234 WRIGHT: A NEW THERMAL MICROSCOPE 



to collect between the two glass windows B and thus to disturb 

 the field of view. To the cylinder part of the jacket a support 

 G is attached with conical center bearing for the crystal holder. 

 The crystal plate whose optical constants are to be determined 

 is held in position by platinum jaws (L fig. 2) which fit into and 

 project beyond the porcelain tube A^, which in turn is held in 

 position by the rotating holder H. The thermoelement wires D 

 are passed thru the larger tube A^ in small porcelain tubes T and 

 extend beyond these so that the thermoelement junction is di- 

 rectly beneath or above and in contact with the mineral plate, 

 thus insuring good temperature readings. The temperatures are 

 read either on a direct reading millivoltmeter (Siemens and Hal- 

 ske type) or in more accurate work by means of a mirror galvano- 

 meter and potentiometer system as commonly used in this lab- 

 oratory. ^ The holder H is equipped with a circle C, graduated 

 to degrees, and with a clamp screw K for keeping the tube N in 

 place. This part of the apparatus is complete. in itself and can 

 be withdrawn from the furnace at any temperature without dis- 

 turbing the other parts. To insure proper adjustment of the 

 crystal plate a second holder (fig. lb) is clamped to the stage of 

 a second microscope and the plate so mounted that the plane of 

 its optic axes is normal to the axis of rotation of the holder. 



Fig. 3 



For this adjustment a device for autocollimation has been 

 used which has proved serviceable and promises to be of value 

 in other autocollimating instruments. A thin cover slip of the 

 shape indicated in figure 3a with one edge vertical and the oppo- 

 site edge inclined 45° to the vertical, is placed in the rear focal 

 plane of the low power objective. Light from a Nernst filament, 

 sent thru the plate to the inchned edge, is totally reflected 



' Carnegie Institution of Washington, Pub. 31, 1905; 157, GS. 1911. 



