apparent. The objects must actually lie close together at a distance
which is small in comparison with the distance at which either of
them is separated from the earth. The fact that the heavens contain
pairs of twin suns in mutual revolution was thus brought to light.

In consequence of this beautiful discovery, the attention of
astronomers was directed to the subject of double stars with a degree
of interest which these objects had never before excited. It was
therefore not unnatural that John Herschel should have been attracted
to this branch of astronomical work. Admiration for his father's
discovery alone might have suggested that the son should strive to
develop this territory newly opened up to research. But it also
happened that the mathematical talents of the younger Herschel
inclined his inquiries in the same direction. He saw clearly that,
when sufficient observations of any particular binary star had been
accumulated, it would then be within the power of the mathematician
to elicit from those observations the shape and the position in space
of the path which each of the revolving stars described around the
other. Indeed, in some cases he would be able to perform the
astonishing feat of determining from his calculations the weight of
these distant suns, and thus be enabled to compare them with the mass
of our own sun.

[PLATE: NEBULA IN SOUTHERN HEMISPHERE, drawn by Sir John Herschel.]

But this work must follow the observations, it could not precede
them. The first step was therefore to observe and to measure with
the utmost care the positions and distances of those particular
double stars which appear to offer the greatest promise in this
particular research. In 1821, Herschel and a friend of his, Mr.