1889.] On the Spectrum of the Great Nebula in Orion. 47 



with D 3 , and a line at X 4476. (See also Mr. Taylor, ' Monthly 

 Notices, R.A.S.,' vol. 49, p. 125.) I defer the consideration of these 

 and other faint lines which I have often suspected in the faint con- 

 tinuous spectrum of the nebula, as in consequence of the great strain 

 upon the eyes from my recent direct comparison of the spectrum of 

 the nebula with the spectrum of burning magnesium, I was not able 

 during the very few fine nights when Orion was favourably situated 

 to undertake an examination for these very faint lines. 



Comparisons with the Magnesium-flame Spectrum. 



In 1882, Dr. Copeland in his paper on Schmidt's Nova Oygni,* 

 remarked in a footnote, "that it is worthy of note that this line 

 (X 5006*5 of burning magnesium) almost absolutely coincides with 

 the brightest line in the planetary nebulae." 



This line, namely, the bright edge of the first band in the mag- 

 nesium-flame spectrum, is very near in position to the brightest 

 nebular line. We have seen that the wave-length of this line in the 

 nebulas (1) is 5004*6 to 5004*8 ; now the wave-length of the end of 

 the magnesium-flame band is 5006" 5, consequently it does not coin- 

 cide with the nebular line but falls on the less refrangible side at 

 a distance of X 0002 nearly from that line. 



The wave-length of the termination of the magnesium-flame band 

 is, as determined by — 



Lecoq de Boisbaudran 5006*0 



Watts 5006*5 



Liveing and Dewarf 5006*4 



I have recently redetermined the position of the end of the band, 

 by direct comparison with the solar iron line given by Angstrom at 

 ' X 5006*58. 



My result places the magnesium-flame band line at X- 5006*5 . . . (5). 



In a paper read before the Hoyal Society in 1887, J Mr. Lockyer 

 says : — " Only seven lines in all have been recorded up to the present 

 in the spectra of nebulae, three of which coincide with lines in the 

 spectrum of hydrogen and three correspond to lines in magnesium. 

 The magnesium lines represented are the ultra-violet low-temperature 

 line at 373, the line at 470 and the remnant of the magnesium fluting 

 at 500, the brightest part of the spectrum at the temperature of the 



* ' Copernicus,' toI. 1, p. 109. 



f ' Roy. Soc. Proc.,' vol. 44, 1888, p. 245. 



X " Researches on the Spectra of Meteorites : a Report to the Solar Physics 

 Committee; communicated to the Royal Society at the request of the Committee," 

 ' Roy, Soc. Proc.,' vol. 43, p. 118. 



