DR. W. M. HICKS; A CRITICAL STUDY OR SPECTRAL SERIES. 
59 
500 lines shorter than 4698. A photograph of a portion of the longer wave-length, 
kindly given me by Sir W. Crookes, contains a few faint lines above Exner and 
Haschek’s limit, as well as a few others not given by the latter. 
Scandium and its group, Yt, La, Yb, show doublet series.* On the contrary. 
Europium, with which they are associated, has a spectrum of triplets witli separations 
2632 and 1004. Moreover, when expressed as fractions of the square of the atomic 
weight (l52'03),t these au’e found to be in correspondence with those for Cd and Hg, 
and therefore suggest that Eu is the member wanting in the Periodic Table between 
those elements; if so, we should expect to find the corresponding S and D series, 
which are well developed in Cd and Hg, as well as other peculiarities. Unfortunately, 
however, the first line Si(l) and 83 ( 1 ) should come about 5400 if they are similar to 
Cd and Hg, and this region is outside the published lists. 
It is possible, however, to allocate observed lines to fit the conditions for the S and 
They are given in the following 
lists:— 
Sharp 
Series. 
A. 
n. 
i'. 
' (5379-1)^ 
(18585-5) 
2632-5 
m — \ < 
(4711-09)t 
(21218) 
1004 
, 4498-81 
22222-00 
f 3322-01 
30093-73 
2629*42 
m = 2 -( 3055-07 
32723-15 
1001-38 
, 2964-35 spark 
33724-53 
" 2909-10 
34365-08 
m = S < 
L 2701-99 
36998-94 
2633-86 
f 2744-36 
36427-75 
m = 4: 
2634-60 
1 2559-30 
39062-35 
m = 5 
2659-50 
37590-17 
II 
2577-69 
38783-00 
* Discussion deferred to a later communication, 
t Jaxtsch, ‘C. R.,’ 1908. 
[I See note at end.] 
