tar had been visible only for a few days before its
discovery, there being no trace of it on earlier plates. Similarly, in
the case of the new star found at Oxford, plates taken on March 1st and
6th, fifteen days and ten days respectively before the discovery-plate of
March 16th, showed the star. But, in this particular instance, greater
interest attaches to two still earlier plates taken elsewhere, and with
exposures much longer than any available at Harvard. One had been obtained
at Heidelberg by Dr. Max Wolf, and another at the Yerkes Observatory of
Chicago University, by Mr. Parkhurst; and on both there appeared to be a
faint star of about the fourteenth or fifteenth magnitude, in the place
subsequently occupied by the Nova; and the question naturally arose, Was
this the object which ultimately blazed up and became the new star? To
settle this point, it was necessary to measure its position, with
reference to neighbouring stars, with extreme precision; and here it was
unfortunate that the photographs did not help us as much as they might,
for they were scarcely capable of being measured with the requisite
precision. The point was an important one, because if the identity of the
Nova with this faint star could be established, it would be the second
instance of the kind; but so far as they went, measurements of the
photographs were distinctly against the identity. Such was the conclusion
of Mr. Parkhurst from his photograph alone; and it was confirmed by
measures made at Oxford on copies of both plates, which were kindly sent
there for the purpose. The conclusion seemed to be that there was a faint
star _very near_, but _not at_, the place of the new star; and it was
therefore probable that, although this faint star was temporarily
invisible from the brightness of the adjacent Nova, as the latter became
fainter (in the way with which we have become familiar in the case of new
stars), it might be possible to see the two stars alongside each other.
This critical observation was ultimately made by the sharp eyes of
Professor Barnard, aided by the giant telescope of the Yerkes Observatory;
and it seems clear therefore that the object which blazed up to become the
Nova of 1903 could not have previously been so bright as a faint star of
the fourteenth magnitude. Although this is merely a negative conclusion,
it is an important one in the history of these bodies.

[Sidenote: Nebula round Nova Persei.]

[Sidenote: Its changes