1882.] Spectrum of the Great Nebula in Orion. 427 



Royal Society, were employed which have been described in my paper 

 on " The Photographic Spectra of Stars." * 



The exposure was limited by the coming up of clouds to forty-five 

 minutes. The opening of the slit was made wider than during my 

 work on the stars. 



The photographic plate shows a spectrum of bright lines, and also a 

 narrower continuous spectrum which I think must be due to stellar light. 

 The bright stars forming the trapezium in the " fish's mouth " of the 

 nebula were kept close to the side of the slit, so that the light from 

 the adjacent brightest part of the nebula might enter the slit. 



Outside this stronger continuous spectrum I suspect an exceedingly 

 faint trace of a continuous spectrum. In the diagram which accom- 

 panies this paper the spectrum of bright lines only is shown, which is 

 certainly due to the light of the nebula. 



In my papers on the visible spectrum of the nebula in Orion, and 

 other nebulae, f I found four bright lines. The brightest line, wave- 

 length 5005, is coincident with the less refrangible component of the 

 double line which is strongest in the spectrum of nitrogen. The 



o 



second line has a wave-length of 495/ on Angstrom's scale. The 

 other two lines are coincident with two lines of hydrogen, II/3 or F, 

 and H7 near G. 



In the photograph these lines which had been observed in the visible 

 spectrum are faint, but can be satisfactorily recognised and measured. 

 In addition to these known lines the photograph shows a relatively 

 strong line in the ultra-violet, which has a wave-length 3730 or nearly 

 so. The wide slit does not permit of quite the same accuracy of 

 determination of position as was possible in the case of the spectra of 

 stars. For the same reason I cannot be certain whether this new line 

 is really single, or is double or multiple. In the diagram this line is 

 represented broad to indicate its great relative intensity. 



This line appears to correspond to £ of the typical spectrum of 

 white stars. J In these stars this line is less strong than the hydrogen 

 line near G ; but in the nebula it is much more intense than H7. In 

 the nebula the hydrogen lines F and H7 are thin and defined, while in 

 the white stars they are broad and winged at the edges. The typical 

 spectrum has been added, for the sake of comparison, to the diagram. 



I cannot say positively that the lines of hydrogen between H<y and 

 the line at 3730 are absent. If they exist in the spectrum of the 

 nebula, they must be relatively very feeble. I suspect, indeed, some 

 very faint lines at this part of the spectrum, and possibly beyond 

 X 3730, but I am not certain of their presence. I hope by longer ex- 



* " Phil. Trans.," 1880, p. 672. 



f lb., 1864, p. 437, and 1868, p. 540. Also " Proc. Eoy. Soc," vol. 14, p. 39, 

 and vol. 20, p. 380. 



% " Phil. Trans.," 1880, p. 677. 



VOL. XXXIII. 2 I 



