[mclennan-petrie] spectra OF HELIUM, ETC. 23 



Â.U., X = 1742.7 Â.U., X = 1649.8 Â.U., X = 1647.4 Â.U. and X = 1495.0 

 Â.U. Regarding the wave-lengths at X = 1134.7 A.U. and X= 1199.7 

 A.U. it is more difficult to decide since the Schumann spectrum of 

 mercury has hitherto been supposed to end at X = 1188 A.U. the limit 

 of the series »*= (1.5,S) — (m,P). If it should turn out that all the 

 above wave-lengths originated in the atoms of mercury it seems 

 strange that they were not obtained in the spectra of hydrogen, since 

 in obtaining the latter, the same discharge tube and the same cir- 

 culatory system was used as with the helium. On the whole it would 

 seem that the wave-lengths X = 1199.7 A.U. and X = 1134.7 A.U. 

 should be attributed to helium. 



The wave-length which Millikan found at X = 1402.9 A.U. in the 

 spark spectrum of carbon, and for which he suggested calcium or 

 silicon as the origin, corresponds approximately to the wave-length 

 X = 1401.6 À.U. found by us in the spectrum of helium. This wave- 

 length, too, may have had mercury as its origin for Lyman records a 

 wave-length of strong intensity at X= 1402.5 A.U. and Wolff also one 

 at ^ = 1402.72 A.U. in the arc spectrum of mercury. The same origin 

 can be attributed to the wave-lengths X= 1481 A.U., X = 1548.8 A.U. , 

 X = 1751.4 A.U. and X = 1825.9 A.U. for Lyman has found correspond- 

 ing wave-lengths at X = 1481.6 A.U., X= 1548.4 A. U., X = 1751.5 A.U. 

 and X= 1826.2 A.U. in the spark spectrum of mercury. It may be, 

 however, that these wave-lengths really originated in carbon, if we 

 can assume that the mercury jn Lyman's experiments was slightly 

 contaminated with that element in some form. The wave-lengths 

 X = 1310.4 A.U., X = 1463 A.U., X = 1577.8 A.U. and X = 1624.3 A.U., 

 found by us in the spectra of helium, do not correspond to any wave- 

 lengths in the list given by Lyman^ for the mercury spark, and they 

 are probably due to carbon as Millikan suggests. Of the eighteen 

 wave-lengths found by us in the spectrum of helium, which approxi- 

 mated to wave-lengths found by Millikan in the spark spectrum of 

 carbon, all but two, namely, X = 1085.2 A.U. and X = 1215.8 A.U. can, 

 therefore, be identified by assuming their origin to be one or other of 

 the elements carbon or mercury. As to X = 1215.8 A.U., all we can 

 say is that in our experiments its origin might have been hydrogen- 

 Regarding the wave-length X = 1085.2 A.U. the origin in our experi- 

 ments was, no doubt, helium. Its origin in Millikan's experiments is 

 difficult to explain unless we adopt his suggestion and assign it to 

 oxygen or nitrogen. If we take this view it is no longer necessary to 



' Lyman, The Spectroscopy of the Ultra-violet, p. 118. 



