Dr. A. Schuster on Spectra of Lightning. 317 



Lines which in one flash appeared especially bright were not 

 seen in another flash. The greater number of flashes, how- 

 ever, gave a different spectrum altogether. In the place of 

 bright lines a great number of bands were seen ; and Prof. 

 Kundt even distinguishes two different band spectra. 



Mr. John Herschel (Proc. R. 8. xvii. p. 61) observed a 

 variable continuous spectrum crossed by bright lines, which 

 also had a variable intensity. He gives the measurements of 

 two lines, which agree very well with nitrogen-lines. 



M. Laborde (Les Mondes, viii. p. 219) observed some lines, 

 especially one near E, which sometimes appeared alone. He 

 also saw a continuous spectrum. 



Dr. H.Yogel (Pogg. Ann. cxliii. p. 653) describes lines only; 

 but in his list I find two which do not coincide with aiy bright 

 lines in the spectrum of the electric spark taken in atmospheric 

 air ; they do, however, coincide with two bands which I have 

 observed in some flashes of lightning, as I shall show. 



Mr. J. P. Joule (' Nature,' vol. xvi. p. 161) also observed 

 some spectra of lightning. Frequently there was only one 

 bright line visible, this being coincident with the brightest 

 nitrogen-line. At other times there were several bright lines 

 visible, sometimes with and sometimes without the green 

 nitrogen-line. A continuous spectrum was also observed. 



Mr. H. P. Proctor (' Nature/ vol. xvi. pp. 161 & 220) gives 

 some measurements of lines which do not lay claim to any 

 accuracy. He observed also a band spectrum, which he finds 

 not to be the band spectrum of nitrogen. 



From conversation with Prof. A. Young, I learned that he 

 also had seen a line spectrum, a band spectrum, and a conti- 

 nuous spectrum. 



During my stay in Colorado last summer, I had some good 

 opportunities of studying the spectra of lightning. It was 

 my intention to get some reliable measurements of the band 

 spectrum which I, in common with most observers, have 

 seen ; and in order to have greater chance of succeeding, I 

 confined myself to one part of the spectrum only. The part I 

 chose extended from X = 5000 to X = 5800, and covered, there- 

 fore, most of the yellow and green. I used a direct-vision 

 spectroscope, with a slit movable by means of a micrometer- 

 screw. A bright line in the principal focus of the telescope 

 formed a fiducial mark. Under ordinary circumstances, the 

 slit is moved until the line to be measured forms a continua- 

 tion of the bright line which reaches down into the centre of 

 the field. I found, however, that the bands I wanted to mea- 

 sure were nearly as broad as the thin glass bar which carries 

 the bright line; and I used the bar therefore simply as pointer. 



Phil. Mag. S. 5. Vol. 7. No. 44. Mag 1879. 2 C 



