806 
ME. J. NORMAN LOCKYER ON SPECTRUM-ANALYSIS 
spectra of calcium, strontium, and barium. For instance, the two longest aluminium 
lines (3943 and 3961) are invariably present in all three spectra, and the long iron lines 
(4045, 4063, and 4071) can in most cases be likewise detected. The lines 4076-9 and 
4215-4 present in all photographs of calcium, and assigned to this element both by 
Angstrom and Thalen, are, as I showed in my last paper, really due to strontium. 
Similarly the two H lines (3933 and 3968), assigned both to iron and calcium by 
Angstrom, are proved to belong to calcium by the following observations : — 
a. The lines are well represented in the spectrum of commercial wrought iron, but 
are absolutely coincident with two thick lines in the spectrum of calcium chloride with 
which the iron spectrum has been confronted. 
b. The lines are represented by mere traces in the spectrum of a ’specimen of pure 
iron prepared by the late Dr. Matthiessen, and obligingly placed at my disposal by 
Dr. Russell. In this photograph (Plate LXXXV. Spectrum 7) both poles of the 
lamp were of iron, the lower pole consisting of an ingot of the metal which had been 
cast in a lime-mould. 
c. The lines are altogether absent in a photograph of pure iron, where both poles of 
the lamp were of the pure metal not cast in lime, and they are likewise absent in a 
photograph of the spectrum of the Lenarto meteorite (Plate LXXXV. Spectrum 1)*. 
These examples serve to illustrate the manner in which large numbers of the coinci- 
dences recorded by former observers have been disposed of in the course of mapping by 
the photographic method. 
In determining the coincidence of very thick lines, such as the H lines just mentioned 
for example, the centre of the thick line is taken, except in instances where the whole 
of a spectrum has been slightly displaced to the right or left by the displacement of 
the apparatus during the act of photographing, in which case a correction has to be 
made for the position of every line in such displaced spectrum. It not unfrequently 
happens that a very thick line will reverse itself, a circumstance which greatly facilitates 
its comparison with confronted lines, since a thin dark line then runs down the centre of 
the thicker bright one (Plate LXXXV. Spectra 2 & 3)f. 
2. Determination of the positions and lengths of new lines . — By eliminating lines due 
to impurities in the manner just described, a spectrum is at length obtained, of which 
every line is assignable to the particular element photographed. 
These lines are then entered in a map, those measured by Angstrom or Thalen being 
placed in the positions assigned by those authors, while the new lines are at first entered 
approximately. The wave-lengths assigned to the new lines, although probably not far 
from the truth, must, in the absence of actual measurement, be regarded only as 
approximate. They have been found in the following manner : — 
The admirable photographic print of the solar spectrum, from H to F, obtained some 
* In both these photographs, however, the longest line of calcium is to be traced. 
f The absorption-line does not always occupy the exact centre of the bright band. This point is occupying 
my attention, as it raises a very interesting question connected with molecular vibrations. 
