MICROMETRY MICROMETRIC 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: IT Dh, 



where 

 and 



io v TOO d 2 

 TT io. - 



Or B.H.- 



10 



-Vioo-d 2 ' 



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 x 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. 



