and this would then give the
desired opportunity of making the telescopic discovery of the planet. We
should expect on such an occasion to observe the planet as a dark spot,
moving slowly across the face of the sun. The two other planets interior
to the earth, namely, Mercury and Venus, are occasionally seen in the
act of transit; and there cannot be a doubt that if Vulcan exists, its
transits across the sun must be more numerous than those of Mercury, and
far more numerous than those of Venus. On the other hand, it may
reasonably be anticipated that Vulcan is a small globe, and as it will
be much more distant from us than Mercury at the time of its transit, we
could not expect that the transit of the planet of romance would be at
all comparable as a spectacle with those of either of the two other
bodies we have named.

The question arises as to whether telescopic research has ever disclosed
anything which can be regarded as a transit of Vulcan. On this point it
is not possible to speak with any certainty. It has, on more than one
occasion, been asserted by observers that a spot has been seen
traversing the sun, and from its shape and general appearance they have
presumed it to have been an intra-Mercurial planet. But a close
examination of the circumstances in which such observations have been
made has not tended to increase confidence in this presumption. Such
discoveries have usually been made by persons little familiar with
telescopic observations. It is certainly a significant fact that,
notwithstanding the diligent scrutiny to which the sun has been
subjected during the past century by astronomers who have specially
devoted themselves to this branch of research, no telescopic discovery
of Vulcan on the sun has been announced by any really experienced
astronomer. The last announcement of a planet having crossed the sun
dates from 1876, and was made by a German amateur, but what he thought
to have been a planet was promptly shown to have been a small sun-spot,
which had been photographed at Greenwich in the course of the daily
routine work, and had also been observed at Madrid. From an examination
of the whole subject, we are inclined to believe that there is not at
this moment any reliable telescopic evidence of the transit of an
intra-Mercurial planet over the face of the central luminary.

[Illustration: Fig. 40.--The Transit of the Planet of Romance.]

But there is still another method by which we might reason