On t/ie Chief Line in the Spectrum of the Nebula. 173 



Three lines were seen, and others suspected. The positions of the 

 lines were only roughly determined even by such an experienced 

 observer. The strongest and widest of the lines was described as 

 situated at two-thirds of the interval between F and &, whilst the 

 second line appeared coincident with F. 



1868. An observation made by Dr. Huggins in 1868 reads : "The 

 determination of the position in the spectrum of the three bright 

 lines was obtained by simultaneous comparison with the lines of 

 hydrogen, nitrogen, and barium. The instrument which I employed 

 had two prisms, each with a refracting angle of 60, and the positions 

 of the lines were trustworthy within the limits of about the breadth of 

 the double line D The coincidence of the line in the 



nebula with the brightest of the lines of nitrogen, though now sub- 

 jected to a much more severe trial, appeared as perfect as it did in my 

 former observations.* 



It will be noticed that in these observations Dr. Huggins informs 

 us to what extent his observations were trustworthy, and, taking 

 Thalen's measures for D, viz., 



5895-0 

 5889-0 



we find the possible error to be sixty units of the scale above sug- 

 gested. In the diagram which accompanies the above description the 

 nebula line is shown coincident with the less refrangible component 

 of the nitrogen double, in contradistinction to the former observation, 

 which, made with less powerful dispersion and in accordance with 

 Dr. Huggins's estimate of the accuracy attained at that time, placed 

 the line midway between them. 



In another paragraph of the same paper (p. 543) Dr. Huggins 

 takes " the wave-length of the nitrogen line at 500*80 millionths of 

 a millimetre." Hence, according to this statement, the nebula line 

 would have a wave-length of 500 80; or 500'51, if Thalen's value for 

 the less refrangible nitrogen line be taken, and by Dr. Huggins's 

 own assertion this value would only be accurate within the interval 

 between the sodium double D, that is, 0006'0. It should also be 

 noticed that the double line of nitrogen is again referred to as if it 

 were a single line. 



18(58. Lieutenant Herschel made some micrometric measures of 

 the chief nebula line in 1868,f and a reduction of them was made by 

 D'Arrest in 1872, with the following result: j 



* ' Phil. Trans.,' 1868, pp. 541-2. 



t ' Roy. Soc. Proc.,' vol. 16, p. 451. 



J ' Undersogelser over de NebuJosestjerner,' Copenhagen, 1872, p. 22. 



