408 
DR. W. M. HICKS ; A CRITICAL STUDY OF SPECTRAL SERIES. 
Consequently in the above list the value of f has no importance in settling the order 
of the displacements-. 
The whole of the foregoing argument is based on the constancy of Aa for all series. 
This matter has been referred to in the introduction in which the question of what is 
to be understood by the atomic weight was brought up. The accuracy in the 
determination of the oun is rendered so great by the constitution of the d and f 
sequences, that the mass of the electrons connected with the nucleus affect it. It may 
be interesting to illustrate the considerations there adduced by a concrete example. 
The example we will take is (l), as the result may possibly throw some light on a 
difficulty which will appear later. The line 19880 is seen to require a displacement 
of — (^. Let us determine the transfer of electrons in order to produce a change by 
one oun. Suppose this transfer changes x to x-\-x'. This means that the mantissa 
as represented in the list must be diminished by 80a:', by putting y — 
Hence 
x' — = ’65, dd = "036. 
80 
The change in the number of electrons (see p. 342) = 925 x ‘036/611 x 130 = 1‘97 = 2 . 
The addition of eight electrons to the mass acting in our standard case, -v^'ould render 
19980 a possible D line with limit ( —^i)D(go) instead of a possible one with 
(-.5)D(®). ^ 
The preceding treatment of the material is only a first step towards unravelling the 
intricacies of these D systems. An exhaustive treatment is here impossible, and 
would involve the consideration of other data—^the triplet separations, linkages, 
similarities of arrangement, dependence on F series and so on. All that can be done 
IS to give a few illustrative instances and to bring into prominence certain problems 
whose solution in tlie future may be of extreme importance. 
(l) The line 20021 is given as requiring the displacement — 3A in the limit, and in 
this case the mantissa is 8 OA 2 . This displacement increases the limit 51025 by 31‘88, 
and the resulting should be greater by 3 x '535 = 1‘60 and = 1779‘50. As a fact the 
line (see p. 382) forms a doublet with separation 1779‘51, and with intensities 1, 10, in 
the proper order for a set. All the tests support each other. Again in the 
doublets 
(7) 20500‘13 1784-68 (2) 22284‘82 
( 8 ) 20559‘08 1785-64 (<Iw) 22344‘72 
the same displacements (or of +3A) are indicated. If we suppose these modified 
values of produced in the same way as in Kr by a relative displacement in the 
sequent, the oun in this case alters the separation by 4-91. The modified therefore 
becomes 1779‘50 + 4-91 = 1784‘41, or within error limits of the observed value for 
20500, but too small for 20559. That for 20559 corresponds to an extra displacement 
of — 2^1 on the limit, or —bS^ in all, which is quite inadmissible on the qualifying test. 
