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 fuer populaere
Mittheilungen_, Bd. i., 1860, p. 201.]

CHAPTER IV

_PLANETARY DISCOVERIES_

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 Koenigsberg, 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]

These speculations were destined before long to assume a more definite
form. Johann Daniel Titius, a professor at Wittenberg (where he died in
1796), pointed out in 1772, in a note to a translation of Bonnet's
_Contemplation de la Nature_,[198] the existence of a remarkable
symmetry in the disposition of the bodies constituting the solar system.
By a certain series of numbers, increasing in regular progression,[199]
he showed that the distances of the six known planets from the sun might
be represented with a close approach to accuracy. But with one striking
interruption. The term of the series succeeding that which corresponded
to the orbit of Mars was without a celestial representative. The orderly
flow of the sequence was thus singularly broken. The space where a
planet should--in fulfilment of the "Law"--have revolved, was, it
appeared, untenanted. Johann Elert Bode, then just about to begin his
long career as leader of astronomical thought and work at Berlin, marked
at once the anomaly, and filled the vacant interval with a hypothetical
planet. The discover