184 Mr. J. N. Lockyer. On the Classification [Jan. 10, 



Line Absorption at Perihelion. 



It lias been seen that the first evidence of the appearance of ab- 

 sorption in comets is that afforded by the flutings of manganese and 

 lead, which mask the citron band of carbon. The next indication of 

 absorption is that of the iron fluting at 615. 



Line absorption was observed in Coggia's Comet (1874) by 

 Christie, on July 14th, but he gives no definite wave-lengths for 

 the lines seen. He says : The spectrum of the nucleus was con- 

 tinuous ; it appeared to have traces of numerous bright bands, and 

 three or four dark lines also were seen on several occasions, but 

 owing to passing clouds, they were lost before their position could be 

 determined. One appeared to be between D and E, another on the 

 blue side of b, and a third near F.* 



The perihelion passage of the comet occurred on July 8th. 



There were also evidences of absorption in Comet Wells, as observed 

 at Greenwich. 



" Two dark spaces were seen near F ; the less refrangible one was 

 measured and its wave-length determined as 4862 tenth-metres. It 

 therefore probably is the F line."f 



Polariscopic observations have shown that part of the light received 

 from comets is reflected light, and it has been assumed that it is 

 reflected sunlight that is in question. Dr. Huggins, in his valuable 

 memoir on the Comet 6, 1881 (' Boy. Soc. Proc.,' vol. 33, p. 1), gives 

 a photograph showing absorption lines which he states to be the 

 reflected lines of Fraunhofer. I have not had an opportunity of 

 seeing the original photograph, and it is therefore impossible to 

 speak with confidence, but if the drawing is exact we are not deal- 

 ing with reflected sunlight, for the hydrogen lines are too strong 

 and the relative thicknesses of H and K are dissimilar. But varia- 

 tions from the solar spectrum are to be noticed in the spectrum of a 

 Cygni, and they should be reproduced in a cometary swarm when 

 near the sun. 



An additional argument for this conclusion with respect to 

 Huggins's photograph is the absence of ultra-violet continuous spec- 

 trum. As shown in the lithograph, the continuous spectrum appears 

 to end rather abruptly, just in front of the group of bright flutings 

 3883. If we had to deal with reflected sunlight this could nob 

 possibly happen. 



In describing the spectrum of the Great Comet of 1882, J as seen on 

 the morning of September 18th, the day after the perihelion passage, 

 Copeland refers to dark lines which he supposes to be the ordinary 



* ' Greenwich Spectroscopic Observations/ 1875, p. 121. 

 f ' Monthly Notices,' vol. 42, p. 410. 

 J ' Copernicus,' \ol. 2, p. 238. 



