138 Royal Society ; — Dr. W. Huggins on the 



the other lines remain very faint. Under these conditions I have 

 always been able, though with some difficulty, on account of the 

 faint light when the necessary dispersive power (spectroscope B with 

 second or third eyepiece) and a narrow slit are used, to see the 

 line to be double ; but it is narrower than when the gas is more dense, 

 and may be easily mistaken for a single line. I have not yet been 

 able to find a condition of luminous nitrogen in which the line has 

 the same characters as those presented by the line in the nebula, 

 where it is single and of the width of the slit. 



Upon the whole I am still inclined to regard the line in the nebula 

 as probably due to nitrogen. 



If this should be found to be the case, and that the nebular line 

 has originally the refrangibility of the middle of the double line of 

 nitrogen, then we should have evidence that the nebula is moving 

 from the earth. The amount of displacement of the nebular line 

 from the middle of the double line of nitrogen corresponds to a velo- 

 city of 55 miles per second from the earth. At the time of observa- 

 tion the part of the earth's orbital motion, which was from the 

 nebula, was 14*9 miles per second. From the remaining 40 miles 

 per second would have to be deducted the probable motion from the 

 nebula due to the motion of the solar system in space. This esti- 

 mation of the possible motion of the nebula can be regarded as only 

 approximate. 



If the want of accordance of the line in the nebula with the middle 

 of the double line of nitrogen be due to a recession of the nebula in 

 the line of sight, there should be a corresponding displacement of 

 the third line as compared with that of hydrogen. For reasons 

 which will be found in a subsequent paragraph, I have not been able 

 to make this comparison with the necessary accuracy. 



In my former paper* I gave reasons against supposing so large a 

 motion in the nebula ; these were based on the circumstance that 

 the nebular line falls upon the double nitrogen line, which the pre- 

 sent observations confirm. I was not then able to use a slit sufficiently 

 narrow to show that the nebular line is single and not coincident 

 with the middle of the double line of nitrogen. 



I am still pursuing the investigation of the parts of this inquiry 

 which remain unsettled. 



Second line, — This line was found by my former comparisons 

 to be a little les3 refrangible than a strong line in the spectrum of 

 barium. Three sets of measures give for this line a wave-length 

 of 4957 on Angstrom's scale ; this would show that the line agrees 

 nearly in position with a strong line of iron. At present I am not 

 able to suggest to what substance this line belongs. 



This line is also narrow and defined. I suspect that the brightness 

 of this line relatively to the first line varies in different nebulee. 



Third and fourth line. — My former observations show that these 

 lines agree in position with two lines of the spectrum of hydrogen, 

 that at F and the line near G. 



* Phil. Trans, 1868, pp. 542, 543. 



