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DR T. E. MERTON AND PEOF. J. W. NICHOLSON ON 



particular pressure we have adopted, its relative intensity is more than doubled. 

 The importance of this result, from the point of view of the conditions of pressure 

 occurring in nebulae, is sufficiently evident, for the behaviour of this line in nebulae as 

 compared with X5876 is in the same sense. It is not unlikely that further reduction 

 of the pressure may carry the process further, and this question forms an important 

 subject for future investigation. 



r 



TABLE XIV. Diffuse Series and Sharp Series (Helium). 



The relative intensity of X4026 with respect to D 3 , on the other hand, is not altered 

 so appreciably. With respect to X4471 this line is much reduced. It is evident that 

 the phenomenon is not correctly described as a transfer of energy to the members of 

 higher term number in the series, and therefore that the effect of low pressure cannot 

 be classed with the phenomena of enhancement of \4472 which we recorded in a 

 previous communication. For in these, cases, the line X4026 also participated in the 

 effect to a much greater extent, and even relatively to X4471. 



Our conclusion must be that reduction of pressure in the tube can enhance the line 

 X4472 to a great degree, but at the same time leaves other members of the series with 

 nearly the same relative intensities. In the case of the Sharp series, the discussion is 

 made rather more difficult by virtue of the disappearance of X7065 and X4121 from our 

 plate at low pressure. But we can calculate the limiting intensities they can have. If 

 their "true" intensity was unity in this case, while X4713 had its true intensity 6 '44, 

 they would be visible. On a scale of intensity 10 for X4713, they become visible if 

 their intensity exceeds the value 10/6'44 or 1'55. We may accordingly assume that 

 it is less. 



Comparing this investigation with the fact of the existence of an intensity 2 '4 on 

 this scale, for the line X4121 in the " ordinary " spectrum, as in the table, it is evident 

 that the phenomenon found in the Diffuse series is present here also, and to the same 

 degree. In the case of nebulae, the Sharp series of Helium is always very weak, but 



