﻿294 Mr. E. J. Evans on the 



exposure of 26 hours was also examined in the same region,, 

 and the wave-lengths o£ two very faint lines, which extended 

 beyond the capillary, were measured. One of these lines, 

 which had a wave-length of 4861*7 A.U., was longer than 

 the 6560 line. The other line was fainter than the 4861 

 line, and its measured wave-length was 4859*5. The fourth 

 member of the series which should appear at 4542 could not 

 be detected on any of the plates. All the lines between 

 7065 and 4713 on the plate taken with an exposure of 12 

 hours were measured, and with the exception of capillary 

 lines at 6721*5, 6641*1, 5739*3,4829*3, 4813*7, all the others 

 could be attributed to helium, oxygen, silicon, sodium, and 

 chlorine. These experiments show that when a condenser 

 discharge was passed through helium, two lines were obtained 

 whose wave-lengths agreed within the limits of experimental 

 error with the values calculated for the first and second 

 members of series (9), and whose intensities fell off with 

 diminishing wave-length. Also a very faint line was obtained 

 whose wave-length was very approximately the same as that 

 calculated for the third member of the series represented by 

 equation (9). However, in view of the faintness of the line 

 and the appearance of another stronger line of unknown 

 origin close to it, the question whether the line is the third 

 member of the series cannot be regarded as settled. 



The experiments described above show that very long ex- 

 posures are required to bring out the lines of the series given 

 by equation (9), for even with exposures of 26 hours only 

 two lines could be detected with certainty. It was therefore 

 considered desirable to obtain photographs with the other 

 prism spectroscope, which gave a lower dispersion. Two 

 photographs were taken, one with an exposure of 21 hours 

 (photograph III.), and the other with an exposure of 31 

 hours. In the latter case, the discharge was reversed in 

 direction at the end of half the exposure. The wave-lengths 

 of the first two lines were measured for each plate, and found 

 to be 6560*69, 5411*95, and 6560*10, 5411*71 respectively. 

 A third line at 4860*5 was also obtained, but its wave-length 

 was 1 A.U. greater than the value calculated theoretically 

 for the third member of the series. Also, the line had too 

 great an intensity if it was to be regarded as the third 

 member of the series in which 6560 and 5411 were the first 

 and second members respectively. It is interesting to 

 point out that the wave-length (4860*5) is approximately 

 the mean of the two values 4861*7 and 4859*5 obtained for 

 the two faint lines photographed with the spectroscope of 

 greater dispersion. The appearance of the line also suggested 



