236 WRIGHT: A NEW THERMAL MICROSCOPE 



plate^ combined with rotating nicols. This plate is useful for 

 determining positions of total extinction of a mineral even at 

 high temperatures where the strong illumination of the furnace 

 field tends to cover up the interference colors from the mineral 

 plate. 



The optic axial angle is measured directly on the graduated 

 circle C (fig. 2), the crystal plate being rotated until each of its 

 two optic axes coincides with the axis of the microscope (tested by 

 rotation of the nicols, as in the universal stage methods); the 

 angular distance between the two positions of coincidence is 

 read directly on the graduated circle. These positions are sharply 

 marked and the error of the measurement of the optic axial angle 

 on a favorable section is not over ±1° even at relatively high 

 temperatures. 



This thermal microscope has proved so satisfactory in practice 

 that an extended series of measurements on selected natural min- 

 erals of known composition has been commenced with a view to 

 determine not only the changes in their optical constants but 

 also their inversion and melting temperatures, these last temper- 

 atures to be checked by means of the accurate temperature meas- 

 uring methods now in use in the Geophysical Laboratory. At 

 the same time the refractive indices and in some instances changes 

 in crystal angles will be ascertained by the use of another fur- 

 nace attached to the two circle goniometer and to be described 

 in a later communication. 



6 Am. J. -Sci. (4) 26, 377, 1908; Carnegie Institution of Washington, Pub. 158, 

 139. 1911. 



