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 }—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 
^ 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 ii, July, 1912. 
