MICROMETRY MICROMETRIC MICROSCOPES 147 



A simpler type of traversing micrometer microscope is shown 

 in Fig. 91. 



Micrometer microscopes of the type shown in Fig. 92 are es- 

 pecially adapted to the measurement of small areas (diameters of 

 circles, etc.), as, for example, in the determination of the hardness 

 of metals by the Brinell method of pressing a hardened steel ball, 

 by suitable appliances, upon the object whose hardness is sought. 

 The area or the depth of the indentation resulting, per kilogram 

 of force applied, is used as a comparative measure of hardness. 1 



In the instrument illustrated in Fig. 92 2 the movement of the 

 microscope is forward and back. Focusing is accomplished by 

 the pinion F. The coarse adjustment of the cross-hairs is made 

 by means of A and the fine adjustment upon the object by means 

 of a. An inclined mirror M illuminates the micrometer scale 

 which is read through the lens R. 



Method 2. Measurements obtained by means of a stage microm- 

 eter and camera lucida. Lay the object upon the stage under the 

 microscope, over the ocular of which some form of drawing camera 

 has been placed. Adjust the illumination even more carefully 

 than in ordinary drawing, using axial light. Focus sharply, and 

 carefully sketch the outline of the object upon drawing board 

 or notebook, using a very hard and sharp-pointed pencil. The 

 object is now removed and replaced by a stage micrometer, 

 the instrument focused and the graduations of the scale traced 

 upon the paper, either across the outline of the object or near 

 by. The distance from the camera to the paper must be identical 

 in each case. The dimensions of the object may thus be ascer- 

 tained easily by comparison. 



This method of indicating the size of different objects in draw- 

 ings of microscopical subjects by means of tracings of a stage 

 micrometer is always preferable to a tabulation of numerical 

 dimensions, since the indication is a graphic one and appeals to 

 the eye at once. Moreover, it enables another investigator to 

 ascertain any dimensions indicated in the drawings. 



1 For a critical discussion of micrometric methods as applied to hardness deter- 

 minations see Devries, Comparison of Five Methods used to Measure Hardness. 

 U. S. Bur. Standards, Tech. Paper n, July, 1912. 



2 Made by C. Reichert, Vienna, Austria. 



