﻿Spectra of Helium and Hydrogen. 293 



lines due to oxygen, silicon, and sulphur made their appear- 

 ance in the spectrum. The photograph also shows the 



presence of the H a (6563), Ha (4861) lines of hydrogen 



and the mercury line at 5460. These lines stretch across 

 the capillary and have the same length as the ordinary 

 helium lines. The 5411 and the 4542 lines also pass through 

 the capillary spectrum, but their lengths are only about 

 one half that of the ordinary helium lines. The line at 5411 

 was also found to be more intense and sharper than the 4542 

 line. The sixth member of series (9) could not be detected 

 on the photographic plate. A thorough examination of the 

 plate near the Ha and H^ lines did not reveal the presence of 

 any faint lines at 6560*4 and 4859*5. Rough measurements, 

 however, showed that the widths of the H a and He lines were 

 approximately 8*1 and 5 A.U. respectively. Also the line at 

 5411 was found to be narrow compared with the hydrogen 

 lines. Since the even members of series (9) were so faint 

 compared with the hydrogen lines, it was decided to repeat 

 the experiments with a prism spectroscope of nearly double 

 the dispersion and tubes provided with thin platinum elec- 

 trodes. 



(B) Experiments tcitk spectrum tubes provided with 

 platinum electrodes. 



A condenser discharge was sent through an end-on tube 

 containing helium at a pressure of *4 mm., and three photo- 

 graphs were taken with the two-prism spectroscope of 

 greatest dispersion after exposures of 5, 12, and 26 hours 

 respectively. In the last case the direction of the discharge 

 was reversed at the end of 13 hours. On all the plates a 

 faint line appeared at 6560, which is the calculated position 

 for the first line of series (9). This line was shorter than 

 the ordinary helium lines, and the oxygen lines at 6455 and 

 6157, but longer than the silicon capillary lines at 6371 and 

 6347. 



The plates taken with exposures of 12 and 26 hours showed 

 in addition to the 6560 line an ill-defined capillary line at 

 6564. The nature of the line made accurate measurements 

 impossible, and the origin of the line is unknown unless it is 

 due to a trace of hydrogen. On the same plate there also 

 appeared a line at 5411, which was of the same general 

 character as the 6560 line but of less intensity. The third 

 member of the series at 4859*5 could not be detected on the 

 plates taken with exposures of 5 and 12 hours, and the only 

 lines present in this region were the oxvgen capillary lines 

 at 4872,4865, 4861, and 4857. The plate taken with an 



