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 1 0*0 inch 

 or two 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. 



Fig. 105. B. C. Ames Co. Dial Gauge. 

 A convenient gauge for measuring 

 the thickness of slides and cover- 

 glasses. 



