COMETS. 



35 



by Arago and myself from the Parisian observ- 

 atory on successive nights with very different 

 appearances('*). The great Konigsberg astron- 

 omer, from numerous measurements and theo- 

 retical considerations, concluded " that the 

 outstreaming cone of light departed distinctly, 

 both to the right and left, from the line of di- 

 rection towards the sun ; but always returned 

 to this line again, to pass over to the opposite 

 side ; that the outstreaming cone of light, 

 therefore, as well as the body of the comet it- 

 self, which engenders and throws it out, has a 

 rotatory, or rather a vibratory motion in the 

 plane of the orbit." He found, further, "that 

 the ordinary attractive force of the sun which is 

 exerted upon heavy bodies, is not adequate to ac- 

 count for these vibrations;" and is of opinion 

 ** that they proclaim a power of polarity in the 

 comet, which keeps one semidiameter of the 

 body turned towards, the other semidiameter 

 turned from, the sun ; that the magnetic proper- 

 ty possessed by the earth may present some- 

 thing of an analogous nature ; and should the op- 

 posites of the telluric polarity inhere in the sun, 

 the influence of this might show itself in the 

 precession of the equinoxes." This is not the 

 place for a more particular development of the 

 grounds upon which explanations that accord 

 with the phenomena have been built ; but ob- 

 servations so remarkable(^«), views of such 

 magnitude in reference to the most wonderful 

 class of bodies that belong to our solar system, 

 could not be passed by unnoticed in this sketch 

 of a general picture of nature. 



Notwithstanding the rule according to which 

 the tails of comets increase in size and bright- 

 ness as the perihelion is approached, and are 

 turned from the central body of our system, 

 the comet of 1823 presented ^he remarkable 

 example of two tails, which finmed an angle of 

 160° with each other, and of which one was 

 turned from the sun, as usual, whilst the oth- 

 er was turned towards him. Peculiar modifi- 

 cations of the polarity, and unequal distribu- 

 tion and conduP<;ion of this, may, in the rare 

 instance just quoted, have occasioned a two- 

 fold and uninterrupted effusion of nebulous 

 matter(i^). 



In the natural philosophy of Aristotle, the 

 phenoiriena of comets and the existence of the 

 milky way may be brought into a most strange 

 juxtaposition or connection. The countless 

 multitude of stars which compose the milky 

 way give off a self-igniting or luminous mass ; 

 and the nebulous streak that divides the vault 

 of the heavens is therefore regarded by the 

 Stagirite as a mighty comet, which ceaseless- 

 ly reproduces itself(^^). 



Occultations of the fixed stars by the head 

 or nucleus of a comet, or its immediate vapor- 

 ous envelope, might throw some light upon the 

 physical constitution of these wonderful heav- 

 enly bodies ; but we have no observations 

 which give us unquestionable assurance that 

 any occultation has been observed which was 

 completely central(^') ; for, as we have above 

 observed, there are alternate concentric scales 

 of dense and very rare vapour in the parts ly- 

 ing near the nucleus. On the other hand, 

 there is no question of the fact, that on the 

 29th of September, 1835, the light of a star of 

 the 10th magnitude passed through an extreme- 



ly dense vapour, at the distance of 7 "78 from 

 the central point in the head of Halley's com- 

 et, according to Bessel's very accurate meas- 

 urements ; and that the light of this star suf- 

 fered not the slightest deflection from its rec- 

 tilinear course at any moment of the passage 

 through this vapour(="'). Such an absence of 

 refractive power, if it actually extends to the 

 centre of the nucleus, renders it difficult to im- 

 agine that the matter of comets is at all of the 

 nature of a gasiform fluid. Or, is the absence 

 of refringent power the mere result of an almost 

 infinite rarity of a fluid of this description 1 or 

 does a comet consist of segregated particles, 

 forming a cosmic cloud, which affects the ray 

 of light passing through it in no greater degree 

 than the clouds of our atmosphere, which have 

 no influence in altering the zenith distance of 

 the fixed stars or the edges of the suni A 

 greater or less diminution of the light of a fixed 

 star has indeed been remarked during the pas- 

 sage of a comet over it, but this has been as- 

 cribed, with great propriety, to the lighter 

 ground from which the star appears to stand 

 out during the occultation. 



The most important and decisive observa- 

 tions which have yet been made upon the na- 

 ture of the light of comets, are those of Arago 

 on its polarization. The polariscope of this 

 distinguis^ied philosopher gives us information 

 of the physical constitution of the sun as well 

 as orthat of comets ; the instrument, in a word, 

 informs us whether a ray of light that reaches 

 us after travelling many millions of miles, is 

 direct or reflected light, and whether, in the 

 former case, the source of the ray is a solid, a 

 liquid, or a gaseous body. The light of Capel- 

 la, and that of the great comet of 1819, were 

 examined by the same apparatus : the comet 

 showed polarized and therefore reflected light ; 

 the brilliant star, as was to have been antici- 

 pated, proclaimed itself a self-luminous sun(=^). 

 The existence of polarized light in connection 

 with the comet, however, was not merely made 

 known by the inequality of the images ; on the 

 reappearance of Halley's comet in the year 

 1835, it was still more distinctly indicated by 

 the striking contrast of complementary colours, 

 in accordance with the laws of chromatic po- 

 larization discovered by Arago, in 1811. But 

 it still remains undetermined, even by the beau- 

 tiful experiments just referred to, whether, be- 

 sides the reflected sun-light, comets have not 

 also a light proper to themselves. In some, at 

 least, of the true planets, Venus for example, 

 it appears to be extremely probable that there 

 is an inherent independent capacity to evolve 

 light. 



The variable brightness of comets is not al- 

 ways to be explained from their position in 

 their orbit, and their distance from the sun. It 

 certainly points, in particular individuals, to 

 internal processes of condensation, and of aug- 

 mented or diminished power of reflecting bor- 

 rowed light. In the case of the comet of 1618, 

 as also of the one with a period of three years, 

 Hevelius observed the nucleus to be lessened 

 as the sun was approached, increased as he 

 was quitted ; and this remarkable phenomenon, 

 so long neglected, has lately been again refer- 

 red to and confirmed by Balz, the able astrono- 

 mer of Nismes. The regularity in the altera- 



