﻿288 Mr. E. J. Evans on the 



of helium. Stark also came to the same conclusion with 

 respect to the line 4686. 



In the above mentioned discussion in ' Nature ' Bohr 

 showed that if his view was correct, the helium spectrum 

 should contain another series of lines, which are represented 

 by the formula 



'- k '{^-Gp} <*> 



where K' is the constant outside the bracket in formula (7). 

 This formula is obtained from (7) by putting n 1 = 4< and 



w 2 = 6, 8, 10 The lines of this series should appear 



near the hydrogen lines of the Balmer series, and their 

 wave-lengths were calculated to be 6560*4, 4859*5, 4338*9, 



4100" 2 These lines and the Pickering lines can be 



represented by one formula 



^~(jH (9> 



and the intensity of all the lines should decrease regularly 

 with increasing values of n. The lines would be difficult 

 to detect, especially if hydrogen was present in any quantity, 

 as it is known from Fowler's experiments that the Pickering 

 lines are faint even when photographed with instruments of 

 small dispersion. The presence of the lines given by formula 

 (8) would greatly strengthen the experimental evidence in 

 favour of Bohr's theory, but their absence would immediately 

 show that the theory was incorrect. The investigation of 

 the lines in question, and the determination of more accurate 

 values for the wave-lengths of the lines of the Pickering 

 series, formed the main object of the present research. 



The Experimental Arrangement. 



The method which was chiefly employed for the production 

 of the spectrum lines was very similar to that previously 

 described by Fowler. A 20-inch coil with a condenser across 

 its terminals was connected to a helium spectrum-tube in 

 series with which was placed an adjustable spark-gap. The 

 spectrum produced was examined visually with a Hilger 

 direct-reading instrument, and the wave-lengths could be 

 read off with an accuracy of 1 or 2 A.U. For the purpose of 

 photographing the lines, four different instruments were 

 employed. In the first series of experiments the lines were 

 photographed by means of a small quartz spectrograph giving 



