HISTORY OF PHYSICAL ASTRONOMY.

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separated the envelope from the head, Herschel concluded that thecomet was encompassed by an elastic atmosphere. In accordance withthis supposition, the semicircular ring of light would represent a stratum ofnebulous matter suspended in the atmosphere of the comet, at a consider-able distance from the head.

Observations of many other comets lead to the belief that they essen-tially resemble in structure the comet of 1811, but in every case theapparent form is modified by circumstances depending on the peculiarconstitution of each body. Thus although the dark interval interposedbetween the envelope and the' head was very apparent in the comet of1811, yet in most instances such a phenomenon is not discernible. Itsabsence may be accounted for by supposing the stratum of nebulous matterto be of sufficient depth to cause it to be everywhere visible. Hence,although it be suspended at a considerable elevation above the more con-densed portion of the comet, the intermediate space will not be anywhereapparent.

The extreme tenuity of the matter composing the head and the sur-rounding nebulosity of comets has been already mentioned. On the otherhand, the dimensions to which these parts swell out is frequently enormous.Herschel found that the head of the comet of 1811 had a diameter of127,000 miles*. The diameter of the envelope was found by the sameastronomer to be at least 64.3,000 miles f.

It has been stated that within the head of a comet there is usually abright point termed the nucleus. This is the only part of its structurewhich excites any suspicion of a solid substance. Some astronomers,however, maintain that the nucleus is composed of nebulous matter, likethe other parts of the comet, differing from them only in the superiorcondensation of its particles. An observation of the comet of 1825, bySir John Herschel , tends to support this opinion. Having examined thecomet with a 10-feet Newtonian telescope, he perceived a stellar pointwhich seemed to indicate a solid nucleus. When he sought, however, toobtain a stronger assurance on this point, by the aid of a 20-feet reflector,the illusion was speedily dispelled. The comet now assumed the appear-ance of a milky irresolvable nebula, the centre exhibiting a round disk,with a brightness equal almost to that of a planet, but without a definiteoutline or the least suspicion of a brilliant point];. Bessel’s observationsof Halley’s comet conducted him to a similar conclusion. When thecomet was examined with a low magnifying power, there appeared withinit a bright nucleus of measurable dimensions, but when a higher powerwas applied to the telescope, no such phenomenon was any longer per-ceptible §.

It would seem from the observations of astronomers, that any indicationof a solid substance presented by cometary bodies is invariably dispelledwhen the comet is examined with a magnifying power adequate to discloseits nebulous structure. A decisive test of the real nature of the nucleusof a comet would be afforded by an observation of its transit over a star.Phenomena of this kind are of very rare occurrence; but instances havebeen mentioned by astronomers which go to support the opinion that thenucleus of a comet is composed of a solid substance. Messier has stated,that on the 18th of August, 1774, while engaged in observing the comet

* Phil. Trans., 1812, p. 121. f Phil. Trans., 1812, p. 123.

i Mem. Ast. Soc., vol. ii., p. 48G, et seq.

§ Connaissance des Temps, 1840, Addit., p. 81.