[50] Newton conjectured that comets formed “the aliment by which suns are sustained,” his opinion being that the former bodies finally coalesced with the suns round which they revolved. He remarked:—“I cannot say when the Comet of 1680 will fall into the Sun,—possibly after five or six revolutions; but whenever that time shall arrive, the heat of the Sun will be raised by it to such a point that our globe will be burnt and all the animals upon it will perish.”
[51] During the seven months from May to November 1890 I noted ninety-five telescopic meteors while engaged in comet-seeking.
[52] A list of these was published in the ‘Monthly Notices,’ vol. 1. p. 466. See also ‘Monthly Notices,’ vol. xlv. pp. 93 et seq.
[53] There are several forms of this instrument: for particulars of construction and use the reader is referred to Thornthwaite’s ‘Hints on Telescopes,’ and Chambers’s ‘Astronomy,’ 4th ed. vol. ii.
[54] Mr. George Knott, of Cuckfield, mentions that the radius of the first bright diffraction-ring of a stellar image, for a 7-1/3-inch aperture, is 1″·01, and for one of 2 inches 3″·70 (‘Observatory,’ vol. vi. p. 19; see also vol. i. pp. 107 and 145). Mr. Dawes is quoted as giving 1″·25 for a 7-inch, 1″·61 for a 5½-inch, and 3″·57 for a 2·4-inch. These figures exceed the theoretical values, if the latter are adopted from Sir G. B. Airy’s ‘Undulatory Theory of Optics,’ where, for mean rays, we have:—
Radius of object-glass in inches × radius of bright ring in seconds = 3·70.
[55] The number visible to different persons varies according to eyesight. Some observers see thirteen or fourteen stars in the Pleiades, while others cannot discern more than six or seven.
[56] About 2 seconds. Sir W. Herschel found the diameter of α Lyræ with a power of 6450 to be 0″·3553. Tycho Brahe, before the invention of telescopes, estimated the diameter of Sirius as 120″. J. D. Cassini, with a telescope 35 feet long, found the diameter of the same star 5″.
[57] Dr. Doberck gives some valuable information with reference to the computation of binary star-orbits in ‘The Observatory,’ vol. ii. pp. 110 and 140.
[58] The star α Canis Minoris (Procyon) was also inferred to be a binary and to have a similar period. Several close companions appear to have been discovered (Ast. Nach. no. 2080). But Prof. Hall, using the 25·8-inch refractor at Washington, says:—“I have never been able to see any of these companions that would stand the test of sliding and changing the eyepiece, turning the micrometer, &c., and am therefore doubtful of their existence. This is an interesting star for the powerful telescopes of the future.” It has been surmised that the companion is a non-luminous one, and therefore invisible.