NATURE 



179 



THURSDAY, JULY 9, 1874 



THE COMET 



TO those who are familiar with the triumphs which 

 that most wonderful of modern instruments of 

 research — the spectroscope — has achieved, the short time 

 during which it has been at work will be most forcibly 

 recalled by a reference to the circumstance that the 

 comet which is now, astronomically speaking, a magni- 

 ficent object in the northern sky, is the first one of any 

 considerable brilliancy which has shown itself since the 

 spectroscope has been adapted to the telescope. 



The truly splendid comets which delighted us dur- 

 ing the autumn of 1S58, and for a brief space in 

 the summer of 1861, made their appearance, in fact) 

 during what we may term the pre-spectroscopic age ; 

 for, however little to the credit of modem science 

 it might have been that the spectroscope w-as no 

 employed in their investigation, the fact remains that 

 they were allowed to pass away mere telescopic ob- 

 jects, and that two opportunities were thus lost such as; 

 perhaps, may not offer themselves again to the present 

 generation of men. 



I propose, in the present paper, to state some points 

 of inquiry regarding comets in which the spectroscope 

 may help us, with a view of showing how much 

 closer is our grip of celestial phenomena when physical 

 astronomy, in its widest sense, is superadded to the 

 older astronomy, and to indicate the numerous gains 

 to knowledge which may be hoped for if adequate tele- 

 scopes, properly armed with spectroscopes, are employed 

 both here and in the southern hemisphere upon the 

 present visitor. 



Omitting all reference to the paths of comets 

 round the sun, with which mechanical astronomy has 

 to do, there are perhaps but few points in which the 

 spectroscope cannot help us ; somewhat unfortunately 

 however, there is one in which it appears powerless, and 

 that precisely one of the greatest difficulty in cometary 

 theory. I allude to the apparent sweep of the tail round 

 the sun when the comet is at its perihelion point, which 

 has suggested to Faye a theory of a repulsive force due 

 to solar heat, and which perhaps is one of the most' 

 mysterious phenomena which we witness in the skies. 

 Leaving this aside, however, there are many questions 

 relating to what Sir John Herschel terms their " inte- 

 rior economy," in which, undoubtedly, the guesses of tele- 

 scopic observers may be turned into hard, detailed fact. 



Let us briefly refer to some of these points. 



Generally speaking, as a comet approaches the sun it 

 gets brighter and its tail lengthens, whether the nucleus 

 is intensely stellar, as in the present case, or not ; in some 

 cases a violent action may be observed ; aigrettes, or jets, 

 make their appearance ; and the nucleus, or head, is sur- 

 rounded, or partly surrounded, by envelopes or shells, 

 very obvious and with marked boundaries, and these are 

 visible in some cases at the commencement of the tail. 



Now, of course, if any or all of these luminous pheno- 

 mena were due to the reflection of sunlight by masses of 

 whatever kind not luminous in themselves, then the spec- 

 trum would be the same from all, differing only in intensity, 

 and the spectrum would be the true solar spectrum if there 

 Vol. X. — No. 245 



were light enough, and a dim continuous spectrum if the 

 part of the comet under examination were dim. 



If, on the other hand, the masses were self-luminous 

 and consisted of vapours not too dense, then we should 

 get a characteristic spectrum proving first the existence 

 of vapours driven into incandescence ; and secondly, if 

 the observations went far enough, the precise quality or 

 nature of the vapour would be determined for us by the 

 spectroscope. Thanks to the labours of Donati, Hug- 

 gins, Secchi, Wolf, Rayct, Vogel, and others, the brightest 

 portions of the comets which have appeared since 1864 

 have been examined with the undoubted result that they 

 consist, in part at least, of not very dense incandescent 

 vapour. I say in part, because in some cases the con- 

 tinuous spectrum, which may denote dense vapours, or 

 perhaps vapours of relatively greater molecular complica- 

 tion, or again even glowing solid substances, has been so 

 strong as almost entirely to mask the bright lines or 

 bands by means of which the presence of the rarer or 

 simpler vapours is determined. 



Nor is this all. Not only have lines been seen, but their 

 positions have been determined with some degree of 

 accuracy, although it must be pointed out that the 

 opinions of authorities do not coincide as to the actual 

 materials indicated or as to the interpretation to be put 

 upon the observations. This is not to be wondered at, 

 considering the amazing delicacy of the research and 

 the few opportunities there have yet been of making per- 

 fectly satisfactory determinations. 



The most searching criticism of the results hitherto 

 obtained appeared some little time ago in Poggendorff's 

 Aniialen from the pen of Dr. Vogel (N,a.ture, vol. 

 ix. p. 193), and it will be well to briefly glance at 

 some points which result from his inquiry. Donati, in 

 the first observations of this nature made in 1864, deter- 

 mined the existence of three bright bands, but made no 

 attempt to determine the substance from which the light 

 proceeded. Huggins in 1866 made the first attempt in 

 this direction, and came to the conclusion that, like the 

 nebuke, the comets might be composed of nitrogen, as in 

 the spectrum of the comet visible in that year there was 

 a single line which nearly, if not quite, coincided with one 

 of the brightest lines of that element. In 1S6S, how- 

 ever, the idea of nitrogen comets was abolished, as the 

 idea of nitrogen nebute has been since ; and the three 

 bands, which were again observed in the comets visible 

 in that year, were found to coincide with those of olefiant 

 gas. Hence it was suggested by Huggins that they con- 

 sisted of carbon vapour. He writes : — " The great fixity 

 of carbon seems indeed to raise some difficulty in the way 

 of accepting the apparently obvious inference of these 

 prismatic observations. Some comets have approached 

 the sun sufficiently near to acquire a temperature high 

 enough to convert carbon into vapour. Indeed, for these 

 comets a body of great fixity seems to be necessary. . . 

 If the substance of the comet be taken to be pure carbon, 

 it would appear that the nucleus had been condensed 

 from the gaseous state in which it existed at some former 

 period. ... If we were to conceive the comet to 

 consist of a compound of carbon and hydrogen . . . 

 other difficulties would arise in connection with the de- 

 composition we must then suppose to take place . . ." 



It is clear that Mr. Huggins' opinion is that a comet 



