[Footnote 174: _Trans. Am. Phil. Soc._, vol. vi., p. 274.]
[Footnote 175: _Memoir of Caroline Herschel_, p. 327.]
[Footnote 176: _Phil. Trans._, vol. xxv., p. 2240.]
[Footnote 177: _Ibid._, vol. xl., p. 182.]
[Footnote 178: _Ibid._, vol. xlv., p. 586.]
[Footnote 179: _Mem. R. A. S._, vol. i., pp. 145, 148.]
[Footnote 180: _American Journal of Science_, vol. xlii., p. 396.]
[Footnote 181: _Phil. Trans._, vol. xxxviii., p. 134. Father Secchi, however, adverted to a distinct mention of a prominence observed in 1239 A.D. A description of a total eclipse of that date includes the remark, "Et quoddam foramen erat ignitum in circulo solis ex parte inferiore" (Muratori, _Rer. It. Scriptores_, t. xiv., col. 1097). The "circulus solis" of course signifies the corona.]
[Footnote 182: _Phil. Trans._, vol. lxix., p. 114.]
[Footnote 183: _Trans. Am. Phil. Soc._, vol. vi., 1809, p. 267.]
[Footnote 184: _Annuaire_, 1846, p. 460.]
[Footnote 187: _Mem. R. A. S._, vol. xxi., p. 82.]
[Footnote 190: _Le Soleil_, t. i., p. 386.]
[Footnote 191: By Williams and Stanistreet, _Mem. R. A. S._, vol. xxi., pp. 54, 56. Santini had made a similar observation at Padua in 1842. Grant, _Hist. Astr._, p. 401.]
[Footnote 192: Lassell in _Month. Not._, vol. xii., p. 53.]
[Footnote 193: _Comptes Rendus_, t. xxxiv., p. 155.]
[Footnote 194: _Optische Untersuchungen_, and _Zeitschrift für populäre Mittheilungen_, Bd. i., 1860, p. 201.]
In the course of his early gropings towards a law of the planetary distances, Kepler tried the experiment of setting a planet, invisible by reason of its smallness, to revolve in the vast region of seemingly desert space separating Mars from Jupiter.[195] The disproportionate magnitude of the same interval was explained by Kant as due to the overweening size of Jupiter. The zone in which each planet moved was, according to the philosopher of Königsberg, to be regarded as the empty storehouse from which its materials had been derived. A definite relation should thus exist between the planetary masses and the planetary intervals.[196] Lambert, on the other hand, sportively suggested that the body or bodies (for it is noticeable that he speaks of them in the plural) which once bridged this portentous gap in the solar system, might, in some remote age, have been swept away by a great comet, and forced to attend its wanderings through space.[197]