472 Royal Society : — Dr. W. Huggins on the Motions of 



and from the earth which I had the honour to present to the Royal 

 Society in 1872 appeared to show, from the position in the heavens 

 of the approaching and receding stars, as well as from the relative 

 velocities of their approach and recession, that the sun's motion in 

 space could not be regarded as the sole cause of these motions. 

 " There can be little doubt but that in the observed stellar move- 

 ments we have to do with two other independent motions — namely, 

 a movement common to certain groups of stars, and also a motion 

 peculiar to each star"*. 



It presented itself to me as a matter of some importance to 

 endeavour to extend this inquiry to the nebula?, as it seemed 

 possible that some light might be thrown on the cosmical relations 

 of the gaseous nebulae to the stars and to our stellar system by 

 observations of their motions of recession and of approach. 



Since the date of the paper to which I have referred, I have 

 availed myself of the nights sufficiently fine (unusually few even 

 for our unfavourable climate) to make observations on this point. 



The inquiry was found to be one of great difficulty, from the faint- 

 ness of the objects and the very minute alteration in position in 

 the spectrum which had to be observed. 



At first the inquiry appeared hopeless, from the circumstance 

 that the brightest line in the nebular spectrum is not sufficiently 

 coincident in character and position with the brightest line in the 

 spectrum of nitrogen to permit this line to be used as a fiducial line 

 of comparison. The line in the spectrum of the nebula? is narrow 

 and defined, while^the line of nitrogen is double, and each com- 

 ponent is nebulous and broader than the line of the nebula?. The 

 nebular line is apparently coincident with the middle of the less 

 refrangible line of the double line of nitrogent. 



The third and fourth lines of the nebular spectrum are un- 

 doubtedly those of hydrogen ; but their great faintness makes it 

 impossible to use them as lines of comparison under the necessary 

 conditions of great dispersive power, except in the case of the 

 brightest nebula?. 



The second line, as I showed in the paper to which I have re- 

 ferred, is sensibly coincident with an iron line, wave-length 495*7; 

 but this line is inconveniently faint, except in the brightest nebula?. 



In the course of some other experiments my attention was di- 

 rected to a line in the spectrum of lead which falls upon the less 

 refrangible of the components of the double line of nitrogen. This 

 line appeared to meet the requirements of the case, as it is narrow, 

 of a width corresponding to the slit, defined at both edges, and in 

 the position in the spectrum of the brightest of the lines of the 

 nebula?. 



In December 1872 I compared this line directly with the first 

 line in the spectrum of the Great Nebula in Orion. I was delighted 

 to find this line sufficiently coincident in position to serve as a fidu- 

 cial line of comparison. 



* Proceedings of the Eoyal Society, toI. xx. p. 392. 

 t Proceedings of the Eoyal Society, vol. xx. p. o80. 



