MICROMETRY MICROMETRIC MICROSCOPES 191 



When employing the fine adjustment for micrometric meas- 

 urements, always make all movements in focusing in the same 

 direction, otherwise a serious error will be introduced due to 

 back-lash. 



If a piece of an object slide is used for calibrating the fine 

 adjustment, it must be remembered that we cannot focus first 

 upon the lower surface through the slide, then upon the upper 

 surface, to obtain its thickness, owing to the displacement of 

 image due to the higher refractive index of the glass than that 

 of air. This phenomenon enables us, however, to determine the 

 thickness of transparent objects when their refractive indices 

 are known by proceeding as described on page 243. 



Micrometric measurements by means of the fine adjustment 

 are often called for in chemical work, as, for example, to ascer- 

 tain the depth of corrosion, weathering, pits, streaks, etc., in 

 the surfaces of many different sorts of materials, or in approxi- 

 mating depths of penetration, or in measuring in transparent 

 bodies the displacement of images due to changes in refractive 

 index. This displacement enables one to calculate the refractive 

 index of the object. 



Measurement of Areas. The methods employed for the 

 determination of the areas occupied by microscopical objects is 

 discussed in Chapter VIII, page 212. 



Special Micrometric Applications. A few of the many 

 commercial applications of micrometric measurements have been 

 selected as illustrations of the way in which microscopic measure- 

 ments are being utilized in the industries. 



Brinell Hardness Number. 1 In this method for determining 

 the hardness of metals and their alloys a hardened steel ball 

 10 millimeters in diameter is pressed upon a smooth surface 

 of the sample under a standard load. For hard materials the 



1 For a new microscopic method for the determination of hardness, with partic- 

 ular reference to the hardness of individual grains see Progress Report of Research 

 Sub-Committee on Bearing Metals, read at annual meeting Amer. Soc. Mech. 

 Eng., Dec. 1920. 



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. 



