s orbital verge has been approximately fixed by two
separate investigators.

Professor George Forbes of Edinburgh adopted in 1880 a novel plan of
search for unknown members of the solar system, the first idea of which
was thrown out by M. Flammarion in November, 1879.[1145] It depends upon
the movements of comets. It is well known that those of moderately short
periods are, for a reason already explained, connected with the larger
planets in such a way that the cometary aphelia fall near some planetary
orbit. Jupiter claims a large retinue of such partial dependents,
Neptune owns six, and there are two considerable groups, the farthest
distances of which from the sun lie respectively near 100 and 300 times
that of the earth. At each of these vast intervals, one involving a
period of 1,000, the other of 5,000 years, Professor Forbes maintains
that an unseen planet circulates. He even computed elements for the
nearer of the two, and fixed its place on the celestial sphere;[1146]
but the photographic searches made for it by Dr. Roberts at Crowborough
and by Mr. Wilson at Daramona proved unavailing. Undeterred by
Deichmueller's discouraging opinion that cometary orbits extending beyond
the recognised bounds of the solar system are too imperfectly known to
serve as the basis of trustworthy conclusions,[1147] the Edinburgh
Professor returned to the attack in 1901.[1148] He now sought to prove
that the lost comet of 1556 actually returned in 1844, but with elements
so transformed by ultra-Neptunian perturbations as to have escaped
immediate identification. If so, the "wanted" planet has just entered
the sign Libra, and, being larger than Jupiter, should be possible to
find.

Almost simultaneously with Forbes, Professor Todd set about groping for
the same object by the help of a totally different set of indications.
Adams's approved method commended itself to him; but the hypothetical
divagations of Neptune having scarcely yet had time to develop, he was
thrown back upon the "residual errors" of Uranus. They gave him a
virtually identical situation for the new planet with that derived from
the clustering of cometary aphelia.[1149] Yet its assigned distance was
little more than half that of the nearer of Professor Forbes's remote
pair, and it completed a revolution in 375 instead of 1,000 years. The
agreement in them between the positions determined, on separate grounds,
for the ultra-Neptunian traveller was merely an odd coinc