146 



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



the interpretation of the exact meaning of the intensities requires considerations 

 which are not strictly relevant to the present communication. 



TABLE II. Helium Bands. 



For a line taken twice on the same plate, but coming from different regions of the 

 discharge tube, a direct comparison of its heights in the two cases is of no value as 

 an indication of relative intensities, for the difference in height corresponding to any 

 definite intensity-ratio in the two cases depends very much upon the density gradient 

 of the wedge, whose variations along the spectrum are considerable. We must 

 accordingly, as a preliminary to any discussion, obtain the photographic intensities of 

 the lines in all cases, according to the formula 



for these, as we have seen, are strictly comparable for the same line on the same plate 

 with two different conditions or regions of excitation. The photographic intensities 

 of the series lines are given in Table III. 



The results of calculation of the photographic intensities of the Helium bands, on 

 the photographs which register them, are as given in Table IV. 



In general, we may say, in connection with these bands, that although in a quali- 

 tative sense they are intensified or weakened together, according to the region from 

 which the spectrum is photographed, this general correspondence is not strictly 

 quantitative, the relative. intensities of any band in two regions being dependent to a 

 small extent on the wave-length. In other words, the regions of maximum emission 

 of these bands, which can appear simultaneously with the series spectrum of Helium, 

 are not identical. This question will not be discussed further in this communication, 

 owing to the difficulty, already indicated, of interpreting, the exact meaning of the 

 intensity in this case. The table already sufficiently indicates the general nature of 

 the phenomena presented by the band heads in this form of experiment. 



