62 Royal Society : — 



which coincides with the double line of the spectrum of nitrogen ; 

 but, as the diagram shows, the band in the comet is in a small de- 

 gree less refrangible than the line of nitrogen. This difference of 

 refrangibility cannot be attributed to the comet's motion, since at the 

 time the observations were made the comet was approaching the 

 earth. 



The band in the blue is considerably more refrangible than F, and 

 is nearly as refrangible as the group of bright lines in the air-spec- 

 trum which have the numbers 2642, 2669 in the map and tables 

 of my paper "On the Spectra of the Chemical Elements"*. 



The least refrangible of the bands occurs in the yellow part of the 

 spectrum, at about the distance from E of one-third of the interval 

 which separates E from D. 



The spectrum of this comet resembles the diagram given by Do- 

 nati of the spectrum of Comet I., 1864f. The positions of the three 

 bands seen by him appear to agree with those which the bright bands 

 of this comet occupy. 



This comet differs remarkably from the two small comets which 

 I examined, in the much smaller relative proportion of the light which 

 forms a continuous spectrum. In Brorsen's comet, as it now appears, 

 the bright middle part of the nebulosity seems to have a constitu- 

 tion analogous at least to that of the nucleus, and to be self-lumi- 

 nous ; in the other comets the coma, which surrounded a distinctly 

 marked nucleus, gave a continuous spectrum. The three comets 

 resemble each other in the circumstance that the light of the bright 

 central part was emitted by the cometary matter, while the surround- 

 ing nebulosity reflected solar light. 



The telescopic observations of the heads of Donati's comet and of 

 other large comets have shown that the luminous material is not at 

 once driven off into the outer portions of the coma and the tail, but 

 usually forms in front of the nucleus a dense luminous cloud, which 

 for a time seems to be identical in the character of its light with 

 that of the nucleus. It is, I believe, the outer portions only of the 

 coma (which are frequently separated by dark spaces from the nu- 

 cleus) and the tail which the polariscope has shown to shine by re- 

 flected light. 



The positions of the bands in this comet would seem to indicate a 

 chemical constitution different from that of the nebulae which give a 

 spectrum of bright lines. It will be seen in the diagram that, though 

 the brightest of the bands in the spectrum of the comet differs but 

 little in position from the brightest line of the nebulae, the other bands 

 are found in parts of the spectrum widely removed from those in 

 which the other lines of the nebulae occur. The suggestion presents 

 itself whether the broad, nebulous bands may not indicate conditions 

 of temperature and molecular state different from those which occur 

 in the gaseous nebulae. Pliicker has shown that nitrogen and some 

 other substances give totally different spectra under different condi- 

 tions of temperature and tension. The spectrum of this comet, how- 



* Philosophical Transactions, 1864, p. 158. 

 t Astronomische Nachrichten, No. 1488. 



