WRIGHT: ELECTRICAL GONIOMETER FURNACE 401 



minerals with temperature rise has been commenced at the Geo- 

 physical Laboratory — the birefringence, extinction angles, and op- 

 tic axial angles being measured on the new thermal microscope 

 while the refractive indices and crystal angles are measured on 

 the thermoelectric goniometer furnace. In each case only min- 

 erals of definitely known composition are to be taken and the 

 temperatures of melting and of inversion controlled by the heating 

 and quenching methods now in use in this laboratory. Accu- 

 rately oriented crystal sections are to be ground on a new crystal 

 grinding goniometer which is now practically finished in the work- 

 shop of the laboratory. With this instrument, in which the 

 device for autocollimation described recently in this Journal* 

 has been adopted, it will be possible to orient and to grind crystal 

 plates with reference either to their crystallographical directions 

 or to their optical directions. 



So far as can be judged from preliminary measurements, the 

 results obtained with the electrical goniometer furnace on fav- 

 orable material, are accurate to about 5° in temperature, about 

 1' in crystal angle readings and 1 or 2 in the fourth decimal 

 place in refractive index measurements. 



The three instruments — new thermal microscope, goniometer 

 furnace attachment, and crystal grinding goniometer — render pos- 

 sible the quantitative study of crystallography and of mineral 

 optics at high temperatures. Such study should add materially 

 to an understanding of the crystallographic forces and of their 

 relation to other physical forces, such as internal friction and 

 surface tension. 



4 This Journal, 3 : 235. 1913. 



