164 



DR. T. R. MERTON AND PROF. J. W. NICHOLSON ON 



But the important fact for our present enquiry is that the role played by a large 

 quantity of Hydrogen is directly contrary to that played by a small trace, and we 

 may argue that the mechanism of inter-action of the two gases is quite different in 

 the two cases. A definite phenomenon has been quantitatively isolated which demands 

 for its appearance only a spectroscopic " trace " of one of the acting gases. 



This reversal of the effect of a trace of Hydrogen, by the admixture of more 

 Hydrogen is, however, interesting in another way, for it introduces us to a striking 

 difference of behaviour between Diffuse and Sharp series. Inspection of Table XIII. 

 indicates that the line A7065 shows very little tendency to change in relation to 

 X4713 by the action of this Hydrogen or at least that the change in the Diffuse 

 series is of quite another order. Moreover, the change among the lines XX7065, 4713, 

 4121, though comparatively small, is quite definitely present as a combination of two 

 effects. For X4121 is reduced relatively to X4713, as by the effect" of the trace 

 of Hydrogen, while X4713, as against A7065, is quite definitely enhanced. It 

 seems that the Sharp series under these circumstances is just ceasing to show the 

 first phenomenon, due to the trace of Hydrogen, and commencing to show the second, 

 so that if the quantity of Hydrogen were increased yet further, the second might 

 predominate. In other words, the essential difference between the Diffuse and Sharp 

 series is that in the latter case a more considerable admixture of impurity is needed 

 to produce the effects observed in the Diffuse series. Sharp series are in fact sensitive, 

 to an equal extent with Diffuse series, to the influence of a trace of Hydrogen, but 

 not to a comparable degree to the different mechanism of interaction with large 

 quantities of Hydrogen. We feel no doubt that the available data can be summarised 

 in this way, for the phenomena shown by the Parhelium spectrum follow the same 

 course throughout. 



The Principal series of Helium, showing only one member X3888 is not, of course, 

 capable of test in this manner by the present experiments. 



Principal Series under the same Conditions. The Diffuse series of Parhelium 

 contains five members on some of our plates, and we can therefore make by its use a 

 much more exhaustive test of the conclusions outlined in the preceding section. It 

 is also possible to obtain information relating to Principal series, and this will be our 

 first object in the present section. Since there are only two visible members, in these 

 experiments, the Table (XV.) is very short. 



TABLE XV. 



