338 DR. W. M. HICKS: A CRITICAL 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 A 2 . The 

 material at disposal is not so comprehensive as in the case of the D series, for, except 

 in the 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 (m = 1, 2...) the / 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 others 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 S 2 (m) = ( A^S^m) or 

 Sj (m} = (A^ S 2 (m) (A, + A 2 ) S 3 (m). With satellites, on the other hand, the similar 

 effect is produced in the sequence terms. In this case it is entered on the right. 

 Thus D 12 (m) = D n (m} (-xS } ) or D u (m) = D 12 (m) (<$,) ; D 22 (m) - (-A,) D J2 (m) 

 = ( A } )D n (m)( xS 1 ). 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 only 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 D 13 , D 23 , Dgg 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 D,,, D 22 ; and, again, most of what should be expected 

 in D, 2 is carried over to become the strongest line D n . Frequently the D 13 line has 

 disappeared and only the fragment D 2S , Dgg of the triplet left. In general, the D n 

 lines of any element are the strongest of the series. But in the present vacuum tube 

 spectra (spark type) we shall find very frequently that the line required for D n 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<^) D ( oo ). It seems a peculiarity of these displacement series that a term 

 of one series may be absent but appear in another. Thus ( 2<Ji) D (m) may not be 

 observed, but a ( + 2^) D (m) may be and vice versd. The presence of a similar effect 



