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PROF. A. FOWLER ON SERIES LINES IN SPARK SPECTRA. 
§ 2. Wave-lengths of the 11 4481 ” Series of Magnesium. 
The members of the “ 4481 ” series are essentially spark, or enhanced lines, but 
they are most conveniently produced in an arc between magnesium poles in vacuo , 
and are best observed in the blue green patch in the region of the negative pole, 
viewed end-on. Under these conditions they are obtained as very narrow lines, well 
adapted for accurate measurement. The new determinations of wave-lengths have 
been based as far as possible on the interferometer values for the lines of iron, or upon 
grating determinations depending upon such standards, among which those given by 
Dr. Keivin Burns # have been especially useful. 
The Line 4481.—-Previous photographs taken in the 1st order of a concave grating 
of 10 feet radius gave no indication of the resolution of 4481, but photographs recently 
obtained in the 3rd and 4th orders show it to consist of two clearly separated lines 
(Plate 3, fig. 6), of which the more refrangible is the stronger, as stated by King. 
The lines have no close resemblance to the side components of a reversed line, as the 
space between them is considerably greater than the thickness of either of them, and, 
as pointed out by King, the measured separation is the same in photographs taken 
with different lengths of exposure, and in different orders of the grating. Iron arc 
comparisons were exposed both before and after the magnesium, and the plates were 
measured in the usual way with red right and red left. The mean results from 
numerous measurements are :— 
International scale. Rowland scale. SA. 
448P327 448P495 
0-198 
4481T29 448P297 
Dr. King’s values for the two components on Rowland’s scale are 4481 "499 ±0"001 
and 4481 "284 ±0"001 (SX = 0'215), but from the reproductions given in his paper, the 
lines do not appear to have been as narrow as those from which the above results 
were obtained. 
[Note .—The new measurements of X 4481 as a doublet permits of a more satisfactory 
conclusion as to its presence among the Fraunhofer lines than has hitherto been 
possible. Rowland tabulates two lines, each of intensity “0” at 4481 "298 and 
4481*515, with an intermediate titanium line (intensity l) at 4481*438. The agreement 
for the stronger component, which is a very distinct line in Rowland’s map, may be 
considered exact, but the discordance on the less refrangible line exceeds the probable 
error of measurement, and the intensity is also a little too high in the sun. It may 
be, however, that both these discordances are caused by a still fainter solar line which 
is nearly superposed on that of magnesium, since such lines are very numerous. The 
presence of Mg 4481 as faint lines in the solar spectrum may therefore be considered 
* ‘Zeitsch. f. Wiss. Photog.,’ 1913, XII., 6, p. 219; ‘Lick Obs. Bull.,’ vol. VIII., No. 247 (1913). 
