368 
MR. E. G. BILHAM ON THE SPARK SPECTRUM OF 
Table IV. 
Class. 
Number of lines. 
Average shift in 
1 A TT 
1UOO ^ * 
(11 atmospheres). 
Ratios. 
I. 
17 
18-9 
1 
II. 
21 
18-6 
1 
III. 
7 
45-9 
2-4 
IV. 
4 
80-8 
4-3 
V. 
7 
97-1 
5-1 
There appears to be some evidence that symmetrical and unsymmetrical reversed lines 
give the same average shifts. The ratios of the average shifts for the different classes 
are of interest in connection with the fact, first noted by Humphreys, that for some 
metals the lines tended to form three groups in which the ratios of the pressure-shifts 
were approximately 1:2:4, when reduced to a standard wave-length. Later work has 
confirmed Humphreys’ view to a certain extent, notably in the case of the iron arc. The 
ratios, however, do not appear to be integral, and the grouping according to pressure- 
shifts is, to a great extent, independent of the classification according to behaviour. 
Under these circumstances it seems doubtful whether the phenomenon is anything more 
than a chance arrangement such as would follow from considering “ small,” k ‘ average,” 
and “ large ” shifts. On the other hand, the relative shifts of lines whose behaviour 
justifies the supposition that they are of radically different types is a question of more 
direct interest, and the results are more likely to be capable of theoretical 
interpretation. Following out this view, and retaining our five original classes, we 
get the ratios in the last column of Table IY. Adopting a more general grouping by 
combining classes I. and II., IY. and Y., we arrive at three main divisions, viz. :— 
(a) Reversed lines.38 lines 
(b) Unreversed lines (narrow). 7 ,, 
(c) Unreversed lines (broad).11 ,, 
giving the displacement ratios, 
a. b. c. 
1:2-4: 4'6. 
If we neglect the enhanced lines and consider only arc lines we obtain 
a. b. c. 
1:2-4: 5T 
9. The Apparent Violet Shift of the Line 3514. 
An interesting case is furnished by the line A 3514*10. At eleven-atmospheres 
pressure this line appears as a very narrow reversal superimposed upon a broad bright 
