INTENSITY RELATIONS IN THE SPECTRUM OF HELIUM. 165 



By comparison of (a) and (6), we see that the second member of the Principal series 

 is not enhanced by low pressure relatively to the first. It is in fact definitely 

 reduced. This series behaves accordingly in a different manner from the others. It 

 would seem that the phenomenon found in the other case that of a selective transfer 

 of energy is not produced. 



We may also notice that the selective enhancement at low pressure of the higher 

 members in any one series which, as we have seen, must be regarded as a phenomenon 

 relating to the Diffuse and Sharp series, but more especially to the former, is one 

 which would be expected on theoretical grounds from a theory such as that of BOHR, 

 which regards lines of higher term number as due to the passage of atoms between 

 " stationary states " of relatively large atomic radius a state of things to be 

 expected with greater frequency under the influence of a considerable reduction of 

 pressure. We may recall, for example, BOHR'S explanation of the great extent of 

 visibility of BALMER'S series of Hydrogen lines in the solar spectrum. It is note- 

 worthy, in this connection, that the Diffuse series of elements are those in which the 

 Rydberg phase constant is nearly unity, so they accord very closely, in quite general 

 terms, with the present principles underlying BOHR'S theory. So far as the present 

 investigation is concerned, the quantitative examination of the alteration of the 

 spectrum of Helium produced by reduction of pressure lends a certain amount of 

 support to BOHR'S theory, at the same time, however, implying that the theory in 

 question, if in its general basis correct as regards Diffuse series, does not furnish any 

 interpretation of the origin of Principal series, in which the Rydberg phase is usually 

 widely different from unity. We do not, however, propose to discuss this question 

 further at the present time, as evidence in the other direction can be adduced also. 

 For example, we showed in a previous communication that the Balmer series of 

 Hydrogen lines does not in fact possess the characteristics of a Diffuse series, for the 

 separations of the doublets which compose it are not constant as regards wave number, 

 but are, on the other hand, appropriate to a Principal series. The question of the 

 relation of our results to BOHR'S theory must therefore be left unsolved at the present 

 time, and we prefer to summarise the selective effect of low pressure in individual 

 series in the Helium spectrum into the statement that while in the Diffuse and Sharp 

 series, there is an energy transfer to higher term numbers, the effect on the Principal 

 series is in the opposite sense. 



We have not, of course, yet examined the effect of low pressure on the Diffuse and 

 Sharp series of Parhelium. This examination will be given briefly after the corre- 

 sponding tables have been exhibited, and will be found to correspond exactly to the 

 similar effects observed in Helium. Meanwhile, we may complete the discussion of 

 Principal series by a short survey of the change produced by admixture of a light gas 

 such as Hydrogen. 



Referring again to Table XIII., photograph (b) and (c), we find that the influence 

 of a trace of Hydrogen decreases X3965 in intensity relatively to X5015. The original 



