816 Messrs. H. Fricke and T. Lyman on the Spectrum 



show varying relative intensities : none of them occur on all 

 our plates. Their behaviour indicates that they are due to 

 impurities. This conclusion is confirmed by the fact that 

 we have found lines at 686 A.U., 860, and 1176 when we 

 employed a high-potential vacuum spark between carbon 

 terminals with our small spectroscope. Millikan * has 

 obtained radiation from the same source at nearly identical 

 wave-lengths, namely 687*3, 858*6, and 1175*6, using a 

 grating of greater dispersion than ours. The line 992 has 

 already been attributed to an unknown impurity f ; 972 

 and 1026 are due to hydrogen f . 



Hicks J lias suggested that as the lines 972, 992, 1026, and 

 1086 fit the formulae for the enhanced spectrum of helium, 

 they must belong to that gas. The fact that the line 1086 

 is not fouud on our plates is sufficient in itself to disprove 

 this idea. 



When we undertook to confirm our observations by means 

 of the large spectroscope, it was only after several trials and 

 with a very long exposure that we were able to obtain a 

 faint but unmistakable record of A. 585. It was accompanied 

 by fairly strong lines at 12 1 6, 1200, 1086, and 1085, all of 

 which have been observed before with the same instrument 

 and are known to belong to hvdrogen or to some other im- 

 purity. In the visible spectrum the discharge showed traces 

 of hydrogen and carbon in about the same intensities as when 

 the small spectroscope was used. In addition, nitrogen 

 bands appeared very faintly at the end of the exposure. 

 This impurity, not present with the smaller grating, may 

 account for the appearance of the lines at 1200, 1086, and 

 1085. The region covered by the photographic plate extended 

 from 500 to 1300 A.U. 



Owing to its faintness and to the fact that it was separated 

 from lines of reference by a considerable distance, an exact 

 measurement of the wave-length of this resonance line is 

 impossible at present; however, the value of 585 + 2 may be 

 regarded with confidence. 



The feeble character of A, 585 may be attributed to the long 

 gas path of the large spectroscope whose grating has a 

 radius of curvature of 97 cm. as against the radius of 20 cm. 

 of our small instrument, if we are willing to admit a certain 

 amount of selective absorption by helium in the neighbourhood 

 of the line in question. This hypothesis of selective absorp- 

 tion is made necessary by the fact that a disruptive discharge 



* R. A. Millikan, Astropliysical Journal, lii. p. 47 (1920). 



t T. Lyinan, Astropliysical Journal, xliii. No. 2, p. 89 (1916). 



+ Hicks, Nature, civ. p. 393 (1919). 



