Scattered about here and there among the stars are certain patches of faint luminosity called by astronomers NebulÆ. These "little clouds" of filmy light are among the most fascinating of all the kaleidoscopic phenomena of the heavens; for it needs but a glance at one of them to give the impression that here before us is the stuff of which worlds are made. All our knowledge of Nature leads us to expect in her finished work the result of a series of gradual processes of development. Highly organized phenomena such as those existing in our solar system did not spring into perfection in an instant. Influential forces, easy to imagine, but difficult to define, must have directed the slow, sure transformation of elemental matter into sun and planets, things and men. Therefore a study of those forces and of their probable action upon nebular material has always exerted a strong attraction upon the acutest thinkers among men of exact science.

Our knowledge of the nebulÆ is of two kinds—that which has been ascertained from observation as to their appearance, size, distribution, and distance; and that which is based upon hypotheses and theoretical reasoning about the condensation of stellar systems out of nebular masses. It so happens that our observational material has received a very important addition quite recently through the application of photography to the delineation of nebulÆ, and this we shall describe farther on.

Two nebulÆ only are visible to the unaided eye. The brighter of these is in the constellation Andromeda; it is of oval or elliptical shape, and has a distinct central condensation or nucleus. Upon a photograph by Roberts it appears to have several concentric rings surrounding the nebula proper, and gives the general impression of a flat round disk foreshortened into an oval shape on account of the observer's position not being square to the surface of the disk. Very recent photographs of this nebula, made with the three-foot reflecting telescope of the Lick Observatory, bring out the fact that it is really spiral in form, and that the outlying nebulous rings are only parts of the spires in a great cosmic whorl.

This Andromeda nebula is the one in which the temporary star of 1885 appeared. It blazed up quite suddenly near the apparent centre of the nebula, and continued in view for six months, fading finally beyond the reach of our most powerful telescopes. There can be little doubt that the star was actually in the nebula, and not merely seen through it, though in reality situated in the extreme outlying part of space at a distance immeasurably greater than that separating us from the nebula itself. Such an accidental superposition of nebula and star might even be due to sudden incandescence of a new star between us and the nebula. In such a case we should see the star projected upon the surface of the nebula, so that the superposition would be identical with that actually observed. Therefore, while it is, indeed, possible that the star may have been either far behind the nebula or in front of it, we must accept as more probable the supposition that there was a real connection between the two. In that case there is little doubt that we have actually observed one of those cataclysms that mark successive steps of cosmic evolution. We have no thoroughly satisfactory theory to account for such an explosive catastrophe within the body of the nebula itself.

The other naked-eye nebula is in the constellation Orion. In the telescope it is a more striking object, perhaps, than the Andromeda nebula; for it has no well-defined geometrical form, but consists of an immense odd-shaped mass of light enclosing and surrounding a number of stars. It is unquestionably of a very complicated structure, and is, therefore, less easily studied and explained than the nebulÆ of simpler form. There is no doubt that the Orion nebula is composed of luminous gas, and is not merely a cluster of small stars too numerous and too near together to be separated from each other, even in our most powerful telescopes. It was, indeed, supposed, until about forty years ago, that all the nebulÆ are simply irresolvable star-clusters; but we now have indisputable evidence, derived from the spectroscope, that many nebulÆ are composed of true gases, similar to those with which we experiment in chemical laboratories. This spectroscopic proof of the gaseous character of nebulÆ is one of the most important discoveries contributed by that instrument to our small stock of facts concerning the structure of the sidereal universe.

Coming now to the smaller nebulÆ, we find a great diversity of form and appearance. Some are ring-shaped, perhaps having a less brilliant nebulosity within the ring. Many show a central condensation of disk-like appearance (planetary nebulÆ), or have simply a star at the centre (nebulous stars). Altogether about ten thousand such objects have been catalogued by successive generations of astronomers since the invention of the telescope, and most of these have been reported as oval in form. Now we have already referred to the important addition to our knowledge of the nebulÆ obtained by recent photographic observations; and this addition consists in the discovery that most of these oval nebulÆ are in reality spirals. Indeed, it appears that the spiral type is the normal type, and that nebulÆ of irregular or other forms are exceptions to the general rule. Even the great Andromeda nebula, as we have seen, is now recognized as a spiral.

The instrument with which its convolute structure was discovered is a three-foot reflecting telescope, made by Common of England, and now mounted at the Lick Observatory, in California. The late Professor Keeler devoted much of his time to photographing nebulÆ during the last year or two. He was able to establish the important fact just mentioned, that most nebulÆ formerly thought to be mere ovals, turn out to be spiral when brought under the more searching scrutiny of the photographic plate applied at the focus of a telescope of great size, and with an exposure to the feeble nebular light extending through three or four consecutive hours.

Many of the spirals have more than a single volute. It is as though one were to attach a number of very flexible rods to an axle, like spokes of a wheel without a rim and then revolve the axle rapidly. The flexible rods would bend under the rapid rotation, and form a series of spiral curves not unlike many of these nebulÆ. Indeed, it is impossible to escape the conviction that these great celestial whorls are whirling around an axis. And it is most important in the study of the growth of worlds, to recognize that the type specimen is a revolving spiral. Therefore, the rotating flattened globe of incandescent matter postulated by Laplace's nebular hypothesis would make of our solar system an exceptional world, and not a type of stellar evolution in general.

Keeler's photographs have taught us one thing more. Scarcely is there a single one of his negatives that does not show nebulÆ previously uncatalogued. It is estimated that if this process of photography could be extended so as to cover the entire sky, the whole number of nebulÆ would add up to the stupendous total of 120,000; and of these the great majority would be spiral.

When we approach the question of the distribution of nebulÆ in different parts of the sky, as shown by their catalogued positions, we are met by a curious fact. It appears that the region in the neighborhood of the Milky Way is especially poor in nebulÆ, whereas these objects seem to cluster in much larger numbers about those points in the sky that are farthest from the Milky Way. But we know that the Milky Way is richer in stars than any other part of the sky, since it is, in fact, made up of stellar bodies clustered so closely that it is wellnigh impossible to see between them in the denser portions. Now, it cannot be the result of chance that the stars should tend to congregate in the Milky Way, while the nebulÆ tend to seek a position as far from it as possible. Whatever may be the cause, we must conclude that the sidereal system, as we see it, is in general constructed upon a single plan, and does not consist of a series of universes scattered at random throughout space. If we are to suppose that nebulÆ turn into stars as a result of condensation or any other change, then it is not astonishing to find a minimum of nebulÆ where there is a maximum of stars, since the nebulÆ will have been consumed, as it were, in the formation of the stars.

It is never advisable to push philosophical speculation very far when supported by too slender a basis of fact. But if we are to regard the visible universe as made up on the whole of a single system of bodies, we may well ask one or two questions to be answered by speculative theory. We have said the stars are not uniformly distributed in space. Their concentration in the Milky Way, forming a narrow band dividing the sky into two very nearly equal parts, must be due to their being actually massed in a thin disk or ring of space within which our solar system is also situated. This thin disk projected upon the sky would then appear as the narrow star-band of the Milky Way. Now, suppose this disk has an axis perpendicular to itself, and let us imagine a rotation of the whole sidereal system about that axis. Then the fact that the visible nebulÆ are congregated far from the Milky Way means that they are actually near the imaginary axis.

Possibly the diminished velocity of motion near the axis may have something to do with the presence of the nebulÆ there. Possibly the nebulÆ themselves have axes perpendicular to the plane of the Milky Way. If so, we should see the spiral nebulÆ near the Milky Way edgewise, and those far from it without foreshortening. Thus, the paucity of nebulÆ near the Milky Way may be due in part to the increased difficulty of seeing them when looked at edgewise. Indeed, there is no limit to the possibilities of hypothetical reasoning about the nebular structure of our universe; unfortunately, the whole question must be placed for the present among those intensely interesting cosmic problems awaiting elucidation, let us hope, in this new century.