ars are not merely optically
in proximity, as being accidentally in the same line of view, but are
actually connected physically, revolving round each other in regular
orbits. The motion of these double suns is, however, in many instances
so slow as to require many years for a satisfactory determination.
[Sidenote: Gravitation of double stars.] Sir J. Herschel therefore
continued the observations of his father, and with other mathematicians,
investigated the characteristics of these motions. The first instance in
which the true elliptic elements of the orbit of a binary star were
determined was given by M. Savary in the case of chi Ursae Majoris,
indicating an elliptic orbit of 58-1/4 years. But the period of others,
since determined, is very much longer; thus, in sigma Coronae, it is,
according to Mr. Hind, more than 736 years. From the fact that the
orbits in which these stars move round each other are elliptical, it
necessarily follows that the law of gravitation, according to the
inverse square, holds good in them. Considering the prodigious distances
of these bodies, and the departure, as regards structure of the systems
to which they belong, from the conditions obtaining in our unisolar
system, we may perhaps assert the prevalence of the law of gravitation
throughout the universe.

[Sidenote: Coloured light of double stars.] If, in association with
these double suns--sometimes, indeed, they are triple, and occasionally,
as in the case of epsilon Lyrae, quadruple--there are opaque planetary
globes, such solar systems differ from ours not only in having several
suns instead of a single one, but, since the light emitted is often of
different tints, one star shining with a crimson and another with a blue
light, the colours not always complementary to one another, a wonderful
variety of phenomena must be the result, especially in their organic
creations; for organic forms, both vegetable and animal, primarily
depend on the relations of coloured light. How varied the effects where
there are double, triple, or even quadruple sunrises, and sunsets, and
noons; and the hours marked off by red, or purple, or blue tints.

[Sidenote: Grandeur of Newton's discoveries.] It is impossible to look
back on the history of the theory of gravitation without sentiments of
admiration and, indeed, of pride. How felicitous has been the manner in
which have been explained the inequalities of a satellite like the moon
under the disturbing i