Myths and Marvels of Astronomy
be seen passing over the sun, and 'this point will very probably be the
planet whose path I observed on March 26, 1859.' 'I am persuaded,' he
added, 'that this body is the planet, or one of the planets, whose
existence in the vicinity of the sun M. Leverrier had made known a few
months ago' (referring to the preliminary announcement of results which
Leverrier published afterwards more definitely).
Leverrier, when the news of Lescarbault's observation first reached him,
was surprised that the observation should not have been announced
earlier. He did not consider the delay sufficiently justified by
Lescarbault's statement that he wished to see the spot again. He
therefore set out for Orgeres, accompanied by M. Vallee. 'The
predominant feeling in Leverrier's mind,' says Abbe Moigno, 'was the
wish to unmask an attempt to impose upon him, as the person more likely
than any other astronomer to listen to the allegation that his prophecy
had been fulfilled.'
'One should have seen M. Lescarbault,' says Moigno, 'so small, so
simple, so modest, and so timid, in order to understand the emotion with
which he was seized, when Leverrier, from his great height, and with
that blunt intonation which he can command, thus addressed him: "It is
then you, sir, who pretend to have observed the intra-mercurial planet,
and who have committed the grave offence of keeping your observation
secret for nine months. I warn you that I have come here with the
intention of doing justice to your pretensions, and of demonstrating
either that you have been dishonest or deceived. Tell me, then,
unequivocally, what you have seen."' This singular address did not bring
the interview, as one might have expected, to an abrupt end. The lamb,
as the Abbe calls the doctor, trembling, stammered out an account of
what he had seen. He explained how he had timed the passage of the black
spot. 'Where is your chronometer?' asked Leverrier. 'It is this watch,
the faithful companion of my professional journeys.' 'What! with that
old watch, showing only minutes, dare you talk of estimating seconds. My
suspicions are already too well confirmed.' 'Pardon me, I have a
pendulum which beats seconds.' 'Show it me.' The doctor brings down a
silk thread to which an ivory ball is attached. Fixing the upper end to
a nail, he draws the ball a little from the vertical, counts the number
of oscillations, and shows that his pendulum beats seconds; he explains
also how his profession, requiring him to feel pulses and count
pulsations, he has no difficulty in mentally keeping record of
successive seconds.
Having been shown the telescope with which the observation was made, the
record of the observation (on a piece of paper covered with grease and
laudanum, and doing service as a marker in the 'Connaissance des Temps,'
or French Nautical Almanac), Leverrier presently inquired if Lescarbault
had attempted to deduce the planet's distance from the sun from the
period of its transit. The doctor admitted that he had attempted this,
but, being no mathematician, had failed to achieve success with the
problem. He showed the rough draughts of his futile attempts at
calculation on a board in his workshop, 'for,' said he naively, 'I am a
joiner as well as an astronomer.'
The interview satisfied Leverrier that a new planet, travelling within
the orbit of Mercury, had really been discovered. 'With a grace and
dignity full of kindness,' says a contemporary narrative of these
events,[55] 'he congratulated Lescarbault on the important discovery
which he had made.' Anxious to obtain some mark of respect for the
discoverer of Vulcan, Leverrier made inquiry concerning his private
character, and learned from the village cure, the juge de paix, and
other functionaries, that he was a skilful physician and a worthy man.
With such high recommendations, M. Leverrier requested from M. Rouland,
the Minister of Public Instruction, the decoration of the Legion of
Honour for M. Lescarbault. The Minister, in a brief but interesting
statement of his claim, communicated this request to the Emperor, who,
by a decree dated January 25, conferred upon the village astronomer the
honours so justly due to him. His professional brethren in Paris were
equally solicitous to testify their regard; and MM. Felix Roubaud,
Legrande, and Caffe, as delegates of the scientific press, proposed to
the medical body, and to the scientific world in Paris, to invite
Lescarbault to a banquet in the Hotel du Louvre on January 18.
The announcement of the supposed discovery caused astronomers to
re-examine records of former observations of black spots moving across
the sun. Several such records existed, but they had gradually come to be
regarded as of no real importance. Wolff of Zurich published a list of
no fewer than twenty such observations made since 1762. Carrington added
many other cases. Comparing together three of these observations, Wolff
found that they would be satisfied by a planet having a period of
revolution of 19 days, agreeing fairly with the period of rather more
than 19-1/3 days inferred by Leverrier for Lescarbault's planet. But
the entire set of observations of black spots require that there should
be at least three new planets travelling between Mercury and the sun.
Many observers also set themselves the task of searching for Vulcan, as
the supposed new planet was called. They have continued fruitlessly to
observe the sun for this purpose until the present time.
While the excitement over Lescarbault's discovery was at its height,
another observer impugned not only the discovery but the honesty of the
discoverer.
M. Liais, a French astronomer of considerable skill, formerly of the
Paris Observatory, but at the time of Lescarbault's achievement in the
service of the Brazilian Government, published a paper, 'Sur la Nouvelle
Planete annoncee par M. Lescarbault,' in which he endeavoured to
establish the four following points:--
First, the observation of Lescarbault was never made.
Secondly, Leverrier was mistaken in considering that a planet such as
Vulcan might have escaped detection when off the sun's face.
Thirdly, that Vulcan would certainly have been seen during total solar
eclipses, if the planet had a real objective existence.
Fourthly, M. Leverrier's reasons for believing that the planet exists
are based on the supposition that astronomical observations are more
precise than they really are.
Probably, Liais's objections would have had more weight with Leverrier
had the fourth point been omitted. It was rash in a former subordinate
to impugn the verdict of the chief of the Paris Observatory on a matter
belonging to that special department of astronomy which an observatory
chief might be expected to understand thoroughly. It is thought daring
in the extreme for one outside the circles of official astronomy (as
Newton in Flamstead's time, Sir W. Herschel in Maskelyne's, and Sir J.
Herschel in the present century), to advance or maintain an opinion
adverse to that of some official chief, but for a subordinate (even
though no longer so), to be guilty of such rash procedure 'is most
tolerable and not to be endured,' as a typical official has said.
Accordingly, very little attention was paid by Leverrier to Liais's
objections.
Yet, in some respects, what M. Liais had to say was very much to the
point.
At the very time when Lescarbault was watching the black spot on the
sun's face, Liais was examining the sun with a telescope of much greater
magnifying power, and saw no such spot. His attention was specially
directed to the edge of the sun (where Lescarbault saw the spot) because
he was engaged in determining the decrease of the sun's brightness near
the edge. Moreover, he was examining the very part of the sun's edge
where Lescarbault saw the planet enter, at a time when it must have been
twelve minutes in time upon the face of the sun, and well within the
margin of the solar disc. The negative evidence here is strong; though
it must always be remembered that negative evidence requires to be
overwhelmingly strong before it can be admitted as effective against
positive evidence. It seems at a first view utterly impossible that
Liais, examining with a more powerful telescope the region where
Lescarbault saw the spot, could have failed to see it had it been there;
but experience shows that it is not impossible for an observer engaged
in examining phenomena of one class to overlook a phenomenon of another
class, even when glaringly obvious. All we can say is that Liais was not
likely to have overlooked Lescarbault's planet had it been there; and we
must combine this probability against Vulcan's existence with arguments
derived from other considerations. There is also the possibility of an
error in time. As the writer in the 'North British Review' remarks,
'twelve minutes is so short a time that it is just possible that the
planet may not have entered upon the sun during the time that Liais
observed it.'
The second and third arguments are stronger. In fact, I do not see how
they can be resisted.
It is, in the first place, clear from Lescarbault's account that Vulcan
must have a considerable diameter--certainly if Vulcan's diameter in
miles were only half the diameter of Mercury, it would have been all but
impossible for Lescarbault with his small telescope to see Vulcan at
all, whereas he saw the black spot very distinctly. Say Vulcan has half
the diameter of Mercury, and let us compare the brightness of these two
planets when at their greatest apparent distances from the sun, that is,
when each looks like a half-moon. The distance of Mercury exceeds the
estimated distance of Vulcan from the sun as 27 exceeds 10, so that
Vulcan is more strongly illuminated in the proportion of 27 times 27 to
10 times 10, or 729 to 100--say at least 7 to 1. But having a diameter
but half as large the disc of Vulcan could be but about a fourth of
Mercury's at the same distance from us (and they would be at about the
same distance from us when seen as half-moons). Hence Vulcan would be
brighter than Mercury in the proportion of 7 to 4. Of course being so
near the sun he would not be so easily seen; and we could never expect
to see him at all, perhaps, with the naked eye--though even this is not
certain. But Mercury, when at the same apparent distance from the sun,
and giving less light than at his greatest seeming distance, is quite
easily seen in the telescope. Much more easily, then, should Vulcan be
seen, if a telescope were rightly directed at such a time, or when
Vulcan was anywhere near his greatest seeming distance from the sun. Now
it is true astronomers do not know precisely when or where to look for
him. But he passes from his greatest distance on one side of the sun to
his greatest distance on the other in less than ten days, according to
the computed period, and certainly (that is, if the planet exists) in a
very short time. The astronomer has then only to examine day after day a
region of small extent on either side of the sun, for ten or twelve days
in succession (an hour's observation each day would suffice), to be sure
of seeing Vulcan. Yet many astronomers have made such search many times
over, without seeing any trace of the planet. During total solar
eclipses, again, the planet has been repeatedly looked for
unsuccessfully--though it should at such a time be a very conspicuous
object, when favourably placed, and could scarcely fail of being very
distinctly seen wherever placed.
The fourth argument of Lescarbault's is not so effective, and in fact he
gets beyond his depth in dealing with it. But it is to be noticed that a
considerable portion of the discrepancy between Mercury's observed and
calculated motions has long since been accounted for by the changed