L891.] On the Chief Line in the Spectrum of the Nebulce. 



, " On the Chief Line in the Spectrum of the Nebulae." By 

 JAMES E. KEELER, Astronomer of the Lick Observatory. 

 Communicated by WILLIAM HuGGiNS, D.C.L., LL.D., F.R.S. 

 Received March 13, 1891. 



As my paper on the Motions of the Planetary Nebulae in the Line 

 of Sight* did not give a final determination of the exact position of 

 the chief nebular line, and might therefore possibly be regarded as 

 leaving in abeyance the question as to whether that line could be 

 regarded as a remnant of the magnesium fluting, I beg to be allowed 

 state briefly the results of some more recent observations, which 

 ,ve enabled me to fix with great accuracy the true position of, the 

 ief nebular line. 



At the time when my paper on the motions of the nebulae was 

 printed, I had not been able to obtain any satisfactory comparisons 

 of the third nebular line with terrestrial hydrogen, all the nebulae 

 in my list having proved to be too faint for the purpose. I was, 

 therefore, compelled to adopt the mean position of the principal line 

 for the ten nebulae observed as the normal position from which to 

 easure displacements, and it was for the reason that the ten nebulae 

 id not have the uniform distribution in the sky which was desirable 

 at the numerical results for their motions were stated as " not to 

 regarded as final." 



In October, 1890, when the Orion nebula came within reach of the 

 lescope, comparisons of the third line with the H/3 line of hydrogen 

 ere made without difficulty, and on the same nights the position of 

 e principal line was determined. One such double observation, if 

 rfect, completely solves the problem, since the displacement of the 

 ird line gives the necessary correction to the position of the first, 

 .e only question is in regard to the accuracy of the observations. 

 It is evident from what has already been written on this subject 

 Dr. and Mrs. Huggins, Professor Lockyer, and myself, that the 

 swer to the question whether the chief nebular line is coincident 

 ith the edge of the magnesium fluting at A, 5006*4 depends 

 pon very small differences of position, differences which would, 

 fact, be considered small even in solar spectroscopy. But 

 eir minuteness, although it increases the practical difficulty of 

 Tvation, does not detract from their importance, since abso- 

 te coincidence of spectral lines is necessary (although not always 

 fficient) to establish a claim to identity of origin. It is there- 

 ire necessary to determine from a careful consideration of the 

 ick Observatory measures whether they are of a sufficiently high 



* 'Publications of the Astronomical Society of the Pacific,' No. 11, p. 265. 



