174 Mr. J. N. Lockyer. 



Qen. Cat. No. \ofchiefline. 



1179 Orion neb. 5017 



1567 501-0 



2102 500-8 



2197 493-4 



2581 499-8 



2917 500-6 



4066 499-8 



4361 497-2 



4390 504-9 



4403 499-8 



4407 499-4 



4510 504-4 



4628 501-9 



The mean wave-length given by this series is 500*36, and the 

 extreme values are 493'4 504 - 9. 



1871. The following observations of the line in question were 

 made by Vogel in 1871.* 



Orion nebula, 13th January, 1871 500"3 



19th March, 1871 500"2 



General Catalogue 4234 500'5 



4373 500-7 



4390 500-5 



4447 500-7 



4510 500-8 



These measurements differ by no less than 60 units of the scale now 

 adopted ; the mean value is 500*53. 



1872. Dr. Huggins writes : " The line of nitrogen, when compared 

 with it (the nebula line), appeared double, and each component 

 nebulous, and broader than the line of the nebula. This latter line 

 was seen on several nights to be apparently coincident with the middle 

 of the less refrangible line of the double line of nitrogen. "f 



This observation, however, obviously left us in the same position as 

 that of 1868, as far as the wave-length of the nebula line was con- 

 cerned. 



1874. In a paper " On the Motions of some of the Nebulae towards 

 or from the Earth" communicated to the Society in 1874, Dr. Huggins 

 wrote : J 



" The brightest line in the nebular spectrum is not sufficiently 

 coincident in character and position with the brightest line in the 



* ' Bothk. Beob..' Leipzig. 1872. 

 t ' Roy. Soc. Proc.,' vol. 20, p. 383. 

 J ' Roy. Soc. Proc.,' vol. 22, p. 252. 



