WRIGHT: A NEW THERMAL MICROSCOPE 235 



and passes thence through the objective to the mineral plate, 

 where it is again reflected and appears, after its return through 

 the objective, as a sharp bright line in the dark field. When the 

 mineral plate is normal to the axis of the lens system, the bright 

 line is practically covered, thus enabling the observer to make 

 very accurate adjustments.- 



This method has been improved by using two cover glasses as 

 indicated by figure 3b, the space between the glasses serving to 

 mark the center. The complete cross has also been obtained 

 by means of cover glasses ground as illustrated in figure 3c. The 

 grinding and mounting of four such cover glasses is, however, 

 difficult and requires considerable skill to be accomplished satis- 

 factorily.^ 



After the crystal plate has been mounted and adjusted and 

 the thermoelement wires correctly placed, the holder H is in- 

 serted into its support G on the water jacket (fig. 2) and the min- 

 eral plate thus introduced into the furnace in a position ready 

 for the measurement both of its optical constants and of its tem- 

 perature. 



The birefringence of a mineral plate is measured on this micro- 

 scope by the method used on the thermal microscope described 

 several years ago.^ The thickness of the plate is determined be- 

 fore insertion into the furnace. The change in thickness due to 

 expansion on heating is so slight that it is negligible for most 

 measurements of birefringence. The path-difference of the emer- 

 gent waves from the crystal plate in the furnace is measured by 

 means of a graduated compensating wedge. ^ 



Extinction angles are measured by means of a biquartz wedge 



^ This scheme for autocoUimation is somewhat similar to that suggested by 

 Nutting in this Journal 2, 404, 1912. In his arrangement a reflecting cover glass 

 is used on end with the result that only a small part of it is in focus at any setting 

 of the eyepiece while in the scheme outlined above the entire cover glass is in 

 sharp focus and the field is less disturbed. 



' These different types of cover glasses were satisfactorily ground for the 

 writer by the Bausch and Lomb Optical Company. 



' Am. J. Sci. (4) 27, 43. 1909. ' 



5 Am. J. Sci. (4) 29, 417. 1910; Carnegie Institution of Washington, Pub. 158, 

 102. 1911. 



