in the Spectrum of the Nebula in Orion. 207 



mast be determined by comparisons of the third line with the corre- 

 sponding bright line of a hydrogen vacuum-tube. The observations 

 I recorded in 1874, as well as those of Mr. Maunder, of Greenwich 

 (loc. cit., p. 60), " show that the nebula has very little, if any, sensible 

 motion in the line of sight." 



The direct comparison was made on several nights with results 

 similar to the observation that Professor Liveing recorded on 

 February 9, namely, that " the coincidence seemed perfect, the one 

 line falling upon the other." 



We have endeavoured to push this observation further, to determine 

 if the coincidence was absolute, or whether there was a very minute 

 overlapping of the edges of the two lines. The adjustment of the 

 apparatus would throw the hydrogen line, to a very minute extent, 

 towards the blue, at the same time that the earth's motion would 

 degrade the nebular line from the hydrogen line towards the red. 



The f aintness of the third line with a narrow slit does not permit us 

 to speak with absolute certainty as to the extent which the hydrogen 

 seemed to overlap the nebular line towards the blue. 



We were quite certain that the hydrogen line did overlap the 

 nebula slightly towards the blue, but we were unable to determine 

 whether the overlapping corresponded accurately to the earth's 

 motion at the time of observation. It appeared to do so approxi- 

 mately, which would support my former conclusion, that the " nebula 

 has very little, if any, sensible motion in the line of sight." 



PART II. 



On the Character of the Principal Line in the Spectrum of the Nebula 



in Orion. 



In our paper last year (loc. cit., p. 53) I stated that " my own obser- 

 vations of this line, since my discovery of it in 1864 show 



the line to become narrow as the slit is made narrow, and to be 

 sharply and perfectly defined at both edges." We gave also the cor- 

 roborative evidence of two accurate observers who have made a 

 special study of the spectrum of the gaseous nebulae, Professor Vogel 

 and Dr. Copeland. 



Since last year the defining power of the spectroscopes has been 

 improved by two new object-glasses by Sir Howard Grubb. The 

 nebular line has been subjected on several nights to a very searching 

 examination with different widths of slit ; and with different magni- 

 fying powers on two spectroscopes the one with a single prism of 

 60, the other, the " four-prism " spectroscope (loc. cit., p 49). 



We came to the conclusion that a marked feature of this line is its 

 sharply-defined character on the more refrangible side ; we were 



