128 Mr. E. 0. Kemble on the Probable 



easily made, while transfers between orbits of the same 

 system having the same angular momentum are not, and the 

 Bohr principle of selection gives a semi-theoretical expla- 

 nation of this fact. It follows that if the normal state were 

 identical with the state (1, S), the strongest series in the 

 ultra-violet emission spectrum would be 



(N)-(m, P) m=2,3,4, (3) 



Other series, such as 



(N)-0,S) ro = 2, 3,4, (4) 



could appear only under the special circumstances required 

 for breaking through the principle of selection. The strongest 

 line of the ultra-violet spectrum would then be (N) — (2, P), 

 whose wave-length is 568 A.U. But the observations of 

 Fricke and Lyman show no trace of this line. On the 

 contrary, the only line which they find is at 585 A.U., the 

 wave-length of the first member of the series (4). It is by 

 no means certain that other lines are not present, but if so, 

 they must be relatively faint, and that fact is sufficient to 

 establish our point. 



Three Hypotheses regarding the Return of Atoms to 

 the Normal State. 



It seems to be possible to account for the known experi- 

 mental facts regarding the excitation of the various lines in the 

 helium arc spectrum by means of the three following hypo- 

 theses : (a) that under ordinary circumstances few, if any, 

 atoms pass directly to the normal state from any of the other 

 stationary states except those of the sharp series crossed orbit 

 system ; (b) that passage from even the above-mentioned most 

 favourable set of stationary states, (m, S), to the normal state 

 takes place with relative difficulty; (c) that the presence of 

 some impurity X commonly occurring in helium introduces 

 one or more stationary states which are not possible in the 

 pure gas, and thus provides an indirect path from the higher 

 energy levels to the normal which is accompanied by the 

 emission of more than one radiation frequency. 



Hypothesis (a) rests on the fact that the only ultra-violet 

 line observed by Fricke and Lyman is the first member of 

 the series (4). Hypothesis (b) is required by the fact that 

 the line (N) — (2, S) has been observed only under the most 

 favourable circumstances in relatively pure helium, and is 

 even then very faint. The work of Paschen affords additional 

 evidence in favour of (b) } which will be discussed later. The 



