OF MICROSCOPIC OBJECTS. 147 
It is generally thought that Canada balsam injures the 
finer markings of these sections, consequently, they are 
almost invariably mounted dry. A thin piece is first cut 
from the tooth with the saw of watch-spring before 
mentioned, if possible; but should the substance be too 
hard for this, the wheel and lathe must be used with 
diamond dust. If this cannot be procured, there is no 
alternative but to rub down the whole substance as thin as 
practicable on some coarse stone or file, or best of all the 
corundum rubber. The surface will then be rough; but 
this may be much reduced by rubbing upon a flat sharp- 
ening stone with the finger, or a small piece of gutta-percha 
upon the object to keep it in contact. The scratches may 
‘be much lessened by this, but not so thoroughly removed as 
microscopic examination requires in dry sections. It must, 
therefore, be polished with the putty-powder and dry strop, 
as recommended in the working of shell-sections. The 
other side of the section of the tooth may then be rubbed 
down to the vequisite thinness, and polished in the same 
manner, when the dust and other impurities must be re- 
moved by washing, after which the section must be carefully 
dried and mounted. Sometimes it may be deemed desirable 
to make a preparation of the teeth in situ; for this purpose © 
take the lower jaw of some animal like the rat, weasel, or 
guinea-pig, and soak it in absolute alcohol first, let that evapo- 
rate out, then soak in the solution of balsam and benzole ; 
when that has evaporated to hardness, grind down the jaw as 
a section, the teeth are fixed in by the balsam. Some of 
these sections are equally interesting as opaque or trans- 
parent objects. 
The dentine of the teeth may be decalcified by immersion 
of the section in dilute muriatic acid; after drying and mount- 
ing in Canada balsam it presents a new and interesting 
appearance, showing the enamel fibres very beautifully 
when magnified about three hundred diameters. A friend 
tells me that after submersion of the whole tooth in the acid 
he has been able to cut sections with a razor. 
i 2 
