448 Proceedings of the Royal Society of Edinburgh. [Sess. 
XXX. — On a New Method for differentiating between Overlapping 
Orders when mapping Grating Spectra. By Alexander D. 
Ross, M.A., B.Sc., Assistant to the Professor of Natural Philosophy 
in the University of Glasgow. 
(MS. received May 19, 1910. Read June 6, 1910.) 
Some time ago the author commenced an investigation of the Zeeman effect 
in the spark spectrum of certain rare elements. The research promised 
results of considerable interest, as the properties and relationships of these 
substances are as yet very imperfectly known, and certain of the elements 
show indication of high magnetic quality.* The following elements were 
examined : uranium, rubidium, gadolinium, samarium, scandium, dysprosium, 
neo-ytterbium, and lutecium. The author is greatly indebted to Sir 
William Crookes for his kindness in supplying the scandium, and to 
Professor Ur bain of the University of Paris for the samples of the three 
last- mentioned elements. 
The photographic work was carried out in the Physikalisches Institut 
of the University of Gottingen. An electric spark was used situated in the 
narrow pole gap of a Du Bois half-ring electro-magnet which had been lent 
for this purpose by the Berlin Akademie der Wissenschaften. The spectra 
were produced by a 21 -foot radius Rowland grating having a ruled area 
of 6 x 2 inches, with 20,000 lines to the inch. 
It was thought probable that difficulties might arise through inaccuracies 
in the published tables of the spectra of the rare elements. No such trouble 
has arisen in the cases of uranium, rubidium, gadolinium, samarium, and 
scandium. In one or two instances it has been found that lines ascribed to 
these elements have evidently been due to impurities in the salts used in 
the investigations of the spectra. A considerable number of hitherto un- 
mapped lines have been photographed and their wave-lengths determined 
by measurement from neighbouring known lines. The position is, however, 
very different with regard to dysprosium, neo-ytterbium, and lutecium. 
The separate existence of the two last-mentioned elements was only dis- 
covered in 1907 by Professor Urbain as the result of a laborious fractional 
crystallisation of ytterbium salts.f Dysprosium, too, was first obtained in 
* S. Meyer, Sitz. Ber. der konigl. Akad. zu Wien , cx. p. 492 (1901), and G. Urbain, 
Comptes Rendus , cxlvi. p. 922 (May 4, 1908). 
t G. Urbain, Comptes Rendus , cxlv. p. 759 (1907). 
