﻿296 



Spectra of Helium and Hydrogen. 



with the condenser discharge and also the ordinary discharge, 

 together with the values calculated from Bohr's theory. 

 The values given for the first four lines were determined from 

 photographic plates taken with the condenser discharge, 

 whilst the last two values were deduced from photographs 

 taken with the ordinary discharge. The dispersion in the 

 particular region of the spectrum and the probable error are 

 also included in the table. 



Wave-length 

 calculated. 



Wave-length 

 observed. 



Limit of 

 error. 



Dispersion. 



6560-38 

 5411-74 

 4859-54 

 4541-80 

 4338-88 

 420003 



6560-43 



5411-67 



? 



4541-93 

 *4339-97 



4199-95 



0-5 

 0-2 



0-2 

 0-4 

 0-4 



75 AU per mm. 

 45 AU per mm. 



35 AU per mm. 



36 AU per mm. 

 30 AU per mm. 



* Two or three oxygen lines in the neighbourhood of this line may 

 the observed value. 



Summary. 



The experiments have shown : — 



(A) That it is possible to observe the first member (4686) 

 of the series 



z; = 109750 



which includes the lines given by formulae (3) and (4), and 

 also a line which agrees in wave-length with the first member 

 (6560*4) of the series 



L = 109750 r 



which includes the Pickering lines and the lines at 6560'4... 

 predicted by Bohr, in a helium tube giving no trace of the 

 hydrogen lines. 



(B) That the experimental values for the wave-lengths 

 of the Pickering lines agree with the theoretical values 

 calculated by Bohr within the limits of experimental error. 



These results strongly point to the conclusion that the 



