﻿292 Mr. E. J. Evans on the 



Experimental Results. 

 The Series given by Formula: (3) and (4). 



The preliminary experiments * shoved that it was possible 

 to obtain photographs showing the 4:686 line very strongly, 

 and no trace of the ordinary hydrogen lines of the Balmer 

 series. As the photographs were taken with a quartz spectro- 

 graph o£ small dispersion, the wave-length could only be 

 determined with an accuracy of about 1 A.U. The line was 

 therefore photographed with the prism spectroscope givino- 

 the greatest dispersion, and the wave-length determined with 

 a greater degree of accuracy. The mean wave-leno-th as 

 determined from these photographs was 4686*00 (±*05), 

 which agreed well with the value 4685*98 (±*01) determined 

 by Fowler from a photograph taken with a 10-foot concave 

 grating. 



It was also possible to obtain this line, employing the same 

 spectroscope, by passing the ordinary discharge without 

 condenser and spark-gap through an end-on tube containino- 

 helium at a pressure of *25 mm. The line is shown in photo^ 

 graph I. (PI. VI.), where it appears fairly strong and broad 

 in the capillary. The wave-length of the" line was measured 

 and found to be 4685*94. The photograph does not, show 

 the presence of the hydrogen lines at 6563,4861, .... but 

 only the ordinary helium lines and the 4686 line. 



These experiments show that it is possible to obtain the 

 first member of series (3) from a helium tube givino- no trace 

 of the hydrogen lines of the Balmer series. 



The Series given by Formula (9). 



(A) Experiments with titles containing aluminium elec- 

 trodes. 



The second member of this series was observed visually 

 for the first time in the spectrum of an end-on tube with a 

 narrow capillary. Two members of the series are shown in 

 photograph II. , which was taken with the two-prism spectro- 

 scope of low dispersion. The wave-lengths of these lines 

 were determined with an accuracy of 0*2 A.U., and were 

 found to agree within the limits of experimental error with 

 the values calculated for the second and fourth members of 

 series (9). The two values obtained were 5411*63 and 

 4541*93 respectively. The wave-lengths of the lines as de- 

 termined by Fowler were 5410'5(±1*0) and 4541'3( = t0*25). 

 In addition to the ordinary helium lines, shorter or capillary 

 * Evans, luc. at. 



