DR. W. M. HICKS: A CRITICAL STUDY OF SPECTRAL SERIES. 
375 
lines, shown on the map, which are linked to S 2 (2) by 3774'14 +1007'23 and another 
by 2458 + 2416, clearly u + v, an enlarged e link is quite a common starting one from 
S and P series lines. Moreover the next link 959'49 is just as much displaced oneway 
from c as 3774'14 is in the other from e, for 3774’14 + 959'49 = 4933'63 = 3771 00 
+ 962’63. Thus the line 18744'28 is exactly e + d + c above S 2 (2). In fact we have 
a case of series inequality.” The link — 966'09 to the next is excessive, but it is 
entered because (see list Nos. 3, 4, 5) the displacement ( —<$) in (3) is destroyed in (4) 
by adding a to it, thus making it normal, and then another $ on the (4) and on the 
—e link reproduces (5). So far all seems satisfactory but other considerations come 
in, in connection with the line 18744. There is an analogous set of doublets of the D 
type (see also p. 372) in each of Au, Ag and Cu with the proper doublet separation for 
each element. That they belong to the D type with negative values in Au and Cu 
is proved by their Zeeman effect, but the pattern for Ag has not been determined. 
The Au set show a satellite while Cu does not, as is usual with the first element in 
a group. In Ag the doublet set are D 21 = —18026, D 12 = —18947 with respective 
intensities of 4 and 1. There is a line 18756 also of intensity 4 which naturally seems 
to be — D n , but its satellite difference is not sufficiently near a multiple of <k to give 
confidence. The line 18744, however, which has just been suggested as belonging to 
the S (2) linkage with such good evidence, might just as well be taken for D n were it 
not that its intensity 2 is less than that of D 21 instead of equal to it or greater. On 
the other hand it makes the satellite difference exactly 29^ and so affords some 
evidence of its being D n . There is also this in its favour, that both it and 18206 
form a kind of combination with S (2) to give two lines in the ultra-red observed by 
Randall, whose wave-numbers are 5944'09 and 574CT40. How close this relation is 
can be seen from the following :— 
18 026'55 —1208 2'63 = 5943'92 dn = T7 
187 44'28 —13003'42 = 5740'86 dn = -'46. 
At first sight Randall’s ultra-red lines look like Ritz combinations but it is not so,* 
since they should be written 
D 21 + S 1 (2) = —5943'92 = p 2 +p 1 - VD 12 -VS (2) 
D u + S 2 (2) = -5740'86 =p 1 +p 8 -VD ai -VS (2) 
* All Randall’s other ultra-red lines are, however, Ritz combinations, though his allocation would 
seem to be doubtful. For instance, using the notation employed throughout this series of papers, he 
gives 5438-65 = YD„ (2) - YF (4), 5460-66 = YD 1S -YF(4), 7965-33 = VD 12 (2) - VF (5). But if so 
the F sequence must be identical with the D series, as is clearly seen from the following :— 
D n (2) - D 12 (3) = 23730 -99 - 18291-06 = 5439 • 93, 
D 12 (2)-D 12 (3) = 23730-99 - 18270-81 = 5460-18, 
D u (2)-D u (4) = 26232-50- 18270-81 = 7961 69, 
in which D i2 (4) is affected with a possible error ± 3'7 and 7965 of ± 3-2. The equality of the F and D 
is pointed out by him. The lines considered in the text are given by him as 5 P 2 (3)-V8>(2) and 
YPj (3) - VS (2), but as the P (3) lines are not known it is not clear how he has obtained their values. 
