the Occurrence of Enhanced Lines in the Arc. 299 



a. Relative intensities of lines : — 



Feeble lines : j, 0, 00, and 000, this last one being at the 



limit of perception. 

 Lines of moderate strength : 1 to 5. 

 Strong lines : 6 to 10. 



Extra strong lines : 12 to 50. 



b. Character of lines : — 



= no line has been observed. 



c = a long line, one which passes practically unaffected 

 from one electrode to the other through the first 

 and second phases. 



d = a discontinuous line ; one which is emitted during 

 the first phase and falls oif rapidly or abruptly at 

 the beginning of the second phase and disappears 

 (abruptly or gradually) before the end of the 

 second phase, 

 dd = a short line ; observed only during the first phase. 



S1 ? 1 ^' j A line which is not specially marked 



n = nebulous and y . . ■, , , n j « «j 



.,, , n i s or n is tolerably well denned. 



nn = broad and hazy. 



b v — winged or broadened towards red. 

 b v = „ „ „ violet. 



b rv = symmetrically winged or broadened. 



r = reversed. 



I = first phase. 



II = second phase. 

 enh = enhanced line. 



These signs are placed behind the numbers indicating the 

 relative intensities of the lines. 



I. Arc spectra of Lead. 



a. Quartz spectrograph X 2150 to \ 3200. Spark lines 

 which are absent from the ordinary carbon-metal 

 arc, but are brought out with a drop of water placed 

 between the electrodes : — 



XX 2204 Very feeble in direct arc, but strong in water 

 arc and spark. 

 2698 Well seen. 

 2950 Strong. 

 3018 Strong. 



