that we are concerned; it is that of sun
round sun--each perhaps, at least in some binary systems, where the
individuals are very remote and their period of revolution very long,
accompanied by its train of planets and their satellites, closely
shrouded from our view by the splendour of their respective suns, and
crowded into a space bearing hardly a greater proportion to the enormous
interval which separates them than the distances of the satellites of
our planets from their primaries bear to their distances from the sun
itself.

A less distinctly characterised subordination would be incompatible with
the stability of their systems and with the planetary nature of their
orbits. Unless close under the protecting wing of their immediate
superior, the sweep of their other sun, in its perihelion passage round
their own, might carry them off or whirl them into orbits utterly
incompatible with conditions necessary for the existence of their
inhabitants.

_IV.--The Nebulae_

It is to Sir William Herschel that we owe the most complete analysis of
the great variety of those objects which are generally classed as
nebulae. The great power of his telescopes disclosed the existence of an
immense number of these objects before unknown, and showed them to be
distributed over the heavens not by any means uniformly, but with a
marked preference to a certain district extending over the northern pole
of the galactic circle. In this region, occupying about one-eighth of
the surface of the sphere, one-third of the entire nebulous contents of
the heavens are situated.

The resolvable nebulae can, of course, only be considered as clusters
either too remote, or consisting of stars intrinsically too faint, to
affect us by their individual light, unless where two or three happen to
be close enough to make a joint impression and give the idea of a point
brighter than the rest. They are almost universally round or oval, their
loose appendages and irregularities of form being, as it were,
extinguished by the distance, and only the general figure of the
condensed parts being discernible. It is under the appearance of objects
of this character that all the greater globular clusters exhibit
themselves in telescopes of insufficient optical power to show them
well.

The first impression which Halley and other early discoverers of
nebulous objects received from their peculiar aspect was that of a
phosphorescent vapour (like the matter of a come