hich we can afford to
neglect. Just when we are on the point of relinquishing it under the
impression that the mine is exhausted, we may be about to find a nugget
worth all our previous and future labour. This rule will not, perhaps,
help us very much in choosing what to work at; indeed, it is no rule at
all, for it leaves us the whole field of choice unlimited. But this
negative result will recur again and again as we examine the lessons
taught by discoveries: there seem to be no rules at all. Whenever we seem
to be able to deduce one from an experience, some other experience will
flatly contradict it. Thus we might think that the discovery of Eros
taught us to proceed patiently with a monotonous duty, and not turn aside
to more novel and attractive work; yet it is often by leaving what is in
hand and apparently has first claim on our attention that we shall do
best, and we shall learn in the next chapter how a failure thus to turn
flexibly aside was repented.

CHAPTER II

THE DISCOVERY OF NEPTUNE

[Sidenote: Search for definite objects.]

In the last chapter we saw that the circumstances under which planets were
discovered varied considerably. Sometimes the discoveries were not
previously expected, occurring during a general examination of the
heavens, or a search for other objects; and, on one occasion at least, the
discovery may be said to have been even contrary to expectation, though,
as the existence of a number of minor planets began to be realised, there
have also been many cases where the discovery has been made as the result
of a definite and deliberate search. But the search cannot be said to have
been inspired by any very clear or certain principle: for the law of Bode,
successful though it has been in indicating the possible existence of new
planets, cannot, as yet, be said to be founded upon a formulated law of
nature. We now come, however, to a discovery made in direct interpretation
of Newton's great law of gravitation--the discovery of Neptune from its
observed disturbance of Uranus. I will first briefly recall the main facts
relating to the actual discovery.

[Sidenote: Disturbance of Uranus.]

After Uranus had been discovered and observed sufficiently long for its
orbit to be calculated, it was found that the subsequent position of the
planet did not always agree with this orbit; and, more serious than this,
some early observations were found which could not be reconciled with the
late