The sky at the time was intensely blue, and the sun shining
with a dazzling brightness unknown in European climates.] -- Tr
This latter circumstance indicates, in particular individuals, a denser
mass, capable of reflecting light with greater intensity. Even in
Herschel's large telescope, only two comets, that discovered in Sicily in
1807, and the splendid one of 1811, exhibited well-defined disks;* the one
at an angle of 1 second, and the other at 0.77 seconds, whence the true
diameters are assumed to be 536 and 428 miles.
[footnote] *'Phil. Trans.' for 1808, Part ii., p. 155, and for 1812, Part
i., p. 118. The diameters found by Herschel for the nuclei were 538 and 428
English miles. For the magnitudes of the comets of 1798 and 1805, see
Arago, 'Annuaire', 1832, p. 203.
The diameters of the less well-defined nuclei of the comets of 1798 and 1805
did not appear to exceed 24 or 28 miles.
In several comets that have been investigated with great care, especially in
the above-named one of 1811, which continued visible for so long a period,
the nucleus and its nebulous envelope were entirely separated from the tail
by a darker space. The intensity of light in the nucleus of comets does not
augment toward the center in any uniform degree, brightly shining zones
being in many cases separated by concentric nebulous envelopes.
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