(34 
DE. W. M. HICKS: A CKITICAL STUDY OF SPECTEAL SERIES. 
very long one, and, in addition, there are the usual narrow triplets depending on the 
D satellites. As an example of a strong triplet the following set may be taken, all 
of intensity 50 on Exner and Hasohek’s scale : 
A. 
n. 
V, 
4435-74 
22537-95 
2630‘30 
3972-16 
25168-25 
1003‘87 
3819-80 
26172-12 
The S and D Lines of Radium. 
Judging from the falling off both in intensity and number of the S and D lines as 
we pass from Mg and Ca to Sr and Ba, it might be expected that the corresponding 
series for Ra—two places farther on in the Periodic Table—would be difficult to 
allocate even if they were present at all. The difficulty is increased the more because 
the only spectral measures published are those of the spark, in which these series are 
always weakened. Spark spectra, in general, show a greatly increased number of 
lines which are related to one another in quite definite ways and many related to 
series lines. This makes the search for doublets much more difficult, as there exist 
whole series of doublets with separations slightly different from one another. This 
necessitates a coniplete study of tlie whole spectrum in order to feel complete 
certainty in any allocation of lines to a series, unless such series is clearly marked by 
many terms. There is, therefore, some degree of uncertainty in the allocation 
proposed below, in spite of the evidence adduced in its favour.* 
The most complete and reliable measurements of the spectrum are those of Runge 
and Precht,! between 6487 and 2709. Their plates were only sensitive up to 6500. 
In addition, Exxer and Haschek publish tables both for the arc and spark, but their 
material can hardly have been pure. They give comparatively few lines, and probably 
not so reliable as their other lists. 
Even in Runge and Precht s work it is probable that most of the lines which 
would be comparaUe with the weaker ones in the Ba lists have not been observed. 
The lesult is that it is not so easy to allocate the D series, as the separations result 
from a pair of lines the second of which is strong and the other is a not observed 
weak satellite of tlie first. In conse(|uence we shall get separations less than the 
true I'l and i/ 2 , and this is exemplified in the list which follows. The wave-numbers 
only are given—^the wave-lengths will lie found in Appendix II. 
i/i = 2050-27, i/2 = 832-00 ? 
There is also some additional evidence drawn from their relation to other lines, of a similar nature to 
that referred to under Europium, 
t ‘Ann. der Phys.,’ 14, p. 419. 
