must follow
up this clue until we have found some explanation which fits the facts,
not being discouraged if we cannot hit upon the explanation at once, since
Bradley himself was puzzled for several years: that after finding one
_vera causa_, and allowing for the effect of it, the observations may show
traces of another which must again be patiently hunted, even though we
spend nineteen years in the chase: and that again we may have to leave the
complete rectification of the observations to posterity. But though we may
admit the general helpfulness of these directions, and that this patient
dealing with residual phenomena seems to be a method capable of frequent
application, we cannot deduce any universal principle of procedure from
them: witness the difficulty of dealing with meteorological observations,
for instance. It is not always possible to find any orderly arrangement of
the residuals which will give us a clue to start with. When such an
arrangement is manifested, we must certainly follow up the clue; it would
almost seem that no expense should be prohibitive, since it is impossible
to foresee the importance of the result.

CHAPTER IV

ACCIDENTAL DISCOVERIES

[Sidenote: The Oxford new star found during work on Astrographic Chart.]

In reviewing various types of astronomical discovery I have laid some
stress upon the fact that they are, generally speaking, far from being
accidental in character. A new planet does not "swim into our ken," at any
rate not usually, but is found only after diligent search, and then only
by an investigator of acute vision, or other special qualifications. But
this is, of course, not always the case. Some discoveries are made by the
merest accident, as we have had occasion to remark incidentally in the
case of the minor planets; and for the sake of completeness it is
desirable to include among our types at least one case of such accidental
discovery. As, however, the selection is a little invidious, I may perhaps
be pardoned for taking the instance from my own experience, which happens
to include a case where one of those remarkable objects called "new stars"
walked deliberately into a net spread for totally different objects. There
is the further reason for choosing this instance: that it will afford me
the opportunity of saying something about the special research in which we
were actually engaged, the work of mapping out the heavens by
photography, or, as it has been call