42 



Dr. W. Huggins and Mrs. Huggins. [May 2, 



which. I found for the first time in 1888, are present ; and on a further 

 scrutiny of the plate I discovered two other pairs of lines, most pro- 

 bably rhythmically connected with them, in the still more refrangible 

 region, the last pair, accompanied by a third line, being near the 

 ultra-violet limit of ex-terrestrial light.* 



I was also able to see faintly two of the bright lines which I have 

 described as present across the continuous spectra of the brighter 

 stars of the Trapezium in my photograph of 1888. It is not quite 

 certain whether these very faint and short lines are really due to the 

 matter of the nebula proper, or have come upon the plate in con- 

 sequence of the stars of the Trapezium having fallen accidentally upon 

 the slit for a time too short to impress the continuous part of their 

 spectra. ISTo trace of a continuous spectrum can be seen upon the 

 plate, but these lines in the plate of 1888 do extend beyond the con- 

 tinuous spectra of the stars of the Trapezium, 



In the diagram which follows I have indicated the positions of the 

 slit upon the nebula relatively to the Trapezium and the well-known 

 three stars near it, for the photographs of 1882, 1888, and 1889. 



I regret extremely that bad weather has made it impossible for me 

 to work out the circumstances on which depended the disappearance 

 of the strong line about X 3730. Both the photographs which show 

 this line include two stars of the Trapezium, and it may possibly be 

 that this strong line is associated with the groups near it in the 

 spectra of the stars, and may therefore come out in those parts of the 

 nebula only which are more condensed. A few photographs with the 

 slit differently placed upon the nebula would doubtless have thrown 

 light upon this point. The suggestion presents itself strongly that 

 the mottled and broken-up character of the nebular matter, shown in 

 Lord Rosse's drawing from eye observations, and much more strik- 

 ingly brought out in the recent photographs of Mr. Common and 

 Mr. Roberts, may be connected with differences of spectrum in the 

 photographic region, though in the visible region there is no known 

 alteration of the spectrum of the four bright lines, except, it may be, 

 some small differences of relative brilliancy of the lines. 



Until next winter we cannot go beyond the new information which 

 these photographs give to us. On the plate of the photograph of 

 1889 two pairs of spectra for comparison were taken : — two spectra, 

 one above and one below the nebular spectrum, of burning magne- 

 sium ; and two spectra, similarly placed, of the light of the sky. 



From the photographs of 1888 taken with a narrow slit, the 

 position which I gave in 1882 to this line is shown to be, as I 

 expected from the wide slit then used, approximate only. I find 

 from the later photograph that the wide slit had caused the strong 



* " On the Limit of Solar and Stellar Light in the Ultra-violet Part of the 

 Spectrum." Infra, p. 133. 



