DR. W. M. HICKS: A CRITICAL STUDY OF SPECTRAL SERIES. 
383 
same time will give some reliable data which have a bearing on the present problem 
of the actual D and F series. The line 20312 and those in its neighbourhood show 
the following sets of separations, viz, :— 
1. (1) 17615-06 
832-13 
(10) 18447-19 
1865-51 
2 . (6) 17638-55 J 
f 827-92 
(2) 18466-47 
1866-75 
I 
,1864-74 
(5) 19503-29 
829-93 
3. (3) 17628-29 
831-70 
(1) 18459-99 
1860 26 
4. (1)17772-38 
835-41 
(8) 18607-79 
1862-96 
r 830-62 
5. (3) 19503-29 
829-93 
(1) 20333-22 . 
1 1864-93 
(6)20312-70 
(1) 20333-22 
(4) 20320-25 (1) 22180-04 
1859-79 
(6) 20470-75 
(2) 21163-84 
(2) 22198-15 (1) 24062-12 (<1) 25925-07 
1863-97 1862-95 
In the general survey at the commencement of the investigation a large number 
of separations of an amount near 1864 was noticed. Suspecting that it indicated 
the existence of a second type of sharp series, a second smaller separation (for 
triplets) was looked for, analogous to the 1 ^ 2 = 815 of the 1778 set. A further 
separation was found for a value about 830."*^ The whole spectrum was searched within 
the limits 1864 + 2. The result is shown in the occurrency curve of Plate 2 , fig. 3 . 
This curve is unique amongst those hitherto observed in its great height above the pure 
chance line and also in the steepness of its rise and culmination to a single definite 
peak. Ihe search brought to light also a very considerable number of long successive 
chains and of meshes (see e.g., Nos. 2 and 5 above) of the same amount, proving that 
1864 enters not only on its original source, as due to a displacement on some 
fundamental sequence, but also as a link. Now the c link of the normal = 1778 
is 1872‘63 and c (3(^) = 1865T6, thus suggesting a possible origin, also a 
corresponding c link for V 2 , i.e., a separation produced by a second Ag displacement 
is 833’84, or with a modified c' {SS) = 83070. These suggest triplets formed by the 
same A^, A., on p ( —Ai) instead of on p, in other words, series whose limits are 
51025 + 1778 = 52803, 54675‘6, 55509'4. But the way in which the separations 
enter with the suggested D line 20312 indicates that they stand in fundamental 
relation to it and neither in a linkage relation, nor with the limits named. For in 
the latter cases it would throw out of gear the whole relation of 20312 with the D set, 
which some provisional work had seemed to establish. In this work they were 
considered as part of the D system through 20312 regarded as a Dj (l) line with 
18447, 17615 as satellites. In this case the sequence is of the order 51025 
— 20312 = 30713. As against this idea is, of course, the greater intensity of 18447, 
the supposed satellite over that of the Du, and also the fact that no normal 
* As a fact, however, there are several others depending on oun multiples also present, but which at 
present we need not deal with. 
3 G 
VOL. CCXX.-A. 
