le to
calculate the orbit of a comet, and carried on such work when Halley left
it, it was probably not congenial to him. Halley, on the other hand,
almost despised accurate observations as finicking. "Be sure you are
correct to a minute," he was wont to say, "and the fractions do not so
much matter." With such a precept Bradley would never have made his
discoveries. No quantity was too small in his eyes, and no sooner was the
explanation of aberration satisfactorily established than he perceived
that though it would account for the main facts, it would not explain all.
There was something left. This is often the case in the history of
science. A few years ago it was thought that we knew the constitution of
our air completely--oxygen, nitrogen, water vapour, and carbonic acid gas;
but a great physicist, Lord Rayleigh, found that after extracting all the
water and carbonic acid gas, all the oxygen and all the nitrogen, there
was something left--a very minute residuum, which a careless experimenter
would have overlooked or neglected, but which a true investigator like
Lord Rayleigh saw the immense importance of. He kept his eye on that
something left, and presently discovered a new gas which we now know as
argon. Had he repeated the process, extracting all the argon after the
nitrogen, he might have found by a scrutiny much more accurate still yet
another gas, helium, which we now know to exist in extremely minute
quantities in the air. But meantime this discovery was made in another
way.

[Sidenote: Still something to be explained.]

[Sidenote: Probably nutation.]

[Sidenote: His nineteen years' campaign.]

When Bradley had extracted all the aberration from his observations he
found that there was something left, another problem to be solved and some
more thinking to be done to solve it. But he was now able to profit by his
previous labours, and the second step was made more easily than the first.
The residuum was not the parallax of which he had originally been in
search, for it did not complete a cycle within the year; it was rather a
progressive change from year to year. But there was an important clue of
another kind. He saw that the apparent movements of all stars were in
this case the same; and he knew that a movement of this kind can be
referred, not to the stars themselves, but to the plumb-line from which
their directions are measured. He had thought out the possible causes of
such a movement of the plumb-li