﻿Spectra of Helium and Hydrogen. 295 



that it might possibly be a double line. A faint fourth line 

 of the right order of intensity in the blue region of the 

 spectrum was also measured, and its wave-length (4542*2) is 

 found to agree within the limits of experimental error with 

 the calculated value (4541*80) for the fourth member of the 

 series. 



When the above exposures were concluded, the helium was 

 pumped out of the spectrum-tube and replaced by oxygen. 

 A condenser discharge was then sent through the tube for 

 4 hours, and a photograph taken. The lines on this plate 

 between 6800 and 6157 were measured, and no evidence 

 was found of the presence of a line at 6560. A line at 

 6563*5, probably due to hydrogen, was however obtained. 

 The two lines at 6721 and 6641, and impurity lines due to 

 nitrogen and silicon, were also present. 



Experiments with the Ordinary Discharge. 



In these experiments, the ordinary discharge was passed 

 through an end-on tube containing helium at a pressure of 

 *25 mm. The same spectroscope was employed as in the 

 experiments described in the last section, but the slit was 

 widened in consequence of the comparatively small intensity 

 of the light. The exposure was varied from 9 to 12 hours 

 and a copper comparison spectrum was sometimes utilized 

 for the determination of wave-lenoths. The discharge was 

 also reversed in direction when half the time of exposure 

 had elapsed. A series of lines with wave-lengths approxi- 

 mately the same as those calculated from equation (9) 

 appeared in the capillary in addition to the 4686 line. The 

 wave-length measurements in this case were not as accurate 

 as before, because of the greater breadth of the lines. The 

 mean values of the wave-lengths ootained for the first four 

 lines are 6560*71, 5411*44, 4860*5, 4542*44. For one of the 

 photographs the measurements were extended still further 

 towards the violet, and lines which are possible members of 

 series (9) were obtained at 4339*97 and 4199-9. All the 

 lines on this photograph (photograph IV.) between 6678 

 and 3819 were measured, and with the exception of the 

 above lines and two others at 4358*7 and 4226*9, they were 

 all due to helium and oxygen. In all the photographs taken 

 with the ordinary discharge the third line at 4860*5 was 

 stronger than it should be if it was the third member of 

 series (9). 



The accompanying table gives the wave-lengths of the 

 lines which have been determined from photographs taken 



