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



lute certainty from observations of either of these two bands whether 

 the spectrum is that of hot or cool carbon. 



If, however, the fluting at 483 is present, we can be certain that 

 we have to deal with cool carbon, because no hot carbon fluting falls 

 near that wave-length. In laboratory experiments with Geissler 

 tubes, the passage from one spectrum to the other is very gradual, 

 so that it is not uncommon to have the two spectra superposed, and 

 we might therefore expect a reproduction of this incometary spectra, 

 and I have 110 doubt that the changes from the cool to the hot carbon 

 spectrum are answerable for many of the apparent discrepancies in 

 different observations of the same comet, as I pointed out in November, 

 1887. 



There is another difficulty which must not be passed over ; indi- 

 vidual observations have not in all cases been recorded. Observers 

 have in many cases been in the habit of giving the means of their 

 several observations, and hence the differences in wave-length of the 

 flutings due to the changes from cool to hot carbon, or vice versa,, if 

 they exist, cannot be certainly followed in many cases. 



A discussion of all the recorded observations at my disposal, how- 

 ever, shows that in some comets we have distinct evidence of cool 

 carbon flutings, but as happens with the magnesium fluting at X 500, 

 the observations recording them are comparatively few. The reason 

 is probably the same in both cases, namely, that the temperature 

 being low, the light is consequently excessively feeble, and obser- 

 vations are very difficult. 



We have evidence of cool carbon in Winnecke's Comet, 1868 (peri- 

 helion passage, June 25th). On the 17th June, M. Wolf* recorded 

 three flutings, the wave-lengths of which, as determined by a curve, 

 are about 480, 517, and 560. These differ from their equivalents in 

 the cool carbon spectrum by almost equal amounts, so there can 

 be little doubt that the comet's spectrum was that of cool 

 carbon. 



At the return of this comet in 1877, cool carbon was again ob- 

 served when it was about a month from perihelion. f The perihelion 

 passage occurred on April 17th, and the observation was made on 

 May 15th. Two bands were measured, one at 517, and the other 

 near 483. Another was also seen near 561. As the criterion for cool 

 carbon is the fluting at 483, there can be no doubt of its identity in 

 this case. 



Again, in Brorsen's Comet (1879), perihelion passage 30th March, 

 KonkolyJ observed three flutings at wave-lengths 482'3, 514*6, and 

 560' 5, the first of which coincides very nearly with the characteristic 



* ' Comptes Bendus,' vol. 66, p. 1336. 

 f ' G-reenwich Observations,' 1887, p. 101. 

 J Astr. Nadir.,' No. 2269. 



