44 



"Dr. W. Huggins and Mrs. Huggins. 



[May 2, 



The position of the pair of lines a little less refrangible than this 

 strong line, seen with it in the photograph of 1888, and present with- 

 out the strong line in the photograph of 1889, and the positions of 

 the two other more refrangible pairs, presumably connected with the 

 first pair, are given in the following table : — 



1st pair about 



2nd pair about 

 Line at about. 

 3rd pair about 



These three pairs of lines are shown in Spectrum !N~o. 3 of the Plate. 



In both photographs I suspect the indications of other lines, which 

 are too faint to permit any certain conclusion to be formed about 

 them, whether they are true lines, or imperfections only of the film. 



[The continuous spectra of the stars of the Trapezium can be seen 

 on the plate from about F to X 3574 ; but they are very faint beyond 

 X 3660.— May 7.] 



The Visible Spectrum. 



a. Brightest line. — In 1872,* I stated as the result of numerous 

 direct comparisons of this line with the brightest line in the spectrum 

 of nitrogen that the nebular line was " sensibly coincident with the 

 middle of the less refrangible line of the double line of nitrogen." To 

 avoid repetition I will call this line JST 1 . Except where it is otherwise 

 stated, I use this line of nitrogen simply as a fiducial point in the 

 spectrum, without any reference to its chemical significance. 



In a still more critical examination of the position of the nebular line 

 for the purpose of determining whether there was any indication of 

 relative motions of the gaseous nebulas in the line of sight, I found 

 some experimental difficulty from the circumstance that the nebular 

 line is narrow and defined while N x is nebulous. I was fortunate to 

 find a more suitable fiducial line of comparison in a narrow line of 

 lead which falls almost upon the middle of N^t In December, 1872, 

 I compared this line directly with N l5 and found it sufficiently near 

 in position to serve as a fiducial line of comparison. 



Six other gaseous nebulae were also examined, each on several 



* " On the Spectrum of the Great Nebula in Orion, &c.," ' Boy. Soc. Proc.,' 

 vol. 20, 1872, p. 383. 



f " On the Motions of some of the Nebulae towards or from the Earth," ' Boy. 

 Soc. Proc.,' vol. 22, 1874, p. 252. 



f X 3752-0 

 \ 3741-0 



/ 3285-0 

 1 3275-0 



.3060-0 



/ 3053-0 

 I 3047-0 



