412 T. A. Jaggar, Jr. — Microsclerometer, etc. 



the thickness of mineral thin-sections ; by making contact 

 with the upper surface of a mineral and then with the object 

 glass level at its side we may measure thickness, for the 

 Chaulnes method of determining the index of refraction. Tests 

 with hypersthene suggested further that by boring through 

 a mineral of high double refraction to the glass beneath we 

 may rapidly get a value for the amount of double refraction or 

 y—a. The conical depression shows on its border the color 

 rings marking various thicknesses ; if we bore to red of the 

 first order, make a depth reading, and then bore through to the 

 glass and read again, the difference in depth gives the thick- 

 ness of the mineral for red of the first order. If we do this 

 on a section cut in the plane of the optic axes, so that the axis 

 of mean elasticity, 6, lies in the plane of polarization of the 

 microscope, we have a thickness value which will give directly, 

 by reference to the calculated tables (v. Rosenbusch) the cate- 

 gory to which the mineral belongs. Lastly, the use of this 

 instrument in treating by actual contact the individual minerals 

 of a rock section, suggests the possibility of an adaptation by 

 which perhaps chemical tests may be applied directly to the 

 dust in the boring. 



