THE PHYSICAL PROPERTIES OF SOILS 99 



References 



Stevenson & Schaub, "Soil Physics Laboratory 

 Guide." 



Bulletin No. i6, Bureau of Soils, pp. 13-15. 



Bulletin No. 24, Bureau of Soils, p. 9. 



STANDARDIZATION OF THE EYE-PIECE 



MICROMETER 



The microscope used in the mechanical analysis of 

 soils must be provided with an eye-piece micrometer 

 for measuring the diameter of the soil grains com- 

 posing the different separates. The value of the di- 

 visions on this eye-piece micrometer will vary with the 

 different magnifying powers of the microscope. It 

 becomes necessary, therefore, to determine the value 

 of the divisions in the eye-piece by comparison with 

 a standard of known value placed on the stage of the 

 microscope. This is done by comparing the eye-piece 

 with a stage micrometer of known value, and com- 

 puting the value of one division on the eye-piece mi- 

 crometer for each optical combination. When this is 

 known, the number of spaces which the soil particles 

 must cover to belong to a given grade is determined 

 by dividing the diameter of the particles by the value 

 of one space. 



Each student is required to make a standardization 

 and compute the value of the divisions of the eye-piece 

 for all of the combinations of the microscope used in 

 the laboratory. The results must be tabulated and 

 kept in this book for reference. 



