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SECTION I.

INTEREST ATTACHING TO THE PHYSICAL STUDY OF COMETARY

LIGHT.

We have seen what the telescope has taught us of the structure of comets, so complex and wonderfully mobile, so different in this respect from that of the planets or the sun. On the one hand we see solid or liquid bodies, bearing the most striking analogy to the terrestrial globe, surrounded like it by atmospheres of comparatively small extent, stable in every portion ; these are the planets, the moon, and the satellites of the planets. As regards the sun and the stars-which shine, like the sun, by their own light, and are, like him, as everything leads us to suppose, foci of light and heat to other planetary groups-if these bodies are incandescent gaseous masses, their condensation is so enormous and their physical constitution is comparatively so stable, that the changes of which they are perpetually the theatre have no appreciable effect upon their equilibrium. In comparison with comets they are permanent stars; while comets seem to be nothing more than clouds-wandering nebulæ, to employ the expression of Laplace, who has but reproduced in a more happy form the term so happily applied by Xenophanes and Theon of Alexandria.

But it is not merely by its concentration in the field of a telescope that the light of a comet may be made subservient

to the study of its physical constitution; the undulations of which it is composed, after passing through the depths of space and arriving at the confines of the atmosphere, after passing through the atmosphere and penetrating the crystal lens of the instrument, retain certain distinctive qualities by which the savant who subjects it to analysis may distinguish whether this light has emanated directly from the body itself or, on the contrary, has undergone reflexion within the cometary mass, and is consequently only light reflected from the sun. Other methods of analysis will permit us to penetrate yet more deeply into the inmost constitution of a comet and its different parts, and the light with which it shines is again the agent to which we have recourse, and which will reveal to us the chemical nature of the cometary matter. So that only one more difficulty remains to be surmounted in order to unveil completely the composition of these once mysterious bodies; that is, to penetrate actually and materially into the interior of a cometary mass. And, as we shall shortly see, there is great probability of such an event being realised, if it has not already partially happened. In any case we have already said enough to show the interest attaching to the study of cometary light, the subject of the following sections of this chapter.

SECTION II.

TRANSPARENCY OF NUCLEI, ATMOSPHERES, AND TAILS.

Visibility of stars through the atmospheres and tails of comets; ancient and modern observations upon this point-Are the nuclei of comets opaque, or transparent like the atmospheres and tails ?-Reported eclipses of the sun and moon produced by comets.

THE visibility of stars, even of very small ones, through the coma and tails of comets is a fact which had been observed by the ancients. Aristotle in his Meteorology mentions the stars seen by Democritus notwithstanding the interposition of a comet. Seneca says likewise, in his Quæstiones Naturales, 'that we may see stars through a comet as through a cloud;' and further on, 'the stars are not transparent, and we can see them through comets-not through the body of the comet where the flame is dense and solid, but through the thin and scattered rays which form the hair; it is through the intervals of the fire, not through the fire itself, that you see.' Humboldt, in quoting this last passage, 'per intervalla ignium, non per ipsos vides,' adds: 'This last remark was unnecessary, for it is possible to see through a flame the thickness of which is not too great.' This is true; but Seneca has merely recorded the fact that up to his time stars had been seen behind the tail or coma, not behind the nucleus itself. The want of the telescope did not, in fact, permit the ancients to distinguish the body or nucleus of a comet, even when the comet had a nucleus.

Modern astronomers themselves are not in a better position, since all observed occultations of stars by comets, one alone excepted, refer to the interposition of the nebulosity forming the coma, not to that of the nucleus properly so called. We here mention the principal instances observed, and more especially those which have been invoked to prove the transparency of cometary light, beginning with the single exception above referred to, of which Arago gives the following account: 'On the 27th of October, 1774, Montaigne saw at Limoges a star of the sixth magnitude in Aquarius through the nucleus of a small comet.'* Let us now proceed to the others.

On November 9, 1795, Sir William Herschel distinctly perceived a double star of the eleventh or twelfth magnitude through the central part of the nebulosity of a comet. The two component stars, one of which was much fainter than the other, were both clearly visible. The comet was Encke's, which is generally destitute of nucleus, and very rarely exhibits more than a faint condensation of light in the centre of its nebulosity. On November 7, 1828, Struve saw in the centre of the same comet a star of the eleventh magnitude, which for a moment he mistook for a cometary nucleus, and whose brightness appeared in no respect diminished. Now, the thickness of the nebulosity interposed was not less than 310,000 miles. This is the observation upon which, as we have seen, M. Babinet has founded his calculation of the mass and density of the nebulosity itself. According to an observa

* [I myself saw the nucleus of Halley's comet at its apparition in 1835 pass over a star, when I was at the Cambridge Observatory. I remember the circumstance distinctly, and my impression is that there was no diminution at all in the brightness of the star. The printed record of my observation runs as follows: 'Sept. 25, 9h 45m to 12h. During the whole time the comet (seen with the equatorial 3 inch aperture) appeared to continue changing its figure. It passed over three stars (the nucleus covering one), which were distinctly visible during the whole time.'-Cambridge Observations, vol. viii., 1835, p. 216.-ED.]

tion made at Geneva twenty-one days later by M. Wartmann a star of the eighth magnitude was, on the contrary, completely eclipsed by the comet. It is interesting to compare these two observations, which show the comet's condensation between the dates mentioned; in this interval the volume of the nebulosity had become reduced to one-eighth, and there must have been a corresponding luminous condensation and increased brilliancy, which would explain the occultation seen by Wartmann. In April 1796 Olbers remarked a similar fact in respect to a star of the sixth or seventh magnitude, which, hardly weakened in intensity, appeared a little to the north of the centre of the nebulosity; the star, therefore, was not occulted by the nucleus, but its light was sufficiently bright to render the nucleus for some time invisible in its vicinity.

Cacciatore observed, at Palermo, the occultation of a star by the comet of 1819. On the 5th of August,' he states, ‘I observed through the nebulosity, very close to the nucleus, a star which at the most was of the tenth magnitude.'

When we add to these observations that of Struve, who, on October 29, 1824, saw a star of the tenth magnitude at 2′′ from the centre of the comet, without the light of the star being at all diminished; those of Pons and Valz, in 1825, who saw, the former a star of the fifth magnitude, and the latter one of the seventh magnitude occulted by the famous comet of Taurus, it will be seen that the light of comets, not only that of their tails, but also that of their nebulosities in close proximity to the nucleus, is transparent in the highest degree. But is the nucleus properly so called equally transparent? This we have not yet data to determine, since we have no observation of the occultation of a star by a comet, which indicates with certainty the interposition of the nucleus, excepting that mentioned by Montaigne. Pons, in 1825,

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