449 
1909-10.] Mapping of Grating Spectra. 
a comparatively pure state by the same chemist in 1906.* Their spectra 
are accordingly not yet fully known. The chief lines of the ultra-violet 
spark spectra of dysprosium,]* neo-ytterbium, and lutecium { have been 
measured by Urbain, while the arc-spectrum of dysprosium § has been 
investigated by Eberhard. As Urbain’s tables of dysprosium, for example, 
contain only some 96 lines of the spark spectrum, while the Zeeman effect 
photographs obtained at Gottingen show over 3000 measurable lines, it 
became necessary to map the ordinary spectrum before proceeding with the 
research on the Zeeman effect. 
The procedure followed in mapping is to photograph the spectrum of 
the substance under examination, and that of iron, on the same plates, the 
Fig. 1 . — Spectra of Iron and Dysprosium. 
two spectral bands overlapping one another to a greater or less extent. 
Fig. 1 shows the portion of the spectrum of dysprosium lying between 
wave-lengths 3930 and 4060, with the comparison spectrum of iron above. 
The wave-lengths of a large number of normal iron lines have been very 
accurately measured by Kayser || and by Exner and Haschek.H These 
being taken as standards, the wave-lengths of the unknown lines on the 
plate can be determined by interpolation from measurements made with 
a comparator microscope. 
* G. Urbain, Comptes Rendus , cxlii. p. 785 (1906). 
t G. Urbain, ibid., cxlvi. p. 922 (1908). f G. Urbain, ibid., cxlv. p. 759 (1907). 
§ G. Eberhard, Publik. des astrophysikalischen Observatoriums zu Potsdam , Nr. 60 (1909). 
|| H. Kayser, Drudds Ann., iii. p. 195 (1900). 
U F. Exner and E. Haschek, Wellenldngen-Tabellen, Vienna (1904). 
