Mr. J. N. Lockyer on Spectrum- Analysis, 387 



to a focus between the poles of the lamp by an extra lens inter- 

 posed between the lamp and the heliostat. 



The use of the shutter enables us to compare either two or 

 more spectra upon a single plate ; or the solar spectrum may be 

 compared with two metallic spectra, being made to occupy the 

 position between the two. 



III. On the Lines coincident in different Spectra, 



The bearing of the former papers on the lengths of the lines 

 of the elements is briefly recapitulated. 



The examination of the various spectra of metals and alloys in- 

 dicated the great impurity of most of the metals used, and suggested 

 the possibility of the coincidences observed by Thalen and others 

 being explained in the light of former work. 



It is observed that coincidences are particularly numerous in 

 the spectra of iron, titanium, and calcium, and that nearly every 

 other solar metallic spectrum has one or more lines coincident with, 

 lines of the last element. These coincident lines are, as a rule, very 

 Variable in length and intensity in various specimens of the metals 

 in which they occur, and are sometimes altogether absent. 



One of the longest calcium lines, that at wave-length 4226-3, is 

 also seen in the strontium spectrum as a line of medium length; and 

 4607*5, a very long line in strontium, appears in calcium as a short 

 line. Another very long strontium line, 4215*3, is asserted by 

 Thalen to be seen in calcium ; but the author has never seen it till 

 lately, and then only in a specimen of calcium known to contain 

 strontium. 



We have here, then, a case of coincident lines, in which the one 

 that is long and bright in one spectrum is short and faint in the 

 other, and a case of a line said to be coincident in two spectra 

 being, though always visible in one, sometimes absent in the other 

 of them, and only appearing in it when the two substances were 

 mixed. The hypothesis of impurity at once explains the whole 

 case, even without the third line, which renders the fact of mixture 

 certain. 



The longest lines of calcium occur in iron, cobalt, nickel, barium, 

 strontium, &c; and the longest lines of iron occur in calcium, 

 strontium, barium, and other metals. 



i Other cases are adduced ; and the following general statements 

 are hazarded, with a premise that further inquiry may modify 

 them. 



1. If the coincident lines of the metals be 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 : those cases are much more 

 frequent in which they are long in one spectrum and shorter in the 

 others. 



2. As a rule, in the instances of those lines of iron, cobalt, 

 nickel, chromium, and manganese which are coincident with lines 

 of calcium, the calcium lines are long, while the lines as they 



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