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
estimate of the earth's mass as compared with the sun's, resulting from
the new determination of the sun's distance. However, the arguments
depending on this consideration would not be suited to these pages.
There was one feature in Liais's paper which was a little unfortunate.
He questioned Lescarbault's honesty. He said 'Lescarbault contradicts
himself in having first asserted that he saw the planet enter upon the
sun's disc, and having afterwards admitted to Leverrier that it had been
on the disc some seconds before he saw it, and that he had merely
inferred the time of its entry from the rate of its motion afterwards.
If this one assertion be fabricated, the whole may be so.' 'He considers
these arguments to be strengthened,' says the 'North British Review,'
'by the assertion which, as we have seen, perplexed Leverrier himself,
that if M. Lescarbault had actually seen a planet on the sun, he could
not have kept it secret for nine months.'
This charge of dishonesty, unfortunate in itself, had the unfortunate
effect of preventing Lescarbault or the Abbe Moigno from replying. The
latter simply remarked that the accusation was of such a nature as to
dispense him from any obligation to refute it. This was an error of
judgment, I cannot but think, if an effective reply was really
available.
The Remarks with which the North British Reviewer closes his account may
be repeated now, so far as they relate to the force of the negative
evidence, with tenfold effect. 'Since the first notice of the discovery
in the beginning of January 1860 the sun has been anxiously observed by
astronomers; and the limited area around him in which the planet _must
be_, if he is not upon the sun, has doubtless been explored with equal
care by telescopes of high power, and processes by which the sun's
direct light has been excluded from the tube of the telescope as well as
the eye of the observer, and yet no planet has been found. This fact
would entitle us to conclude that no such planet exists if its existence
had been merely conjectured, or if it had been deduced from any of the
laws of planetary distance, or even if Leverrier or Adams had announced
it as the probable result of planetary perturbations. If the finest
telescopes cannot rediscover a planet which with the small power used by
Lescarbault has a visible disc, within so limited an area of which the
sun is the centre, or rather within a narrow belt of that circle, we
should unhesitatingly declare that no such planet exists. But the
question assumes a very different aspect when it involves moral
considerations. If,' proceeds the Reviewer, writing in August 1860,
'after the severe scrutiny which the sun and its vicinity will undergo
before and after and during his total eclipse in July, no planet shall
be seen; and if no round black spot distinctly separable from the usual
solar spots shall be seen on the solar spots' (_sic_, presumably solar
disc was intended), 'we will not dare to say that it does not exist. We
cannot doubt the honesty of M. Lescarbault, and we can hardly believe
that he was mistaken. No solar spot, no floating scoria, could maintain
in its passage over the sun a circular and uniform shape, and we are
confident that no other hypothesis but that of an intra-mercurial planet
can explain the phenomena seen and measured by M. Lescarbault, a man of
high character, possessing excellent instruments, and in every way
competent to use them well, and to describe clearly and correctly the
results of his observations. Time, however, tries facts as well as
speculations. The phenomena observed by the French astronomer may never
be again seen, and the disturbance of Mercury which rendered it probable
may be otherwise explained. Should this be the case, we must refer the
round spot on the sun to some of those illusions of the eye or of the
brain which have sometimes disturbed the tranquillity of science.'
The evidence which has accumulated against Vulcan in the interval since
this was written is not negative only, but partly positive, as the
following instance, which I take from my own narrative at the time in a
weekly journal, serves to show:--After more than sixteen years of
fruitless watching, astronomers learned last August (1876) that in the
month of April Vulcan had been seen on the sun's disc in China. On April
4, it appeared, Herr Weber, an observer of considerable skill, stationed
at Pecheli, had seen a small round spot on the sun, looking very much as
a small planet might be expected to look. A few hours later he turned
his telescope upon the sun, and lo! the spot had vanished, precisely as
though the planet had passed away after the manner of planets in
transit. He forwarded the news of his observation to Europe. The
astronomer Wolff, well known for his sun-spot studies, carefully
calculated the interval which had passed since Lescarbault saw Vulcan on
March 26, 1859, and to his intense satisfaction was enabled to announce
that this interval contained the calculated period of the planet an
exact number of times. Leverrier at Paris received the announcement
still more joyfully; while the Abbe Moigno, who gave Vulcan its name,
and has always staunchly believed in the planet's existence,
congratulated Lescarbault warmly upon this new view of the shamefaced
Vulcan. Not one of those who already believed in the planet had the
least doubt as to the reality of Weber's observations, and of these only
Lescarbault himself received the news without pleasure. He, it seems,
has never forgiven the Germans for destroying his observatory and
library during the invasion of France in 1870, and apparently would
prefer that his planet should never be seen again rather than that a
German astronomer should have seen it. But the joy of the rest and
Lescarbault's sorrow were alike premature. It was found that the spot
seen by Weber had not only been observed at the Madrid observatory,
where careful watch is kept upon the sun, but had been photographed at
Greenwich; and when the description of its appearance, as seen in a
powerful telescope at one station, and its picture as photographed by a
fine telescope at the other, came to be examined, it was proved
unmistakably that the spot was an ordinary sun-spot (not even quite
round), which had after a few hours disappeared, as even larger
sun-spots have been known to do in even a shorter time.
It is clear that had not Weber's spot been fortunately seen at Madrid
and photographed at Greenwich, his observation would have been added to
the list of recorded apparitions of Vulcan in transit, for it fitted in
perfectly with the theory of Vulcan's real existence. I think, indeed,
for my own part, that the good fortune was Weber's. Had it so chanced
that thick weather in Madrid and at Greenwich had destroyed the evidence
actually obtained to show that what Weber described he really saw,
although it was not what he thought, some of the more suspicious would
have questioned whether, in the euphonious language of the North British
Reviewer, 'the round spot on the sun' was not due 'to one of those
illusions of the eye or of the brain which have sometimes disturbed the
tranquillity of science.' Of course no one acquainted with M. Weber's
antecedents would imagine for a moment that he had invented the
observation, even though the objective reality of his spot had not been
established. But if a person who is entirely unknown, states that he has
seen Vulcan, there is antecedently some degree of probability in favour
of the belief that the observation is as much a myth as the planet
itself. Some observations of Vulcan have certainly been invented. I have
received several letters purporting to describe observations of bodies
in transit over the sun's face, either the rate of transit, the size of
the body, or the path along which it was said to move, being utterly
inconsistent with the theory that it was an intra-mercurial planet,
while yet (herein is the suspicious circumstance of such narratives) the
epoch of transit accorded in the most remarkable manner with the period
assigned to Vulcan. A paradoxist in America (of Louisville, Kentucky)
who had invented a theory of the weather, in which the planets, by their
influence on the sun, were supposed to produce all weather-changes, the
nearer planets being the most effective, found his theory wanted Vulcan
very much. Accordingly, he saw Vulcan crossing the sun's face in
September, which, being half a year from March, is a month wherein,
according to Lescarbault's observation, Vulcan may be seen in transit,
and by a strange coincidence the interval between our paradoxist's
observation and Lescarbault's exactly contained a certain number of
times the period calculated by Leverrier for Vulcan. This was a noble
achievement on the part of our paradoxist. At one stroke it established
his theory of the weather, and promised to ensure him text-book
immortality as one of the observers of Vulcan. But, unfortunately, a
student of science residing in St. Louis, after leaving the Louisville
paradoxist full time to parade his discovery, heartlessly pointed out
that an exact number of revolutions of Vulcan after Lescarbault's March
observation, must of necessity have brought the planet on that side of
the sun on which the earth lies in March, so that to see Vulcan so
placed on the sun's face in September was to see Vulcan through the sun,
a very remarkable achievement indeed. The paradoxist was abashed, the
reader perhaps imagines. Not in the least. The planet's period must have
been wrongly calculated by Leverrier--that was all: the real period was
less than half as long as Leverrier had supposed; and instead of having
gone a certain number of times round since Lescarbault had seen it,
Vulcan had gone twice as many times round and half once round again. The
circumstance that if Vulcan's period had been thus short, the time of
crossing the sun's face would have been much less than, according to
Lescarbault's account, it actually was, had not occurred to the
Louisville weather-prophet.[56]
Leverrier's faith in Vulcan, however, has remained unshaken. He has used
all the observations of spots which, like Weber's, have been seen only
for a short time. At least he has used all which have not, like
Weber's, been proved to be only transient sun-spots. Selecting those
which fit in well with Lescarbault's observation, he has pointed out how
remarkable it is that they show this accord. The possibility that some
of them might be explicable as Weber's proved to be, and that some even
may have been explicable as completely, but less satisfactorily, in
another way, seems to have been thought scarcely worth considering.
Using the imperfect materials available, but with exquisite skill--as a
Phidias might model an exquisite figure of materials that would
presently crumble into dust--Leverrier came to the conclusion that
Vulcan would cross the sun's disc on or about March 22, 1876. 'He,
therefore,' said Sir G. Airy, addressing the Astronomical Society,
'circulated a despatch among his friends, asking them carefully to
observe the sun on March 22.' Sir G. Airy, humouring his honoured
friend, sent telegrams to India, Australia, and New Zealand, requesting
that observations might be made every two hours or oftener. Leverrier
himself wrote to Santiago de Chili and other places, so that, including
American and European observations, the sun could be watched all through
the twenty-four hours on March 21, 22, and 23. 'Without saying
positively that he believed or disbelieved in the existence of the
planet,' proceeds the report, 'Sir G. Airy thought, since M. Leverrier
was so confident, that the opportunity ought not to be neglected by
anybody who professed to take an interest in the progress of planetary
astronomy.'
It is perhaps unnecessary to add that observations were made as
requested. Many photographs of the sun also were taken during the hours
when Vulcan, if he exists at all, might be expected to cross the sun's
face. But the 'planet of romance,' as Abbe Moigno has called Vulcan,
failed to appear, and the opinion I had expressed last October
('English Mechanic and World of Science,' for October 27, 1876, p. 160),
that Vulcan might perhaps better be called the 'planet of fiction' was
_pro tanto_ confirmed. Nevertheless, I would not be understood to mean
by the word 'fiction' aught savouring of fraud so far as Lescarbault is
concerned--I prefer the North Briton's view of Lescarbault's spot, that
so to speak, it was
... the blot upon his brain,
That _would_ show itself without.
I have left small space to treat of other fancied discoveries among the
orbs of heaven. Yet there are some which are not only interesting but
instructive, as showing how even the most careful observers may be led
astray. In this respect the mistakes into which observers of great and
well deserved eminence have fallen are specially worthy of attention.
With the description of three such mistakes, made by no less an
astronomer than Sir W. Herschel, I shall bring this paper to a close.
When Sir W. Herschel examined the planet Uranus with his most powerful
telescope he saw the planet to all appearance girt about by two rings at
right angles to one another. The illusion was so complete that Herschel
for several years remained in the belief that the rings were real. They
were, however, mere optical illusions, due to the imperfect defining
qualities of the telescope with which he observed the planet. Later he
wrote that 'the observations which tend to ascertain' (indicate?) 'the
existence of rings not being satisfactorily supported, it will be proper
that surmises of them should either be given up, as ill-founded, or at
least reserved till superior instruments can be provided.'