rds; I have therefore given the
measures of the comet's diameter in seconds and thirds. And the parts
of my micrometer being thus reduced, I have also given all the rest
of the measures in the same manner; though in large distances, such
as one, two, or three minutes, so great an exactness, for several
reasons, is not pretended to."

[Sidenote: Called first a comet.]

[Sidenote: Other observers would not have found it at all.]

At first sight this seems to be the wrong reference, for it speaks of a
new comet, not a new planet. But it is indeed of Uranus that Herschel is
speaking; and so little did he realise the full magnitude of his
discovery at once, that he announced it as that of a comet; and a comet
the object was called for some months. Attempts were made to calculate its
orbit as a comet, and broke down; and it was only after much work of this
kind had been done that the real nature of the object began to be
suspected. But far more striking than this misconception is the display of
skill necessary to detect any peculiarity in the object at all. Among a
number of stars one seemed somewhat exceptional in size, but the
difference was only just sufficient to awaken suspicion in a keen-eyed
Herschel. Would any other observer have noticed the difference at all?
Certainly several good observers had looked at the object before, and
looked at it with the care necessary to record its position, without
noting any peculiarity. Their observations were recovered subsequently and
used to fix the orbit of the new planet more accurately. I shall remind
you in the next chapter that Uranus had been observed in this way no less
than seventeen times by first-rate observers without exciting their
attention to anything remarkable. The first occasion was in 1690, nearly a
century before Herschel's grand discovery, and these chance observations,
which lay so long unnoticed as in some way erroneous, subsequently proved
to be of the utmost value in fixing the orbit of the new planet. But there
is even more striking testimony than this to the exceptional nature of
Herschel's achievement. It is a common experience in astronomy that an
observer may fail to notice in a general scrutiny some phenomenon which he
can see perfectly well when his attention is directed to it: when a man
has made a discovery and others are told what to look for, they often see
it so easily that they are filled with amazement and chagrin that