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XXIV. Researches in Spectrum- Analysis in connexion with the Spectrum 
of the Sun. — No. IV. By J. Norman Lockyer, F.R.S. 
Received May 11, — Read June 18, 1874. 
I beg to lay before the Royal Society a Map of that portion of the spectra of calcium, 
strontium, and barium comprised between wave-length 3900 and 4500. This Map has 
been constructed from photographs of the spectra taken by the method described by 
me in a former communication (the third paper of this series) to the Society, and I am 
induced to send it in as a specimen of the results to be obtained by the method in the 
hope that other observers will co-operate ; for I am of opinion that it will be necessary 
to construct similar maps for all the metallic elements before either our knowledge of 
the composition of the sun’s reversing layer can be said to be in any way perfect, or we 
can be said to have a ready means of determining cyclical changes in its composition. 
The great labour attending and long time required for the construction of these maps 
results from the universal presence of impurities, even in the purest specimens of the 
metals or metallic salts prepared by the ordinary chemical methods ; and although the 
method employed is the only one which enables us to eliminate them eventually, this 
elimination necessitates a photographic comparison of the spectrum mapped with those 
of all the substances present as impurities. Hence there are numerous records to be 
discussed, and the discussion requires special treatment. 
Method of Mapping. 
The method of treatment which I have employed in constructing the Maps is as 
follows : — 
1 . Elimination of lines due to impurities. — The spectrum of the element is first con- 
fronted with the spectra of the substances most likely to be present as impurities, and 
with those of metals which, according to Thalen’s measurements, contain in their spectra 
coincident lines. Lines due to impurities, if any are thus traced, are marked for 
omission from the Map and their true sources recorded, while any line that is observed 
to vary in length and thickness in the various photographs is at once suspected to be 
an impurity line, and if traced to such is likewise marked for omission. 
The retention or rejection of lines coincident in two or more spectra is determined 
by observing in which spectrum the line is thickest ; and it is then, as indicated in my 
last paper, assigned to that element. Where, as in the present case, several elements 
are mapped at once, all their spectra are confronted on the same Plate, as by this means 
the presence of one of the substances as an impurity in the others can be at once detected. 
Many lines due to impurities have in this manner been traced in the photographed 
