376 CAROLhVE IV. BALDWIN. [Vol. III. 



as impurities in the carbons, are superposed upon this under spec- 

 trum. The arc spectrum thus seems to consist of two spectra. 



The general arrangement of the periodic lines is worthy of 

 note. They are finer and nearer together toward the violet end of 

 the spectrum; the bright groups are also nearer together in the 

 regions of greater refrangibility. In each group, however, the 

 lines are finer and nearer together toward the red of the spectrum, 

 the maximum being toward the longer wave lengths, on which 

 side the termination of the group is sudden. Each of the groups 

 shows secondary maxima, and between each of these the same 

 general law is followed, as in the case of the group as a whole. 

 We shall speak of these groups hereafter under the general 

 name of the band spectrum. 



Many of the metallic lines are stronger near the negative car- 

 bon and are weak or invisible at the positive carbon, ^ while the 

 lines of the band spectrum are strongest at the positive carbon. 



As we pass from the center of the arc to the edge, the band 

 spectrum grows fainter and finally disappears. This is also true 

 of many of the metallic lines, while other metallic lines seem to 

 be equally bright in all parts of the arc, and a considerable 

 number are relatively stronger in the outer sheath. This effect 

 is enough to change the entire aspect of the spectrum in many 

 places. The lines which show the last effect do not belong 

 exclusively to any one metal, neither do all the lines of a metal 

 seem to act thus. In the case of the calcium lines, all the trip- 

 lets of the 2d series given by Kayser and Runge ^ are strongest 

 in the center, while the pairs 



3968.63] ^^^ 3737-08 



3933-83) 3706.18. 



and certain other lines as X = 4226.91 are much stronger in the 

 yellow part of the arc flame. A few of the triplets of the first 

 series are stronger toward the edge. Such are 



4318.80- 



4302.68 



4299.14, 



3644-45 

 and 3630.82 

 3624.15 j 



1 Lockyer, Proc. Roy. Soc, XXVIII., 1879. 



2 Kayser and Runge, Ueber die Spectren der Elemente. 



