252 . Dr. W. Huggins on the Motions of some [Mar. 26, 



myself of the nights sufficiently fine (unusually few even for our un- 

 favourable climate) to make observations on this point. 



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

 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 nebulae is narrow and defined, while the line of 

 nitrogen is double, and each component is nebulous and broader than 

 the line of the nebulae. The nebular line is apparently coincident 

 with the middle of the less refrangible line of the double line of ni- 

 trogen*. 



The third and fourth lines of the nebular spectrum are undoubtedly 

 those of hydrogen ; but their great faintness makes it impossible to use 

 them as lines of comparison under the necessary conditions of great dis- 

 persive power, except in the case of the brightest nebulae. 



The second line, as I showed in the paper to which I have referred, is 

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

 inconveniently faint, except in the brightest nebulse. 



In the course of some other experiments my attention was directed 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 nebulae. 



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 fiducial line of com- 

 parison. 



I am. not prepared to say that the coincidence is perfect ; on the con- 

 trary, I believe that if greater prism-power could be brought to bear 

 upon the nebulae, the line in the lead spectrum would be found to be in 

 a small degree more refrangible than the line in the nebulae. 



The spectroscope employed in these observations contains two com- 

 pound prisms, each giving a dispersion of 9 6' from A to H. A mag- 

 nifying-power of 16 diameters was used. 



In the simultaneous observation of the two lines it was found that if 

 the lead line was made rather less bright than the nebular line, the small 

 excess of apparent breadth of this latter line, from its greater brightness, 

 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. This line could be 

 * Proceedings of the Royal Society, vol. xx. p. 380. 



