410 T. A. Jag gar, Jr. — Microsclerometer, for 



tricity of the diamond point to be -08 mm , with the filings most 

 abundantly heaped outside the periphery of the ring, and a 

 smaller ridge within. 



2. Gypsum. Cleavage. 



Weight 10 gm. 



Depth 10/* 



Rate 6 '5 rev. per sec. 



Hardness (revolutions) 8 # 3. 



Under such weight the diamond penetrated to the glass 

 under the first thin section used almost instantly : a cleavage 

 fragment was tried, and the above value is an average of bor- 

 ings to several different depths, it being found impossible to 

 stop the boring at exactly 10^ depth, so great was the oscilla- 

 tion of the scale m in the field of the microscope : this was 

 caused by the eccentricity of the diamond, the point penetrat- 

 ing too rapidly and meeting unequal resistances in different 

 cleavage directions. This difficulty was less noticeable in the 

 hard minerals. 



3. Calcite. Cleavage face. 



H 50. 



In both gypsum and calcite the groove formed was some- 

 what elliptical, instead of circular. This is due to diverse 

 cohesion values for different cleavage directions as worked out 

 by Exner. The initial variations in resistance force the dia- 

 mond out of its normally circular path. Three tests on the 

 same calcite face showed these elliptical grooves to be similarly 

 oriented with reference to the cleavage fissures, the longer axis 

 of the ellipse occupying an oblique position between the long 

 and short diagonals of the rhomb face. This coincides with 

 the determination by Franz of maximum and minimum hard- 

 ness in the azimuth of the short diagonal in opposite directions. 



4. Fluorite. Octahedral cleavage. 



H.... 143. 



The depth was checked by direct focal measurement in a 

 series of observations on fluorite, and was found to conform 

 within 0*5/* with the record given by the inclined micrometer 

 m. One groove had sub-elliptical form, with a slight flatten- 

 ing on one side. A series of observations to test the increment 

 of resistance with increased depth gave the following values : 



h in //. 



Revolutions. 



Rev. 



for each 5ju 



10 



148 





74 



15 



273 





125 



20 



459 





186 



25 



666 





207 



