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PHYSICAL CONSTITUTION OF COMETS. 



Herschel, however, distinctly saw a star of the 16th magnitude through the 

 very centre of the head of the comet which appeared in the year 1795. Prof. 

 Struve, on the 28th of Nov., 1828, saw a star of the llth magnitude, so small 

 as to be invisible to the naked eye, through the centra of Encke's comet. 



The parts of the nebulosity which immediately surround the nucleus appear 

 to be much less luminous than the more distant parts, as if the nebulous atmo- 

 sphere became less dense and more transparent near its surface. At some dis- 

 tance from its centre the luminous effect suddenly increases so as to assume the 

 appearance of rings of light aromid the nucleus ; sometimes two, three, or 

 more, such concentric rings have been perceived surrounding comets, separated 

 by dark intervals. 



It must be understood, that the arrangement which produces the appearance 

 of these concentric rings, is. in reality, a succession of spherical shells of va- 

 por or nebulous matter, which alternately increases and decreases in density, 

 forming an atmosphere of various densities around the comet. This has been 

 illustrated by Arago by comparing it to successive layers of clouds of different 

 heights surrounding our globe. To perfect the analogy we have only to im- 

 agine three transparent spherical shells, still retaining the peculiar optical quality 

 which distinguishes them from the pure air by which they are separated. 



The memorable comet of 181] was enveloped by a nebulosity the thickness 

 of which measured 30,000 miles above the surface or nucleus of the comet. 



The thickness of the nebulosity of the comet of 1807 was 36,000 miles; 

 that of 1799 was 24,000 miles 



In comets which have a tail, the rings we have now adverted to are not com- 

 plete : they terminate at the edges of the tail, and are open through the space 

 where the tail abuts upon the head. 



THE NUCLEUS. 



Some difference of opinion prevails among observers whether comets real- 

 ly have nuclei at all. W r hen, however, they are supposed to have them, they 

 are generally admitted to be small, and of doubtful magnitude. The following . 

 measurements are given by Arago as having been ascertained, or, at least, as- i 

 sumed : 



The comet of 1798 had a nucleus whose diameter was 30 miles ; that of 

 1805, 35 miles ; the comet of 1799, 450 miles ; the comet of 1807, 650 miles ; 

 and the second comet of 1811, about 3,000 miles. 



Those who deny the existence of solid matter within the nebulosity of comets, 

 maintain that even the most brilliant and most conspicuous of those bodies, and 

 those which have presented the strongest resemblance to planets, are complete- 

 ly transparent. It might be supposed that a fact so simple as this, in this age 

 of astronomical activity, could not remain doubtful ; but it must be considered, 

 that the combination of circumstances which alone would test the truth of this 

 doctrine, is of rare occurrence. It would be necessary that the centre of the 

 head of the comet, although very small, should pass critically over a star, in 

 order to ascertain whether such star is visible through it. With comets having 

 extensive nebulosity without nuclei, this has sometimes occurred ; but. we have 

 not had such satis> r 'ictory examples in the more rare instances of those which 

 have distinct nuclei. The following examples are, however, adduced : 



On the 23d of October, 177 1, Montaigne, at Limoges, saw a star of the 6th 

 magnitude through the nucleus of a small comet ; but, unfortunately, he has 

 not stated through what part, of the nucleus he saw it, and the power of the 

 telescope he used was too limited to entitle his observations to much consider- 

 ation. 



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