INTENSITY RELATIONS IN THE SPECTRUM OF HEITUM. 
157 
Other formulge 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 sliown 
in fig. 2. The abscissm are distances from the cathode, while tlie 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 
o 
two Principal series, A\3888, 5015, is very strikingly difterent from the behaviour of 
tlie curves obtained for lines of the associated series, which in themselves are similar. 
50 : 
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 b}^ 
processes introducing excessive photographic contrast which, whilst accentuating the 
outstanding features, have obliterated fainter lines such as Ha and HgA5461, 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.” Tlie Mercury lines reach their maximum of intensity at 
