168 
ELEMENTARY CHEMICAL MICROSCOPY 
Cover-glass and Slide Gauge. — In dark-field illumination 
it is necessary to employ the proper slide thickness for which 
the reflecting condenser has been designed. So too when using 
high-power dry objectives, especially those with correction col¬ 
lars, it is necessary that we ascertain the thickness of the cover- 
glass and correct for this thickness either by means of the cor¬ 
rection collar of the objective or by lengthening or shortening 
the draw tube as the case may require. The most satisfactory 
gauge with which the author is familiar is shown in Fig. 105.1 
Pressure upon the handle H 
opens the jaws, J, the slide or 
cover-glass is inserted between 
the jaws, the pressure released 
and the thickness read upon 
the dial. A small multiplying 
dial, as shown, indicates the 
number of complete revolutions 
of the indicator of the large 
dial. These instruments are 
very accurate and may be ob¬ 
tained graduated in ro^o 0 inch 
or rho millimeter. 
Microtomes. — Although it is rare that the chemist is called 
upon to prepare serial sections of great precision, the necessity 
frequently arises of cutting slices, of many varieties of materials, 
of sufficient thinness to permit of their study by transmitted 
light. Many of these materials are so tough and hard that 
precision microtomes are impracticable. A sturdy microtome of 
simple construction (Fig. 106) which can be clamped firmly 
to the table top answers admirably. The jaws for holding the 
specimens in instruments of this type will accommodate as large 
pieces as it is feasible to cut. Cutting may be practiced with any 
sort of keen edged knife ground flat on one side and with chisel 
edge on the other, or with a razor having an extra large blade 
(section razors sold under the name of “ botany ” section razors). 
These razors must have one side of the blade flat and true and 
1 “ Pocket Dial Gauge ” made by the B. C. Ames Co., Waltham, Mass. 
J 
Fig. 105. B. C. Ames Co. Dial Gauge. 
A convenient gauge for measuring 
the thickness of slides and cover- 
glasses. 
