er any of the plates could be admitted as _just_
coming up to the minimum requirements. And I devoted a morning to this
inquiry. In the course of it I came across one plate which certainly
seemed worth an inclusion among our series from the point of view of the
number of stars shown upon it. It seemed quite rich in stars, perhaps even
a little richer than might have been expected. On inquiry I was told that
this was not one of the originally condemned plates, but one which had
been taken since the failure in sensitiveness of the plates had been
detected; was from a new and specially sensitive batch with which the
courteous makers had supplied us; but though there were certainly a
sufficient number of stars upon the plate, owing to some unexplained cause
the telescope had been erroneously pointed, and the region taken did not
correspond to the region required. To investigate the cause of the
discrepancy I thereupon took down from our store of plates the other one
of the same region which had been rejected for insufficiency of stars, and
on comparing the two it was at once evident that there was a strange
object on the plate taken later of the two, a bright star or other
heavenly body, which was not on the former plate. I have explained that by
repeating the exposure more than once, it is easily possible to recognise
whether a mark upon the plate is really a celestial body or is an
accidental blot or dust speck, and there was no doubt that this was the
image of some strange celestial body. It might, of course, be a new
planet, or even an old one which had wandered into the region; but a few
measures soon showed that it was not in movement. The measures consisted
in comparing the separation of the three exposures with the separation of
the corresponding exposures of obvious stars, for the exposures were not,
of course, simultaneous, and if the body were a planet and had moved in
the interval between them, this would be made manifest on measuring the
separations. No such movements could be detected; and the possibilities
were thus restricted to two. So far as we knew the object was a star, but
might be either a star of the class known as _variable_ or of that known
as _new_. In the former case it would become bright and faint at more or
less regular intervals, and might possibly have been already catalogued;
for the number of these bodies already known amounts to some hundreds.
Search being made in the catalogues, no entry of