THE ULTRA-VIOLET SPECTRA OP THE ELEMENTS. 
213 
Part II. 
(Received June 15, 1882.) 
The account of the ultra-violet spectra of fifteen metals here recorded is a first 
instalment of the results of observations which we have accumulated during the past 
threte years, but have not heretofore been able to reduce. During that time we have 
taken some thousands of photographs of the electric arc under various conditions, and 
especially in crucibles of lime and magnesia (as previously described by us), and in the 
presence of most of the known metals; but with the exception of Cornu’s map of the 
ultra-violet solar spectrum giving the chief iron lines and a few of those of other 
elements, up to the line U we have had little to aid us in the exploration of a new 
field and the assignment of the several lines to the elements producing them, and the 
measurement of our many photographs lias cost both time and patience. Dr. 
W. A. Miller, long ago published an account of his photographs of the spark spectra 
of the elements, and Mr. Hartley has recently (Trans. Boy. Dublin Soc.) published 
photographs of the spark spectra of several elements which are a great improvement 
on those previously published. But those give spark spectra only, are taken with an 
apparatus of small dispersion, and are not reduced to scale, so that they give qualitative 
rather than quantitative results. The spectra which we here describe are those of the 
arc up to the wave-length 2200, and we give in each case the approximate wave¬ 
lengths of the lines observed. For some few of the lines of tin and aluminium the 
wave-lengths have been determined by means of a grating as described in the first 
part of this paper, but in all other cases they have been derived by interpolation from 
the wave-lengths of the neighbouring iron lines. In the map which accompanies this 
paper we have given in the top line the principal lines of iron for convenience of refer¬ 
ence, and in the lowest line the arc lines of carbon with which it is necessary to be 
acquainted as they are always present, though varying much in intensity, in the arc 
taken between carbon electrodes. The scale of this map is one-half that of Angstrom’s 
“ Normal Solar Spectrum.” 
We have already, in describing the visible spectra of the alkali metals and that of 
magnesium, called attention to probable harmonic relations between the lines. This 
relation manifests itself in three ways—first, by the repetition of similar groups of lines; 
secondly, by a law of sequence in distance, producing a diminishing distance between 
successive repetitions of the same group as they decrease in wave-length; and thirdly, 
a law of sequence as regards quality, an alternation of sharper and more diffuse groups, 
with a gradually increasing diffuseness and diminishing intensity of all the related 
groups as the wave-length diminishes. 
The first relationship has long since been noticed in the case of the sodium lines 
which recur in pairs, and we have observed that the potassium lines between the 
extreme red and violet pairs are repetitions of a quadruple group, while the lithium 
