those of his father.

But nevertheless John Herschel did great work during his "sweeps." He
was specially particular to note all the double stars which presented
themselves to his observation. Of course some little discretion must
be allowed in deciding as to what degree of proximity in adjacent
stars does actually bring them within the category of "double
stars." Sir John set down all such objects as seemed to him likely
to be of interest, and the results of his discoveries in this branch
of astronomy amount to some thousands. Six or seven great memoirs in
the TRANSACTIONS of the Royal Astronomical Society have been devoted
to giving an account of his labours in this department of astronomy.

[PLATE: THE CLUSTER IN THE CENTAUR, drawn by Sir John Herschel.]

One of the achievements by which Sir John Herschel is best known is
his invention of a method by which the orbits of binary stars could
be determined. It will be observed that when one star revolves
around another in consequence of the law of gravitation, the orbit
described must be an ellipse. This ellipse, however, generally
speaking, appears to us more or less foreshortened, for it is easily
seen that only under highly exceptional circumstances would the plane
in which the stars move happen to be directly square to the line of
view. It therefore follows that what we observe is not exactly the
track of one star around the other; it is rather the projection of
that track as seen on the surface of the sky. Now it is remarkable
that this apparent path is still an ellipse. Herschel contrived a
very ingenious and simple method by which he could discover from the
observations the size and position of the ellipse in which the
revolution actually takes place. He showed how, from the study of