PROF. A. FOWLER ON SERIES LINES IN SPARK SPECTRA. 
255 
by strong discharges in helium tubes, to which reference has already been made. Some 
of these were at first regarded as being identical with the lines of the supposed P series 
of hydrogen calculated by Rydberg on the assumption that the Pickering (£ Puppis) 
lines formed the S series. The lines in question are compared with Rydberg’s values 
(Rowland scale) in the appended table :— 
Table XIY.—The “ 4686 ” Series. 
Observed A 
(Fowler). 
Calculated A 
(Rydberg). 
Difference 
AA. 
4685-98 
4687-88 
1-90 
3203-30 
2733-34 
2734-55 
1-21 
2511-31 
2385-47 
2386-50 
1-03 
2306-20 
2252-88 
2253-74 
0-86 
In order to conform with Rydberg’s theory, and to adapt the formulae then 
ordinarily employed, it was necessary, as in the first discussion of the Mg “ 4481 ” 
series, to divide the series into two parts by taking alternate lines. One series, as will 
be seen from the table, then agreed very closely with the calculated values for the 
P series of hydrogen, and the other was regarded as a second P series having a simple 
relation to the first. 
The conditions of appearance of these lines in helium tubes indicates that they are 
enhanced lines, and calculation shows that they can be united in a single series of the 
4N type, as in the case of the other series of lines of this class which have been 
discussed. Thus, the first line, which has been most accurately measured, gives the 
equation 
. = 438,879-1 (I- A). 
The differences 0 — C (An) for the seven observed lines are O'O, —0'3, +1‘4, +0'6, 
+ l'O, +2'3, —1‘4. All are within the estimated limits of error of observation, or very 
o 
nearly so, except the third, and even in this case a correction of O'l A.U. to the 
observed wave-length would reduce the difference to zero. 
The “ 4686 ” series is accordingly a series of the enhanced line, or 4N, type, and 
can no longer be considered to belong to the same group as the Balmer series of 
hydrogen which is of the arc, or N, type. The numerical relations indicated by 
Rydberg’s calculations are therefore not significant, and, in view of the experimental 
evidence, it must he concluded that the “ 4686 ” series is not due to hydrogen, but to 
helium, as first indicated by Dr. Bohr from theoretical considerations. This conclusion 
is further supported by Evans’s observation of the line 4686 in a helium tube in 
