800 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



surface of P is attached a small prism P'. This allows the rays from 

 the crystal on tho stage to pass up to the eye which views them sur- 

 rounded hy the rays from the quartz, and a ready comparison is thus 

 made. 



By turning the milled head the quartz wedge can be moved in a 

 horizontal direction, and a graduated scale enables the displacement 

 to be measured in order to identify the tints. 



Fig. 173. 



The light for the small mirror should be taken from the same 

 source as that for the mirror of the Microscope, so as to avoid any 

 error in the value of the tints. The apparatus is applied with best 

 advantage to very small thin plates of minerals, not going beyond the 

 tints of the first three Newtonian orders. 



Israel's Warming 1 Apparatus as a substitute for the Hot Stage.* 

 — Dr. 0. Israel's apparatus is so constructed that the illumination of 



Fig. 174. 



the objects is not interfered with, and the slide is warmed from above. 

 It consists of a slide O (fig. 174), having a central hollow-ground well, 



* Zeitsch. f. Wis?. Mikr., ii. (1885) pp. 459-G3 (3 figs.). 



