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THE ROYAL SOCIETY OF CANADA 



the telescope U. The source of light in use at present is a mercury 

 lamp vS used with various Wratten filters F to give a monochromatic 

 beam. The source is at the focus of the lens L, so that a parallel 

 beam passes through the instrument. The mirrors, the tubes, and 

 the compensating plates are all mounted on soft bronze stands D, 

 which can be moved between brass guides BB. There are two 

 positions for the mirrors so that tubes of length 12" or 33" may be 



O 



» 



I 



used. Fig. 2 shows in detail the stand which holds the mirrors. 

 The mirror D is held in the bronze part 5 by a brass ring C. This part 

 is held to the upright A by three screws RE, which with steel springs 

 FF give a very fine adjustment of the mirror. The screw G is used 

 to clamp the stand to the iron bed of the instrument. 



For use with metallic vapours one tube is enclosed in an electric 

 furnace as shown in Fig. 3. The tube B is made of fused quartz 

 with quartz plates AA sealed in to it. The furnace of nichrome wire, 

 insulated with asbestos, is built up around the tube, leaving the side 

 piece projecting. The temperature is measured by thermocouples 

 DD, and the windings are adjusted so that when a steady state is 

 reached the ends are about 900°C. and the centre about 850°C. 



Such a quantity of metal is used that it is entirely vapourized 

 about 800°C., so that the density is known. Following the practice of 

 Cuthbertson and Metcalfe^ part of the tube, the small projection, is 



^Phil. Trans. Ser. A. 207, Feb., 1907; Proc. Roy. Soc, Ser. A79, May, 1907; 

 Proc. Roy. Soc, Ser. A80, May, 1908. 



