No. 5.] STUDY OF ARC SPECTRA. 377 



In the carbon band, ending at A. = 473 5, the maxima remain 

 the same as we pass from center to edge of the flame, but the 

 law seems to be changed. In the outer sheath the maximum 

 intensity is toward the violet and the strongest maximum is 

 \ = 4645. The shading of the band is thus completely reversed. 

 Another band with maxima toward the violet is seen in the outer 

 sheath from A, = 4840; the maxima being A. = 4840, X = 4865 and 

 X = 4892. The strongest part of the group at 3590 becomes 

 the weakest in the outer part of the arc, and many groups 

 have the faintest lines at the center, the strongest at the edge. 



When the photographs are taken giving the spectrum of the 

 flame as blown out from between the carbons by the action of 

 a magnet, the slit extends across the different sheaths of the arc, 

 and those lines which are stronger in the outer parts of the arc 

 reach through the spectrum with their brightness undiminished or 

 increased at the edges, while the bands and many of the metallic 

 lines appear only in the central part. 



When the slit is arranged to pass through the image of the arc 

 horizontally, this effect is more marked, owing to the greater 

 steadiness of the flame. In watching the developing of the nega- 

 tives, it was observed that many of the lines, as 4226, 3583, 

 3571, were first visible at the edge and then gradually extended 

 toward the center, while the fine lines of the band spectra and 

 most of the other lines appeared first at the center and then 

 extended part way to the edge. The photographs taken by the 

 three methods thus show complete agreement in their results. 



Since the spectrum of the arc has already been mapped with 

 more powerful apparatus than that used in these experiments, 

 only such points will be given as may serve to indicate the differ- 

 ences between the spectra of the different sheaths. 



(A) Region lying bettveen 2263 and 31 00. 



135 lines were mapped between 2263 and 3016, all of which 

 belonged to the central violet sheath [(i) Fig. 2]. 



The first lines which could be detected in the blue and yellow- 

 sheaths lay between 3018 and 3099; eleven lines among the 

 nineteen mapped were visible in the blue and four in the yellow. 



