. 
It is, however, by the use of polarized light and appropriate 
crystals that colour studies are best made with the microscope. 
It is necessary to have, in addition to the usual polarizing 
and analyzing prisms, a set of selenite films of different 
thicknesses, to vary the tints. Darker’s films well mounted are 
the best, but another arrangement, devised by Mr. Ackland 
(Horne and Thornthwaite) will do exceedingly well. This 
consists in an ebony frame carrying a selenite plate, revolving 
easily by applying the finger to a toothed wheel. Above this 
can be placed one of three separately mounted films, marked 
respectively according to their wave retarding power. 
Each of these is readily rotated. The microscope should have 
a rotating stage, and the analyser mounted so as to rotate with 
facility. 
A series of crystals of double refracting substances should be 
prepared either with colloid silica or a little sugar, or by such 
manipulation as does not allow the crystals to form until the 
mother fluid is in a viscid state. The silica is used by dissolving 
a pure water-glass in distilled water, gently adding hydro- 
chloric acid to unite with the alkali, and allowing the resulting 
salt to dialize out of a parchment-paper drum, leaving 2 or 
3 per cent, of pure colloid silica in solution. Those who are 
accustomed to chemical operations will know how to do this ; 
others will find it best to get it done for them. Having 
obtained the silica solution, dissolve the salt to be crystallized 
in it, and prepare a slide in the usual way over a spirit-lamp, 
not being satisfied unless complicated patterns are obtained, and 
a good many tertiary tints obtainable with the polarising appa- 
ratus. These slides should be mounted with Canada Balsam.* 
Let the experimenter take for a beginning an ordinary shop 
slide of salicine which, when the polariser and analyser are 
crossed so as to give a dark field, exhibits the well known 
circles with strong black and coloured crosses. The effect is not 
altogether pleasant ; though it could not be called discordant, 
* Tlie sort of patterns to be obtained are shown in the plates to tbe 
author’s paper on Crystalline Forms modified by Colloid Silica } u Monthly 
Microscopical Journal,” March, 1871. 
