fore estimates the whole number in the heavens capable of being photographed at several hundred thousand. It may be assumed that only a moderate fraction of these are visible to the eye, even aided by the largest telescopes.

Among the most singular of these objects are large diffused nebulæ, sometimes extending through a region of several degrees. A number of these were discovered by Herschel. Barnard, W. H. Pickering, and others have photographed these for us. One of the most remarkable of them winds around in the constellation Orion in such a way that at first sight one might be disposed to inquire whether the impression on the photographic plate might not have been the result of some defect in the apparatus or some reflection of the light of the neighbouring stars, which is so apt to occur in these delicate photographic operations. But its existence happens to be completely confirmed by independent testimony. It was first detected by W. H. Pickering and afterwards independently by Barnard.

A curious fact connected with the distribution of nebulæ over the sky is that it is in a certain sense the reverse of that of the stars. The latter are vastly more numerous in the regions near the Milky Way and fewer in number near the poles of that belt. But the reverse is the case with the nebulæ proper. They are least numerous in the Milky Way and increase in number as we go from it in either direction. Precisely what this signifies one would not at the present time be able to say. Perhaps the most obvious suggestion

would be that in these two opposite nebulous regions the nebulæ have not yet condensed into stars. This, however, would be a purely speculative explanation. On the other hand, star-clusters are more numerous in the galactic region. This, however, is little more than saying that in the regions where the stars are so much more numerous than elsewhere many of them naturally tend to collect in clusters. It is, however, a curious fact that, so far as has yet been noticed, the large diffused nebula which we have mentioned are more numerous in or near the Milky Way. If this tendency is established it will mark a curious distinction between them and the smaller nebulæ.

The most interesting question connected with these objects is that of their physical constitution. When, about 1866, the spectroscope was first applied to astronomical investigation by Huggins he found that the light of the great nebula of Orion formed a spectrum of bright lines, thus showing the object to be gaseous. This was soon found to be true of the nebulæ generally. There is, however, a very curious exception in the case of the great nebula of Andromeda. This object gives a more or less continuous spectrum. The bright lines in the spectrum of a nebula are seldom or never more than four in number. The wave-lengths are 4341, 4861, 4957, 5004. The first of these is the violet, is very faint, and visible only in the brightest nebulæ. The last is the brightest, and in faint nebula is the only one that can be distinguished. None of these lines can be certainly identified with those of any terrestrial substance.

The supposed matter which produces them has, therefore, been called nebulum.

Beyond the general fact that the light of a nebula does not come from solid matter, but from matter of a gaseous or other attenuated form, we have no certain knowledge of the physical constitution of these bodies. Certain features of their constitution can, however, be established with a fair approach to accuracy. Not only the spectroscopic evidence of bright lines but the aspect of the objects themselves show that they are transparent through and through. This is remarkable when taken in connection with their inconceivable size. Leaving out the large diffused nebula which we have mentioned, these objects are frequently several minutes in diameter. Of their distance we know nothing, except that they are probably situated in the distant stellar regions. Their parallax can be but a small fraction of a second. We shall probably err greatly in excess if we assume that it varies between one-hundredth and one-tenth of a second. To assign this parallax is the same thing as saying that at the distance of the nebula the dimensions of the earth's orbit would show a diameter which might range between one-fiftieth and one-fifth of a second, while that of Neptune would be more or less than one second. Great numbers of these objects are, therefore, thousands of times the dimensions of the earth's orbit, and probably most of them are thousands of times the dimensions of the whole solar system. That they should be completely transparent through such enormous dimensions shows

their extreme tenuity. Were our solar system placed in the midst of one of them, it is probable that we should not be able to find any evidence of its existence.

A form of matter so different from any that can be found or produced on the surface of the earth can hardly be explained by our ordinary views of matter. A theory has, however, been propounded by Sir Nor man Lockyer, so ingenious as to be at least worthy of mention. It is that these objects are vast collections of meteorites in rapid motion relatively, to each other, which come into constant collision. Their velocity is such that at each collision heat and light are produced. In the language of our progenitors, who in the absence of matches used flint and steel, they "strike fire" against each other. The idea of such a process originated with Prof. P. G. Tait, in an attempt to explain the tail of a comet, but it was elaborated and developed by Mr. Lockyer in his work on the Meteoritic Theory.

The objections to this theory seem insuperable. A velocity so great, at such a distance from the centre of the nebulæ, would be incompatible with the extreme tenuity of these objects. Every time that two meteors came into collision they would lose velocity, and, therefore, if the mass was sufficient to hold them from flying through space, would rapidly fall toward a common centre. The amount of light produced by the collision of two such objects is only a minute fraction of the energy lost. The meteors which fall on the earth are mostly of iron, and, were the theory true, numerous lines of iron should be most conspicuous in the spectrum.

THE

CHAPTER XII

CONSTITUTION OF THE STARS

Doubt thou the stars are fire.-SHAKESpeare.

'HE spectroscope shows that, although the constitution of the stars offers an infinite variety of detail, we may say, in a general way, that these bodies are suns. It would, perhaps, be more correct to say that the sun is one of the stars and does not differ essentially from them in its constitution. The problems of the physical constitution of the sun and stars may, therefore, be regarded as one, all these being bodies. of the same general nature, consisting of vast masses of incandescent matter at so exalted a temperature as to shine by their own light.

Diversities

among the Stars.

This similarity in general constitution does not, however, preclude very great differences in detail. The spectra of the stars show that hardly any two are exactly alike in the substances of which they are composed, and in the temperature and density of these substances. remarkable is the diversity of their actual luminosities. or the amount of light and heat which they individually emit. The whole tendency of recent research has been to accentuate this diversity. It was once

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