I may so express myself) the grain is much coarser, and the intervals darker; and the llocculi, instead of being generally round, are drawn into little wisps. They pre sent, however, no appearance of being composed of small stars, and their aspect is altogether different from resolvable nebulae. In the latter we fancy that we see stars, or that could we strain our sight a little more, we should see them ; but the former suggest no idea of stars, but rather of
something quite distinct from them."
Another very remarkable nebula is that in the girdle of Andromeda, which, being visible to the naked eye, was known in the earliest ages of astronomy. It was rediscovered in 1612 by Simon Marius, who describes it as having the appearance of a candle seen through horn, that is, a diluted light, increasing in density towards a centre. Le Gentil mentions that its figure had appeared to him for many years round, but that in 1757 it had become oval. He also remarks that its light was perfectly uniform in all parts, a fact which is quite at variance with its present appearance, and which, if true, argues that the nebulous matter is in a rapid state of condensation. " At present," says Sir J. Herschel in the volume above referred to, " it has not, indeed, a star or any well-defined disk in its centre ; but the brightness, which increases by regular gradations from the circumference, suddenly acquires a great accession, so as to offer the appearance of a nipple, as it were, in the middle, of very small diameter (10" or 12"), but totally devoid of any distinct outline, so that it is impossible to say precisely where the nucleus ends and the nebula begins. Its nebulosity is of the most perfect milky, absolutely irresolvable kind, without the .slightest tendency to that separation into flocculi above described in the nebula of Orion ; nor is there any sort of appearance of the smallest star in the centre of the nipple." This nebula has, however, been resolved into stars, at least partially, by the Harvard reflector.
Messier formed a list of 103 nebulas. But Sir W. Herschel, by means of his large telescopes, discovered no less than 2500 ; and Sir John Herschel added 500 northern and 1700 southern nebulas (staying seven years at the Cape to catalogue these last). In all, about 3700 nebulas are known. They have been thus classified:—
(1.) Clusters of stars (globular or irregular) in which the stars are clearly distinguishable.
(2.) Resolvable nebulae, and such as, from their appearance, probably require only increased power to be resolved into separate stars.
(3.) Nebulae properly so called, showing no appearance of stars.
(4.) Planetary nebulae.
(5.) Stellar nebulae.
(6.) Nebulous stars.
Among the varieties of form may be noted spiral, elliptic, and ring nebulae, double nebulae, and irregular nebulae.
Sir W. Herschel classified the nebulas according to his conception of the probable stage of their development from purely nebulous matter into stars, single or multiple, as follows:—
(1.) Widely-spread nebulosity.
(2.) Irregular nebuke.
(3.) Planetary nebulae.
(4.) Nebulous stars.
(5.) Diffused clusters.
(G.) Stellar nebulas.
(7.) Rich star-clusters.
Examined with the spectroscope by Mr Huggins, the stellar and resolvable nebulae are found to give a spectrum resembling the stellar spectrum in general characteristics. But the irregular nebulae, planetary nebulae, and a con siderable proportion of the irresolvable nebulae, give a spectrum of three bright lines, indicating that they consist of glowing gas. One of these lines is the F line of the hydrogen spectrum, another is a line of nitrogen, and the third is undetermined. A few of the gaseous nebulas show a fourth line.
Some nebulas are variable. A large irregular nebula surrounding the singular variable star 77 Argus appears to undergo extensive variations ; and we have seen that the great nebula in Orion is strongly suspected of variability. Some nebulas have vanished ; others have appeared where formerly no nebulosity had been recognised.
As there are double and variable nebulas as well as double and variable stars, so also are there groupings and aggregations of nebulae as of stars. It is noteworthy that the zone region occupied by the milky-way shows very few nebulae, and those mostly of the resolvable order. The regions farthest from the milky-way, that is, the regions around the poles of the galactic circle, are those richest in nebulae, speaking generally; but the distribution of the nebulae is in reality too irregular to be sufficiently described in such terms. Streams and branches and clustering aggregations of nebulae are found in both of the two hemispheres into which the galaxy divides the heavens. Most remarkable among these aggregations are the Nubeculce or Magellanic Clouds, in the southern heavens. These resemble the milky- way in aspect, but on telescopic examination are found to consist, not of stars alone, like the milky-way, but of stars mixed with nebulas. Within the Nubectila Major, Sir J. Herschel counted 278 nebulas, besides more than 50 out lying nebulas. The Nubecula Minor is not so rich in nebula}, but still deserves to be regarded as an aggregation of these objects.
theories of the constitution of the sidereal heavens advanced by Wright, Kant, Lambert, the Hcrschels, Struve, Macller, Herbert Spencer, Proctor, and others. But so far as discussion of this subject depends on observation, it may be thus presented : The zone of the milky-way being rich in stars, we must either infer great extension of the sidereal system in the direction of that zone, or a real aggregation of stars within a ring-shaped or spirally- shaped region around the earth. If we adopt the former view, we shall naturally be led to regard the nebulae, except only those known to be gaseous, as other sidereal systems outside our own, and so distant as to appear like small cloudlets of stars. But if we adopt the latter view, we may regard many of the nebulae, if not all, as cluster ing aggregations of stars belonging to the same system. Sir W. Herschel adopted the former hypothesis at the beginning of his career as an observer ; but later (though nearly all writers on astronomy overlook the fact), his opinion turned towards the latter. It was while he still supposed the stars spread with tolerable uniformity through out the sidereal system that he trusted to a method of star- gauging depending on the number of stars seen in equal telescopic fields in different directions. It is manifest that, if the telescope reaches to the limits of a system of stars scattered uniformly, the greater the distance at which the limit lies in any direction the greater will be the number of stars in that direction. Gauging the sidereal system on this principle, Sir W. Herschel deduced the inference that it is shaped like a cloven flat disk. But later, perceiving that the rich regions are often seemingly circular in shape, he inferred that such groups are really globular in form. It was natural to extend this principle, and, seeing that parts of the milky-way are stream-shaped, to infer that this appearance indicates the existence of real streams of stars. Also, since the clustering aggregations are various in size and constitution, it might be assumed that the nebulas themselves are only clustering aggregations
