OF MICROSCOPIC OBJECTS. Til 
that the glass cover must be applied lightly, otherwise the 
grains may be crushed. There are some objects which 
cannot be shown in a perfect manner when mounted dry, 
but when immersed in balsam become so very transparent 
that they are almost useless. ‘To avoid this, it has been re-. 
commended to stain the objects any colour that may be con- 
venient, and afterwards mount in balsam in the ordinary 
manner. 
Permanent dyes, however, for these minute objects are 
not so readily procurable. My friend Mr. Abbey showed 
me that what was permanent with vegetable matter of one 
kind was totally untrustworthy with another. The most 
useful that I have tried is Magenta, and the colour is a 
convenient one. Whatever is used for this purpose should 
be in solution, and the object steeped for awhile and after- 
wards thoroughly washed, in order that no superfluous salt 
may remain. There are many liquids now sold by every 
chemist which will help the student in this respect. 
Most objects intended for the polariscope may be mounted 
in Canada balsam ; but there are some exceptions to this. 
Many of the salts are soluble in this medium, or their forms 
so injured by it, that glycerine or oil has to be used (see 
Chapter V.): others must be left in the dry form, as before 
mentioned; and some few it is impossible to preserve un- 
changed for any length of time. Crystals, however, are 
amongst the most beautiful and interesting subjects for 
polarization; and it is very probable that, by the aid of the 
polariscope, new and valuable facts are yet to be made 
known. For one who finds pleasure in form and colour, 
there is a field here which will only open wider upon him as 
he advances; and instead of being in anywise a merely 
mechanical occupation, it requires deep and careful study. 
The little here said on the subject will show this in some 
degree. 
With almost every salt the method of crystallization must 
be modified to obtain the best forms; I may even go further 
than this, and say that it is possible to change these forms 
