physical relation existing between the stars composing such an "archipelago of worlds," but its nature is a mystery. They seem generally crowded together toward the center, blending into a continuous blaze of light. Yet, although they appear so densely compacted, it is probable that, if we could change our stand-point and penetrate one of these groups of suns, we should find it, on our approach, opening up and spreading out before us, until, in the midst, the suns would shine down upon us from the heavens as the stars do in our own sky.

6. Nebulæ are faint, misty objects, like specks of luminous clouds. A few are visible to the naked eye, but the telescope reveals thousands. They differ from clusters in not being resolvable into stars when viewed through the largest telescopes. With the con

stant improvement made in these instruments, however, many so-called nebulæ have been resolved, and thus the number of clusters has been increased, while new nebulæ have been discovered.

Until of late, it was thought that all nebulæ were simply groups of stars, which would be ultimately discerned in the more powerful telescopes yet to be made. Spectrum analysis shows, however, that many of these luminous clouds are gaseous, and are not composed of stars.

Since all the nebulæ maintain the same position with respect to the stars, their distance must be inconceivably great, and, in order to be visible to us, their magnitude must be proportionately vast. They are most abundant at the two poles of the Milky Way, but are more uniformly distributed over the heavens lying near the south pole.

It is now generally believed that nebulæ constitute the material for making stars,-are, in fact, sungerms; that all stars originally existed as nebulæ ; and that every nebula will, in time, be changed into stars.

Nebulæ are divided, according to their form, into six classes-elliptic, annular, spiral, planetary, irregular nebulæ, and nebulous stars.†

THE ELLIPTIC or merely oval nebulæ are the most abundant. Under this head is classed the Great Nebula in Andromeda, which was discovered over

+ This division of the nebulæ is purely arbitrary, and used only to introduce some order of arrangement. The shape of the nebulæ changes with the power of the telescope through which they are seen. Thus the Great Nebula in Andromeda, as resolved by Bond, is no longer oval, but irregular in form. The Ring-Nebula of Lyra, seen through the large telescope of to-day, is egg-shaped; while the Dumb-bell Nebula assumes the outline of a chemical retort.

a thousand years ago, and is visible to the naked Prof. Bond, of the Cambridge Observatory,

eye.

the enormous number of 30,000,000."

The distance of such nebulæ from the earth 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 teach us something of the limitless expanse of that space in which God is working the mysterious problem of creation.

is a representation of one in Lyra,-first, as seen by Herschel, having in the center a nebulous film like a

"bit of gauze stretched over a hoop;" second, as shown in Lord Rosse's telescope (p. II), which resolves the filmy parts of the nebula into minute stars, and reveals a fringe of stars along the edge.

THE SPIRAL OF WHIRLPOOL NEBULA are exceedingly curious. The most remarkable one is 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. 100.

planets of our system.

PLANETARY NEBULA, by their circular form and pale, uniform light, resemble the disks of the distant Their edges are generally well defined, though sometimes slightly furred. There is one in Ursa Major, which, if located at the distance of 61 Cygni, would "fill a space equal to seven times the entire orbit of Neptune."

Planetary Nebula.

IRREGULAR NEBULA are those which have no definite form. Many present the irregularities of clouds torn by the tempest. Some of the likenesses which may be traced are strangely fantastic for example, the Dumb-bell Nebula, in the constellation Vulpecula, and the Crab Nebula, near the southern horn of Taurus. There is also one known

:

* Columbus discovered a new continent, and so immortalized his name; what shall we say of the astronomer who discovers a system of worlds?