1889.] 



On the Spectrum of the Great Nebula in Orion. 



49 



magnesium-flame band, and so make the determination more dim- 

 es • 



cult ; but if under such circumstances, the nebular line was seen 

 on the more refrangible side of that of magnesium the observation 

 would be much more trustworthy, for in the case of coincidence with 

 magnesium the line would have appeared towards the opposite and 

 less refrangible side of the magnesium line, broadening the mag- 

 nesium line on this side. I considered that the comparison could be 

 made most satisfactorily by the complete superposition of the two 

 spectra, that from burning magnesium being gradually reduced in 

 brightness by the interposition of coloured glass screens, uutil the 

 ground of the spectrum between the successive bright lines of the 

 band of the magnesium-flame spectrum was sufficiently subdued to 

 allow of the nebular line being seen upon it. 



Under these circumstances, if the nebular line had the position 

 which my direct comparisons and the micrometric observations of 

 other observers assign to it, it would be seeu as a bright line at a 

 very small interval within the line ending the band, and to the 

 observer the band would appear to commence with a double line. 



This direct comparison was first successfully made on March 6, 

 1889. The observations were made with the 15" refractor belonging 

 to the Iloyal Society. The spectroscope used has two compound 

 Grubb prisms, each with 5 square inches of base, and giving nearly 

 twice the dispersion of a single prism of 60°, namely, 9° 20' from A 

 to H ; and collimator and telescope of 1'25-inch aperture. An eye- 

 piece magnifying eighteen times was employed. The nebular line 

 was brought upon the cross- wires, and when carefully focussed and 

 clearly seen, the light from burning magnesium was thrown in. This 

 observation is one of great difficulty, especially as the interval to be 

 observed had been purposely reduced by causing the magnesium to 

 fall, for the sake of the greater trustworthiness of the observations, 

 on the more refrangible side of its true position. Although I con- 

 sider the results to be satisfactory, I prefer to say that I, and 

 Mrs. Huggins independently, believed fully at the time that we saw 

 the appearance which all former observations of this line led me to 

 expect, namely, the nebular line to fall within the termination of 

 the magnesium band, and to form with the band-boundary a double 

 line. The relative positions of the two spectra are represented in 

 the diagram across the page. The line at the end of the magnesium 

 band was then brought upon the cross- wires, without any attention 

 being given to the nebular line; when the burning magnesium 

 went out, the nebular line was seen to be at a measurable distance to 

 the left of the intersection of the wires, namely, on the more re- 

 frangible side. 



When the object-glass of the telescope was covered, the magnesium 

 band presented its usual appearance, namely, terminating in a single 

 VOL. xlvi. E 



