246 



Mr. A. Le Sueur on the Spectra of 



[Mar. 3, 



lines I make out to be, if not coincident with, very near to C, D, b, F, and 

 the principal green nitrogen line. There are possibly other lines, but those 

 mentioned are the only ones manageable. 



Direct spark comparison has hitherto been found impossible ; though 

 plainly marked at moments the lines require concentrated attention, and 

 will not permit the disturbing effect of other light in the field ; attempts 

 were made to diminish the brilliancy of the spark spectrum but with no 

 good result, a different method was therefore adopted. 



By watching for good moments the pointer was placed on a particular 

 star line, the spark spectrum was then turned on, and the position of the 

 pointer noted. 



By this means it was seen in repeated trials that star lines within the 

 limits of the dispersion used (about 7°) were coincident with C, F, the 

 principal green nitrogen line, and b, or rather (the spark employed was 

 platinum in air) the air-band involved in b group. It cannot be deter- 

 mined whether the coincidence is with the magnesium group or the air- 

 band ; nothing more definite can be said than that the star line lies within 

 the limits of the group. 



The comparison spectrum employed does not show F, but the position 

 of the previously adjusted pointer, with reference to air lines in the 

 neighbourhood, leaves little doubt as to the identity of the blue star line 

 with F, due regard being had to the collateral evidence (when such close 

 limits are reached) that C coincides with a red star line. The yellow (or 

 orange ?) line in the star has not yet received sufficient attention ; it is, 

 however, very near D. 



With the dispersion employed, D and the bright air line on less re- 

 frangible side of D are well separated ; so that, notwithstanding the delicacy 

 of the star line, I hope, if not to get satisfactory evidence of coincidence 

 with a particular line, at least to eliminate one of the competitors ; at pre- 

 sent it cannot even be said whether the line may not be slightly more re- 

 frangible than D ; the limits are, however, very small, placing the bright air 

 group about 1180 of Mr. Huggins's scale completely outside the possible 

 range. 



I would remark that the very faint nebulosity (if any) in the immediate 

 neighbourhood of the star v is incompetent to give a trace of spectral lines 

 with even a wide slit ; for a considerable space s. and f. of y no lines are 

 at all visible ; the nearest nebula bright enough to show a line (the three 

 usual lines are now easily seen on a good night over the brighter parts) is 

 reached in the direction about 45° n. p. from rj y and even then the distance 

 from t/, as judged by the appearance in the spectroscope with r\ threaded 

 on the thus directed slit, is little less than one minute. This remark is of 

 some importance in connexion with the ordinary telescopic observations of 

 the nebula, but is mentioned at this point to relieve any impression which 

 might arise that the nitrogen line seen on the star spectrum is merely the 

 chief nebula line crossing it. 



