Section III, 1922 • [145] Trans. R.S.C. 



On the Prism Method of Determining the Refractive Indices of Metallic 



Vapours 



By H. G. Smith, M.A. 



(Read May Meeting, 1922) 



An account has been given by Professor J. C. McLennan' of a 

 new method of determining the refractive indices of metallic vapours 

 by means of the deflection of a monochromatic beam of light, caused 

 by a prism of the vapour enclosed between two quartz plates in a 

 fused quartz tube. 



Fig. 1 



A cross-section of one of these tubes is shown in Fig. 1. The 

 oval plates were sealed into the tube at an angle as shown. A small 

 quantity of the metal was placed in the space between them, and 

 this space exhausted. An electric furnace was then built up around 

 the entire tube. As made for the later experiments, the furnace gave 

 a uniform and steady temperature up to 900°C., the variation being 

 seldom more than 10° within an hour. 



In the former account were also given some results obtained 

 with Mercury vapour at about 300°C., and with Thallium vapour 

 at about 450°C. These results for Mercury agreed moderately well 

 with the results given by Cuthbertson and Metcalfe.^ who used a 

 Jamin refractometer. No attempt was made to determine the 

 absolute refractivity of Thallium vapour, but relative values for 

 light of different wave-lengths were obtained and a dispersion curve 

 drawn. 



It was recognized at this time that the quartz plates were slightly 

 distorted, deflecting the light even when cold, and also altering the 

 focus by a small amount. But, on account of the small temperature 

 coefficient of quartz it was thought that the difference between cold 



iProc. Roy. Soc, Ser. A. 100, p. 191, 1921. 

 2Phil. Trans., Ser. A. 207, 1907. 



10— c 



