488 
MR. J. NORMAN LOCKYER ON SPECTRUM-ANALYSIS 
coincident in different spectra is long and bright in only one of them, and that in others 
it is short, or faint, or both ; or it may even, in certain specimens of the substances, be 
altogether absent from the spectrum. 
As an instance of this difference of behaviour, the following cases in the spectra of 
calcium and strontium may be considered. The numbers are wave-lengths taken from 
Thalen’s list. 
The longest line in the visible portion of the calcium spectrum, wave-length 4226*3, 
is found in the strontium spectrum as a line of medium length. 4607*5, one of the 
longest lines of strontium, appears in the calcium spectrum as a short line. 
Another very long line of strontium occurs at 4215*3, in close proximity to the longest 
calcium line, and, according to Thalen, occurs also in the spectrum of that metal. 
I have, however, never, until lately, succeeded in obtaining any evidence of its presence 
in the calcium spectrum ; but there is evidence to show that the metal I employed was 
very pure. 
We have here, then, two metals with two lines common to their spectra ; and it is found 
that the line which was long and bright in one spectrum, is faint in the other ; and with 
regard to a third line, one observer finds it in both spectra, the other in one only, and 
after many attempts succeeds in observing it in the second, but only in a specimen known 
to be contaminated with the first. 
The simplest explanation of the case, bearing in mind the facts already established in 
my first paper, is that the calcium used to produce the spectrum was contaminated to a 
certain extent with strontium, the strontium in turn containing calcium, a state of things 
which a moment’s consideration will show to be not only possible but most probable, the 
close chemical relation of the two metals, and the extreme difficulty of making even an 
approximate separation when mixed, being well known. 
Even if we knew nothing of the probability of mixture occurring in the cases of the 
two metals in question, the behaviour of the line at w. 1. 4215*3 is sufficient to show 
what is the true cause of the coincidences. 
The longest lines of calcium at wave-lengths 4226, 3968, and 3933 occur also in iron, 
cobalt, nickel, barium, &c., and assume very considerable proportions, equalling or sur- 
passing in length many undoubted lines of those elements ; on the other hand, the iron 
lines at wave-lengths 4071, 4063, and 4045 occur in calcium, strontium, and barium, 
and in other metals. 
Again, the longest lines of aluminium at wave-lengths 3961 and 3943 occur usually 
in the spectrum of iron as longish lines, and are to be found in the spectra of cobalt, 
nickel, calcium, strontium, and barium, and in other metals, where they are even longer 
than some of the true lines of the metals in which they occur. 
As a result of these considerations the following general statements may be hazarded, 
premising that it is possible that further inquiry may modify them. 
1st. If the coincident lines of the metals are considered, those cases are rare in which 
the lines are of the first order of length in all the spectra to which they are common : 
