MICROMETRY — MTCROMETRIC MICROSCOPES 
193 
From the measurement of the diameter the Brinell number 
is computed thus: 
Let D be the diameter of the steel ball, d the average diameter 
of the indentation as measured. The spherical surface of the 
indentation will be: tt Dh, 
D- VD--d^ 
where 
2 
and 
3000 
B.H.= 
TT 10. 
Or 
10 —V100 — 
Determinations of Grain Size. — In the metallurgical industries 
statements as to the “ grain size ” of the crystals forming our 
commercial alloys are entering more and more into contract 
specifications, and it is quite universally recognized that the 
importance of grain size cannot be overestimated as a check 
upon the heat treatment and also upon the nature of the mechan¬ 
ical treatment the alloy has subsequently received, particularly 
in the matter of commercial brasses. 
In America the Jeffries method ^ has been recommended by 
the American Society for Testing Materials. 
The image of the polished and etched preparation is projected 
by a microscope upon a plate of finely ground glass which has 
a circle 79.8 mm. in diameter drawn upon it. Such a circle 
has an area of 5000 sq, mm. The number of grains wholly 
within this circle is first counted and recorded; the number 
of grains through which the circumference of the circle passes 
is next determined, this number is divided by two and added 
to the number of whole grains. The sum is taken as represent¬ 
ing the number of crystal grains within the circle. 
In practice the ground glass is turned polished side up and the 
1 Jeffries: Determination of Grain Size in Metals; Trans. Amer. Inst. Min. 
Eng. Feb. 1916. Grain Size Measurements, Met. Chem. Engr. 18 , 185. 
