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 requisite 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 ir> 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. 



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