section of
the vast number of stars with which the vault of the heavens is studded.
That the sun is no more than a star, and the stars are no less than
suns, is a cardinal doctrine of astronomy. The imposing magnificence of
this truth is only realised when we attempt to estimate the countless
myriads of stars. This is a problem on which our calculations are
necessarily vain. Let us, therefore, invoke the aid of the poet to
attempt to express the innumerable, and conclude this chapter with the
following lines of Mr. Allingham:--

"But number every grain of sand,
Wherever salt wave touches land;
Number in single drops the sea;
Number the leaves on every tree,
Number earth's living creatures, all
That run, that fly, that swim, that crawl;
Of sands, drops, leaves, and lives, the count
Add up into one vast amount,
And then for every separate one
Of all those, let a flaming SUN
Whirl in the boundless skies, with each
Its massy planets, to outreach
All sight, all thought: for all we see
Encircled with infinity,
Is but an island."

CHAPTER XX.

DOUBLE STARS.

Interesting Stellar Objects--Stars Optically Double--The Great
Discovery of the Binary Stars made by Herschel--The Binary Stars
describe Elliptic Paths--Why is this so important?--The Law of
Gravitation--Special Double Stars--Castor--Mizar--The Coloured
Double Stars--b Cygni.

The sidereal heavens contain few more interesting objects for the
telescope than can be found in the numerous class of double stars. They
are to be counted in thousands; indeed, _many_ thousands can be found in
the catalogues devoted to this special branch of astronomy. Many of
these objects are, no doubt, small and comparatively uninteresting, but
some of them are among the most conspicuous stars in the heavens, such
as Sirius, whose system we have already described. We shall in this
brief account select for special discussion and illustration a few of
the more remarkable double stars. We shall particularly notice some of
those that can be readily observed with a small telescope, and we have
indicated on the sketches of the constellations in a previous chapter
how the positions of these objects in the heavens can be ascertained.

It had been shown by Cassini in 1678 that certain stars, which appeared
to the unaided eye as single points of light, really consisted of two or
more stars, so close together that the telescope was