DE. W. M. HICKS: A CRITICAL STUDY OF SPECTRAL SERIES. 
(LS 
and others be observed displaced according to certain laws. The matter is far too 
large to deal with here and will require a paper to itself. I mention it iiere, however, 
in order that judgment may be suspended if any natural doubt is felt concerning the 
small intensities of the series, the mingling of the arc aiid spark lines, and the various 
lacunse which appear. That lacunae actually do occur in Exnee and Hasohek’s list is 
shown by the absence of the line 4552'6 from the arc, its appearance as a weak line in 
the spark, whilst it is of considerable strength in Crookes’ photograph. 
In the list of D lines those in iR’ackets have not been observed. Tliey are either 
Di 2 lines deduced from Dgj, or Dgi deduced from Dj 2 . The lines in italics are those 
calculated from the formula for D^. The values of and D ,2 gradually converge, 
as they should do, their differences being 
75-83, 37-91 ?, 11-49, 5-92, 5-30, 3-25. 
There may be some doubt whether the series generally assigned to tlie diffuse for 
Zn, Cd, Hg are really diffuse. The a term is negative, and agreement within obser¬ 
vational errors is obtained only by using m — 'b instead of m, but they are clearly of 
the same type for all these elements. The formula for Eu was therefore taken of tliis 
form. The constants were determined by using the same limits as for S, viz., 40364-08, 
and only the two first lines. The result for is 
n = 40364-08-N/{?n + -95G04G--0G8460/(2w-l)}2. 
D 2 i is determined from this by finding the difference between the ^I’s for the first line 
of Dji and D 12 . This is practically the same as Aj/G, and tlie altered fx is used for the 
calculation of the succeeding values. The table gives the difference between the 
observed and calculated values, on the supposition that I'l is 2632-5. The third 
column is the result if l^e taken as 2633. As throughout the paper, * denotes a 
line used for calculation. 
m. 
D„. 
D21. 
D21. 
2 
* 
* 
'¥r 
3 
* 
• 53 
4 
- -29 
- -25 
5 
0 
1 
- -22 
- -18 
6 
- -02 
•00 
7 
- -03 
•00 
8 
•60 
- -52 
- -49 
9 
+ -76 
+ -79 
(The last two are probably not the real lines.) 
In this note I am not going into the question of the constitution of the Europium 
spectrum in general. Indeed, the list of triplets with separations 2630 and 1004 is a 
