( 547 ) 



well-known dispersion curves of Langley and of Rubens show 

 considerable differences, and although at first sight Langley's deter- 

 minations seem to be much preferable, yet on closer examination 

 their excellence must be doubted, especially for the longer wave- 

 lengths. To prefer one of the dispersion curves to the other seems 

 to be at present a matter of arbitrary choice. So I have given 

 in the tables besides the observed refractive indices, the wave- 

 lengths, calculated from them as well by Langley's as by Rubens' 

 formula. The refractive indices hold good for a temperature of 20°; 

 their determination is based on the index 1.54429 for the ZMine, 

 a value, derived from very accurate determinations by Langley. 



The tables given below contain the lines of Na, K, Rb and Cs 

 (I have been unable to obtain reliable results with Li in the arc) 

 and of Hg. The results were derived from a large number of . spec- 

 trograms (10 to 12 for each metal). For the investigation of the 

 mercury spectrum a mercury arc-lamp was devised, furnished with 

 a rock-salt window. The spectrum of mercury has been repeatedly 

 investigated as far as lG\u ; no measurable emission has been found 

 beyond J.7jg 



In the tables the first column gives the refractive index n of 

 rock-salt, the second and third the wave-length ft of the line, 

 according to the formulae of Langley and Rubens, and the fourth 

 the approximate value I of the intensity. 



For the lines of which the exact position was difficult to ascer- 

 tain, the refractive index is only given in four decimals. 



SODIUM. POTASSIUM. 



37* 



