46 



Dr. W. Huggms and Mrs. Huggins. 



[May 2, 



and with Thalen's measure of the lead line which falls upon it, and 

 also with the maps of Angstrom and of Kirchhoff. 



The wave-length of the brightest nebular line may therefore be 

 taken at from 



\ 5004-6 to X 5004-8 (1). 



The micrometric measures of this line, given by D' Arrest, Vogel, 

 and Copeland, agree closely with this value. 



D 'Arrest's* mean value 5004 



Vogel's „ 5004 



Copeland'sf „ 5004 



b. Second line. — In 1872, J I stated that I had found this line, by 

 comparison with a line of barium and subsequently with an iron line, 

 to have 



A wave-length of X 4957'0 (2). 



D'Arrest's mean value from micrometric measures A 4956*6 

 Copeland's „ „ „ 4958"0 



c. Third line. — In my original paper§ " On the Spectra of some of 

 the Nebulas," in 1864, I showed, by direct comparison with hydrogen, 

 that this line is undoubtedly the line of that gas at F of the solar 

 spectrum. This observation was afterwards repeated, |[ and has been 

 confirmed by the photographs of 1882 and 1888. 



The wave-length of this line is therefore X 4860' 7 (3). 



D'Arrest's value from measures 4860'6 



Copeland's ,, 4861 - 



d. Fourth line. — In 1872,^[ I stated that I had satisfied myself of the 

 coincidence of this line with H7, but, on account of its faintness, it is 

 very satisfactory to find this observation of coincidence confirmed by 

 the photographs taken in 1888 and 1889. There can be no doubt that 

 this is a line of hydrogen, and that 



The wave-length therefore is 4340'1 (4). 



Copeland's mean value 4342 "0 



Dr. Copeland gives the measures of two still fainter lines which he 

 has seen in this nebula, namely, one at X 5874, possibly coincident 



* ' Undersogelser over de nebulose Stjerner.' Copenhagen, 1872, p. 23. 



f ' Monthly Notices, K.A.S.,' vol. 48, p. 361. 



X 1 Roy. Soc. Proc.,' vol. 20, 1872, p. 385. 



§ < Phil. Trans.,' 1864, p. 437. 



|| < Phil. Trans.,' 1868, p. 545. 



% ' Eoy. Soc. Proe.,' vol. 20, 1872, p. 385. 



