ength. His own
discovery came to bridge over the interval, and quite confirmed him in
his belief. He returned to the subject of the revolution of stars about
their axes again and again, and connected it with the revolution of
satellites.

He found that the satellites of _Jupiter_ and one of _Saturn's_
periodically changed in brightness, and by quite simple means showed
that their periods of rotation were at least approximately the same as
their periods of revolution about their primaries. In this case, as in
every other, he considered a discovery in each and every one of its
possible bearings. There are no instances where he has singularly
overlooked the consequences of his observations.

_Researches on Double Stars._

The double stars were the subject of HERSCHEL'S earliest and of his
latest papers. In 1782 he published his "_Catalogue of Double Stars_,"
and his last published memoir (1822) was on the same subject.

The question of determining the parallax of stars first brought HERSCHEL
to the discovery of double stars. If two stars, A and B, appear very
close together, and if, in reality, the star B is very many times more
distant from the earth than A, although seen along the same line of
sight, then the revolution of the earth in its orbit will produce
changes in the relative situation of A and B, and, in fact, B will
describe a small orbit about A, due to this revolution. This idea had
been proposed by GALILEO, and measures on this plan had been made by
LONG, with negative results. But HERSCHEL, in reviewing their work,
declares that the stars chosen by LONG were not suitable to the purpose.
It is necessary, among other things, to the success of this method, that
it should be certain that the star B is really very much more distant
than the star A. The only general test of the distance of stars is their
brilliancy, and HERSCHEL decided to use only stars for this research
which had two components very greatly different in brightness. A must be
very bright (and presumably near to us), and B must be very close to A,
and very faint (and thus, presumably, very distant).

It was in the search for such pairs of stars that the _Catalogue of
Double Stars_ (1782) was formed. HERSCHEL'S first idea of a double star
made such pairs as he found, to consist of two stars _accidentally_ near
to each other. A was near to us, and appeared projected in a certain
place on the celestial sphere. B was many tim