•220 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



■distorted image of a plane object beino; obtained ; and in order that a 

 proper and definite distance may be maintained dnring nse, the instru- 

 ment is provided with an eye-cap, which should be pressed close to the 

 margin of the orbit, as shown in the illustration (fig. '28). 



(3) Illuminating- and other Apparatus. 



Spectroscope for the " Allan Dick " Petrological Microscope.* 

 This modification of spectroscope (fig. 2\)) will be found useful when 

 examining minerals of the monazite class giving faint absorption bands ; 

 such bands can be seen best with a prism of moderate dispersion. 

 The apparatus consists of a prism mounted to fit over the ocular. 

 Fig. 30 is a brass plate with an adjustable slit, and working above this 



Fig. 29. 



Fig. 30. 



is a dovetailed plate with a V cut in tlie centre for restricting the length 

 of the slit. On the right-hand side of the slit is a perforation for locating 

 the object previous to pushing the slit into position. This piece of ap- 

 paratus slides into the opening cut into the ocular just above the 

 diaphragm. Made by Swift and Son. 



Draper's Improved Dichroiscope. t — The improvement (fig. 31) 

 •consists of a revohing st^tge carrying a small cup filled with wax. The 



Fig. 31. 



■crystal can be moved in the horizontal or vertical direction or brought 

 into focus by means of the sliding tube carrying the prism eye-piece. 

 The crystal can also be revolved on the horizontal and vertical axes 



♦ Swift and Son's Special Catalogue, 1906, fig. 19. t Tom. cit., p. 27, fig. 22. 



