No. 5.] STUDY OF ARC SPECTRA. 379 



(E) Region lying between 45 50 and 4960. 



Photographs were taken in three regions (1,2, and 3, Fig. 2). 

 ' 129 lines were mapped, of which 



129 Hnes were visible in the violet sheath, 

 103 lines were visible in the blue sheath, 

 95 lines were visible in the yellow sheath. 



(F) Region lying betzveen 4960 aiid 6042. 



115 lines were mapped, of which 



1 1 5 lines were visible in the violet sheath, 

 42 lines were visible in the blue sheath. 



If we sum up these data, we find 



Total lines mapped 799,^ 



Visible in central violet jZj, 

 Visible in blue 431, 



Visible in yellow 232. 



Attempts to classify these lines according to their sources led 

 to no satisfactory results. A study of the photographs, however, 

 showed that the lines of the arc spectrum may be grouped as 

 follows : 



\.a. Lines which grow gradually weaker as we move from the 

 center to the edge, and are usually invisible in the outer sheath. 



\.b. Lines which preserve about the same intensity in the center 

 and blue of the arc, but are suddenly much weaker or invisible in 

 the yellow. 



Forty lines of this class were counted. They were distributed 

 throughout the spectrum from 3024 to 5616. 



The lines of the band spectra grow fainter through the blue, but 

 disappear quite suddenly there, not being seen in the yellow. 



\.c. Faint lines which are only found in the first two positions 

 {a and 2, Fig. 3). Invisible in the blue. One hundred and sixteen 

 of these were counted. 



^ Exclusive of the fine lines of the band spectrum which were not counted. 



