most glorious objects in the heavens. "If we suppose this nebula to be one continuous bed of stars of different sizes for its entire extent, it must comprise the enormous number of 30,000,000." The distance of such nebulæ from the earth entirely passes our comprehension. Some astronomers have estimated that a ray of light would require 800,000 years to span the gulf that intervenes. Imagination wearies itself in the attempt to understand these figures. They only teach us something of the limitless expanses of that space in which God is working the mysterious problem of creation.

The annular nebulae have the form of a ring. There are but four of these "ring universes." In

the cut is a representation of one in Lyra-first as seen by Herschel, and having in the centre a nebulous film like a "bit of gauze stretched over a hoop;" second, as shown in Lord Rosse's great telescope, which resolves the filmy parts of the nebula into excessively minute stars, and reveals a fringe of stars

along the edge. Though apparently so small, its dimensions must be enormous. If no further from the earth than 61 Cygni, the diameter would be 2,000,000,000 miles. It is probably immensely further distant.

The spiral or "whirlpool nebula" are exceedingly curious in their appearance. The most remarkable one is that in Canes Venatici. It consists of brilliant spirals sweeping outward from a central nucleus, and all overspread with a multitude of stars. One is lost in attempting to imagine the distance of such a mass, and the forces which produce such a "tremendous hurricane of matter-perhaps of suns."

Fig. 86

Planetary nebula, by their circular form and pale uniform light, resemble the disks of the most distant planets of our system. Their edges are generally well defined, though sometimes slightly furred. Threefourths of them are in the southern hemisphere. Several have a blue tinge. There is one in Ursa Major, which if located at the distance named before-that of 61 Cygniwould fill a space equal to

three times the entire orbit of

PLANETARY NEBULA.

Neptune. About twenty-five of these "island universes" have been found scattered through the ocean of space. Columbus discovered a new continent, and so immortalized his name: what shall we

say of the astronomer who discovers a universe of worlds?

Fig. 87.

Irregular nebulae are those which have no definite form. Many of them present all the irregularities of clouds torn and rent by the tempest. Some of the likenesses which may be traced by the fancy are strangely fantastic: for example, the "dumb-bell nebula" in the constellation Vulpecula, and the

"crab nebula"

near the southern

horn of Taurus.

DUMB-BELL NEBULA.

There is also one known as "the great nebula in the sword-handle of Orion," in which may be seen a faint resemblance to the wings of a bird.

Nebulous stars are so called because they are enveloped by a faint nebula, usually of a circular form. The star is generally seen at the centre, although some which are elliptical surround two stars, one in each focus. It is thought that these may be suns possessing immense atmospheres, which are rendered visible somewhat as that of our sun is in the zodiacal light; and that in like manner our sun

itself to those in space presents the appearance of a nebulous star. The luminous atmosphere of the star in Cygnus, if located at the distance of a Centauri, is of an extent equal to "fifteen times the distance of Neptune from the sun.