INTENSITY RELATIONS IN THE SPECTRUM OF HELIUM. 



157 



Other formulae of interpolation which have been used, though not described in this 

 communication, give 2'0 mm. as the position of maximum emission of the Diffuse series 

 of Helium in these experiments. 



In the case of other series, the position is not the same for each member, and the 

 best method of studying the phenomena is apparently by means of a graph, as shown 

 in fig. 2. The abscissa are distances from the cathode, while the ordinates represent 

 photographic intensities of lines at these distances. Graphs for several of the more 

 important lines in the spectrum are given. The general features of these typical 

 curves can at once be seen. The close resemblance between the leading lines of the 

 two Principal series, XX3888, 5015, is very strikingly different from the behaviour of 

 the curves obtained for lines of the associated series, which in themselves are similar. 



Fig. 2. 



A qualitative survey of the variation in distribution of the lines in front of the 

 cathode is given in fig. 3. This plate was obtained by photographing, without the 

 wedge, the glow in front of the cathode, which was in this case perpendicular to the 

 length of the slit. It presents certain remarkable features which have not hitherto 

 been discussed. In order to enhance the effects, the plate has been reproduced by 

 processes introducing excessive photographic contrast which, whilst accentuating the 

 outstanding features, have obliterated fainter lines such as Ha and HgX5461, which 

 were clearly visible on the original plate. It will be noticed that whilst the Helium 

 lines start from the cathode and fade away continuously, the Hydrogen lines show a 

 very definite " dark space." The Mercury lines reach their maximum of intensity at 



