176 Mr. J. N. Lockyer. 



Four additional measurements were made by Dr. Copeland and 

 Lord Lindsay respectively on September 2, 1877. The reduction of 

 the micrometric measures by means of curves gave the following wave- 

 lengths : 



Lord Lindsay. Dr. Copeland. 



499-5 ' 498-6 



500-1 496-2 



498-5 496-4 



499-0 497-2 



Mean 499'3 49M 



Here, again, it is obvious that the wave-length of the line was by no 

 means certain even to the first three figures. 



1880. Dr. Copeland observed the spectrum of a new planetary 

 nebula in 1880, and obtained the following measures for the chief 

 nebula line : * 



1880, December 3 501-1 



6 501-2 



Dr. Lohse measured the line at 500'6. 



The spectrum of the Stephen- Webb nebula was also observed at the 

 same time, and the line measured at 501"9. 



From these observations the mean wave-length of the nebula line 

 is found to be 501 '2, a value differing widely from that given by 

 Dr. Huggins. 



1882. In Dr. Huggins's important paper on the photographic 

 spectrum of the Orion nebula the only reference to the chief line is as 

 followsrf "The brightest line, wave-length 5005, is coincident with 

 the less refrangible component of the double line which is strongest 

 in the spectrum of nitrogen." The change of wave-length from 

 500'80, the value given in 1868, due to a change in the assumed value 

 of the nitrogen line is made without explanation, which shows that 

 Dr. Huggins did not at that time attach as much importance to such 

 variations as he now seems inclined to do. The latest measures of 

 this line, so far as I know, are those given by Dr. Copeland in 1888. J 

 Although the dispersion employed is not definitely stated, it is 

 remarked that " a sufficiently powerful spectroscope was used." The 

 measures he gives are as follow : 



1886, December . . 5007 Two measures 



1887, January 5003 



1887 5003 One measure. 



* ' Copernicus,' vol. 1, p. 2. 



t ' Roy. Soc. Proc.,' vol. 33, p. 427. 



I ' Monthly Notices,' vol. 48, p. 360. 



