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 diam