1889.] On the Spectrum of the Great Nebula in Orion. 45 



nights, with the result that "in no instance was any change of relative 

 position of the nebular line and the lead line detected."* 



In the simultaneous observation of the nebular line and the lead 

 line it was found if the lead line was made rather less bright than 

 the line of the nebula, the small excess of apparent breadth of this 

 latter line appeared to overlap the lead line to a very small amount on 

 its less refrangible side, so that the more refrangible sides of the two 

 lines appeared to be in a straight line across the spectrum. The 

 closeness of position of the two lines was shown by the observation 

 that when the line of the nebula passed across the field of the spec- 

 troscope, and the lead line was thrown in, the lead line was not seen, but 

 only an increase in brightness of the nebular line. By comparing the 

 end of the nebular line near the Trapezium where it is refined to a 

 point, I estimated that the difference of position of the middle of the 

 lead line and that of the nebular line might be possibly from A, 0000*2 

 to X 0000*3. j Some recent measures of the position of the lead line 

 with the middle of 'N 1 show that the lead line is about X 0000*12 more 

 refrangible. 



These direct comparisons of the nebular line with the lead line 

 confirmed, therefore, my former conclusion, that the brightest line in 

 the gaseous nebulse is very near Is^, when seen under a dispersion equal 

 to nearly eight prisms of 60, namely, 36° 25' from A to H. 



This result is based on direct comparisons, on twenty-four different 

 nights, with N-, or with the line of lead. 



The wave-length of N 1 has been determined by Kirchhoff, Thalen, 

 and by myself. Watts' reduction of my measure to wave-lengths is 

 clearly not accordant with my measures of air lines immediately pre- 

 ceding and following this line. I have therefore reduced my original 

 measure to wave-lengths and find for N l the value A. 5004*5. 



Kirchhoff 5004-6 



Thalen 5005-1 



Thalen's value is clearly too high, as Thalen gives for the lead line 

 coincident nearly with N x X 5004*6, and 'N 1 is seen on the more refran- 

 gible side of the solar iron line given by Angstrom as X 5004*9. In 

 Angstrom's map N l is laid down on the more refrangible side of the 

 iron line 5004*9, at about 5004*5. The same position is given to N l 

 in Kirchhoff' s map. 



I have made a new determination of the position of N l5 using the 

 second spectrum of a grating 17,300 to the inch, relatively to the solar 

 iron line at 5004*9 according to Angstrom. 



The value came out X 5004*6, which agrees with Kirchhoff's value, 



# " On the Motions of some of the Nebulae towards or from the Earth," ' Roy. 

 Soc. Proc.,' vol. 22, 1874, p. 253. 

 f Ibid., p. 252. 



