]90 SELECTION AITO USE 



to receive the attention that it deserves. Where a thin, flexi- 

 ble blade is used, a moderate change in the amount of pressure 

 employed will make a great difference in the thickness of the 

 section, even so far as to double it. When the blade is stiff, a 

 change in the degree of pressure has but little effect. 



Soft substances must first be hardened either by immersion 

 in alcohol or other means, and in general must be supported by 

 being surrounded with melted wax or paraffin. Where the 

 specimen is very slender (such as a hair) it must be carefully 

 supported between firm and rigid clamps. Corks and similar 

 yielding substances, which are recommended in most books, 

 never give a cross section accurately taken at right angles. The 

 same is true of the plan so much recommended for obtaining 

 sections of hair, viz. : to pass the razor over the face shortly 

 after shaving. We get sections it is true, but they are all 

 oblique. The best way to get true sections is to imbed the sub- 

 stances in glue, gum, paraffin, wax or some such material. 



Sections of bone are prepared by sawing off a thin slice in the 

 first place, and cementing it to a slide by means of thick or old 

 balsam; one side is then filed or ground flat, and polished on 

 buff leather, after which the section is transferred to another 

 slide so as to expose the other side, which is then filed down 

 and polished as before. Great care must be taken so as to hit 

 just the right thickness, and the operation of cementing to the 

 slide must be performed expeditiously, so that the balsam may 

 not saturate the section, and render it too transparent, as when 

 this occurs certain very important features become invisible. 



Very hard substances require special apparatus, and consid- 

 erable skill. Still it is astonishing what may be accomplished 

 by means of good files, Avhetstones and grindstones in the way 

 of preparing thin and transparent sections even of such sub- 

 stances as rocks and stones. 



In order to acquire correct ideas in regard to the structure of 

 objects, of which sections are examined, the student should fa- 

 miliarize himself with the geometrical forms produced by cut- 

 ting cylinders, cones, spheroids, etc., in various directions. 

 Thus a cylindrical vessel, cut square across, shows a circle; when 

 cut obliquely it shows :vn oval (ellipse) of greater or less length, 

 and when cut longitudinally it shows two lines which have no 



