OF MICROSCOPIC OBJECTS. Ill 



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 



