338 
I)K. W. M. HICKS: A CIHTICAL STUDY OF SPECTRAL SERIES. 
sufficiently accurate to prove this, although they obey the rule within error limits. 
[Ill, p. 340 .] 
The f Sequence. —This sequent of the first order also has the multiple of A2. The 
material at disposal is not so comprehensive as in the case of the D series, for, except 
in tlie second group of the periodic table, the F lines occur chiefly in the ultra-red. 
The proof of the above statement is perhaps, therefore, not so conclusive as in the 
case of the d sequence. It completely stands the test however in the rare gases. 
There seems some evidence that F series also show a satellite effect in a small degree 
—of one or two ouns. In the second group it seems to be a general rule that in 
many of the low orders {rn = 1, 2...) the f sequents receive very large displacements 
from their normal value, so that a normal line is much weaker or is altogether absent 
and replaced by others separated from it by considerable numbers. This also is found 
to be the rule in the present case. 
Displacement .—Regarding the ordinary doubtlet or triplet series we may consider 
the second (or third) as displaced from the first by the deduction of a certain 
number of ouns from the mantissa of the limit; or better perhaps regard the last 
satellite set as the fundamental one and the othei'S as displaced by the addition 
of ouns. When such displacements occur in the limit of one line the new one is 
indicated by .writing the displacement on the left. Thus 82(^1) = ( —Ai)Si{m) or 
Si(? 7 i) = {Ai)S2(w) = (Aj + Ag) 83(777). With satellites, on the other hand, the similar 
effect is 23roduced in the sequence terms. In this case it is entered on the right. 
Thus Di2(?n) = Dll (777.) (-a-( 5 i) 01’ = D12 (777) (iCc^i) ; D22(m) == (-Ai) D12 (m) 
= ( — Ai) Dll (777) ( — x^i). Displacements of both kinds are very common in spark 
spectra and put themselves specially in evidence in the succeeding pages. A normal 
line may not oidy show lines displaced from it, but often it appears to be replaced by 
them, and, in general, when it does not disappear its intensity is abnormally low. 
This is practically what happens in the D satellites. The D13, D23, D33 appear to be 
the normal lines in which we should expect descending order of intensity; but most 
of the energy (or the majority of the emitting centres) appears carried over to the 
more intense and displaced set Djo, D22; and, again, most of what should be expected 
in Di 2 is carried over to become the strongest line Du. Frequently the D13 line has 
disappeared and only the fragment D23, D33 of the triplet left. In general, the Dn 
lines of any element are the strongest of the series. But in the present vacuum tube 
spectra (spark type) Ave shall find very frequently that the line required for D^ is 
comparatively weak, and in this case there appear other lines related to it by oun 
displacements chiefly in the limit. As the real existence of these displaced series is a 
matter of some importance considerable space has been given in the discussion of the 
X spectrum (p. 399 ) to its demonstration in the case of two series depending on the 
limits (±2 ^i)D(go). R seems a peculiarity of these displacement series that a term 
of one series may be absent but appear in another. Thus (— 2(^i) D (777) may not be 
observed, but a (-t 2 ( 5 i) D (777) may be and vice versd. The presence of a similar effect 
