disturbance was produced revolved in an orbit which must lie outside
that of Uranus. It seemed thus certain that the planet could not be
a body visible to the unaided eye. Indeed, had it been at all
conspicuous its planetary character would doubtless have been
detected ages ago. The unknown body must therefore be a planet which
would have to be sought for by telescopic aid.

There is, of course, a profound physical difference between a planet
and a star, for the star is a luminous sun, and the planet is merely
a dark body, rendered visible by the sunlight which falls upon it.
Notwithstanding that a star is a sun thousands of times larger than
the planet and millions of times more remote, yet it is a singular
fact that telescopic planets possess an illusory resemblance to the
stars among which their course happens to lie. So far as actual
appearance goes, there is indeed only one criterion by which a planet
of this kind can be discriminated from a star. If the planet be
large enough the telescope will show that it possesses a disc, and
has a visible and measurable circular outline. This feature a star
does not exhibit. The stars are indeed so remote that no matter how
large they may be intrinsically, they only exhibit radiant points of
light, which the utmost powers of the telescope fail to magnify into
objects with an appreciable diameter. The older and well-known
planets, such as Jupiter and Mars, possess discs, which, though not
visible to the unaided eye, were clearly enough discernible with the
slightest telescopic power. But a very remote planet like Uranus,
though it possessed a disc large enough to be quickly appreciated by
the consummate observing skill of Herschel, was nevertheless so
stellar in its appearance, that it had been observed no fewer than
seventeen times by experienced astronomers prior to Herschel. In
each case the planetary nature of the object had been overlooked, and