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DR. T. E. MEKTON AND PROF. J. W. NICHOLSON ON 



intensity is, however, more than restored by the addition of more Hydrogen. This 

 behaviour is precisely similar to that shown by the Diffuse series of Helium, although 

 the actual changes are of a smaller order of magnitude. 



Diffuse and Sharp Series of Parhelium. We stated at the beginning of the last 

 section that the Diffuse series of Parhelium supplied a peculiarly exhaustive test of 

 the more general applicability of some of our conclusions. The main details regarding 

 the intensities of the lines under the conditions in question are given in Table XVI. 



TABLE XVI. Diffuse Parhelium under Various Conditions. 



The enhancement of X4922 and X4388 relatively to X6678 is at once obvious, by 

 inspection of the table, in the case of the low pressure spectrum. It is in fact even 

 more remarkable than in the corresponding Helium series. Moreover, X4388 is 

 enhanced relatively to X4922. The remarks which we made earlier regarding X4472 

 in the nebular spectrum apply with greater force to X4388. The behaviour of these 

 two lines in nebulae is thus correlated, in the light of these experiments, by the fact 

 that nebulae are in a state of extremely low pressure and certainly much lower than 

 in the present investigation, so that the relative enhancement of X4471 and X4388 

 may be expected to be much greater. But the degree to which the phenomenon 

 occurs, even with the present exhaustion of the tube, is sufficiently convincing. 



As in the case of the Diffuse series of Helium, this effect again cannot be described 

 as a continuous transfer of energy down the series, for if this were the case, X4144 

 would become visible when enhanced to a greater degree than X4388. It is actually 

 invisible, and calculation shows that this fact implies that its intensity relatively to 

 X6678 is not more than doubled. We must therefore repeat the former conclusion 

 that the strong enhancements of particular lines at low pressure are peculiar to these 

 lines, and in fact to the three lines XX4471, 4922, 4388, of the two Diffuse series. 

 Under still lower pressure, X4922 may be expected to become quite subordinated to 

 X4388, which is already as strong in pur experiments, and in the very low conditions 

 of pressure in nebulae, XX4471, 4388, should therefore be the two most prominent 

 Helium lines of the Diffuse series. This is a well-known fact of observation in 

 astrophysics. 



We may now take up the consideration of the effect of a trace of Hydrogen. 

 Inspection of the table is almost sufficient to show that the energy-transfer to longer 



