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only 68 nebulæ had been detected. Singularly enough, 42 of these belonged to the southern hemisphere. In 1771, Messier published a list of 103 nebulæ. But all prior investigations sink into insignificence in the presence of the discoveries made by the two Herschels. In 1786, Sir W. Herschel published a list of 1000 nebulæ ; three years later he added another 1000; and finally, in 1802, he formed a supplementary list of 500 new nebulæ. Sir J. Herschel examined no less than 2306 northern nebulæ, of which 500 were discovered by himself; then journeying to the Cape of Good Hope, he formed a catalogue of 1708 southern nebulæ. Of the 6000 known nebulæ, the Herschels have discovered more than four-fifths.
The views entertained respecting nebulæ are so vague that it is hardly possible to present them in any systematic form. But, perhaps, we shall be able to form a tolerably fair estimate of modern notions by considering the two chief hypotheses which have been held by astronomers.
First, we must draw a line of demarcation between nebulous objects which are assumed to belong to the galaxy, and true nebulæ. This is not so easy a matter as it might seem at first sight. In the sidereal universe we see binary, triple, and multiple systems; we see also star-clusters, such as the Pleiades, and Præsepe, and that wonderful cluster which adorns the sword-handle of Perseus; we see also clustering collections of stars, extending far more widely than any of these. But already we have touched upon a difficulty. Such spots of light as the cluster in Perseus—are these to be looked on as certainly belonging to the galactic system? I have no doubt myself upon the point; nor, so far as I am aware, has any astronomer ever expressed any. Yet it is not easy to see on what grounds we can assume that the spot in Perseus belongs to the Milky Way, while we look upon other clusters—the great cluster in Hercules for example-as lying far out in space beyond the confines of our galaxy. For the present, however, we may content ourselves with the distinction usually drawn by astronomers, and admit irregular and widely-distributed clusters as belonging to the sidereal system, while we place the globular and closely-compacted star-clusters in the list of true nebulæ.
Thus there remain five classes of nebulæ :
Resolvable nebulæ, or nebulæ which exhibit such an appearance as leads the experienced astronomer to suspect that with an increase of telescopic power they can be resolved into discrete stars.
Nebulæ which exhibit no trace of resolvability.
Now, according to one view, which was maintained until quite recently as by far the most probable theory, objects belonging to all these classes (including globular star-clusters), were looked upon as in reality composed of Suns, resembling our own in magnitude and splendour, and separated from each other by distances comparable to, perhaps surpassing, the distances which separate our Sun from neighbouring fixed stars. Nebulæ, in fact, were looked upon as galaxies resembling our own, some exceeding it, others falling short of it, in richness and splendour : but all of them “island-universes,” to use Humboldt's expressive verbiage, and all of them-even the nearest-removed from us by distances which exceed, in an enormous proportion, the dimensions of our galaxy. The resolvable clusters were of course considered to be the nearest of the outlying universes. Accordingly, when it is remembered that some of these are clearly resolved by pigmy tubes, while there exist irresolvable nebulæ of great apparent extent, which have defied the power of the great Parsonstown reflector, it will be seen how largely—on the hypothesis we are considering—the “islanduniverses vary in their distances from us, and in their own dimensions.
According to the other hypothesis, multitudes of the nebulæ are outlying universes, but not all of them. Sir W. Herschel, in the beginning of his career, had held the former opinion, following in this respect Cassini and Mitchell. But in the first year of the present century, he began to express different views. Admitting that the majority of the nebulæ are aggregations of Suns, rendered nebulous only through excessive distance, Herschel was led to the belief that many nebulæ are formed in reality—as in appearancefrom cosmical vapour. Half a century before, Kant and Lambert had expressed similar views; but their speculations had not been founded, as Herschel's theories were, upon a long process of research among nebulæ. Herschel's treatment of his observations was characterized by his usual clear-sightedness. He pointed out the remarkable contrast that exists between the small yet easily resolved clusters, and such objects as the great Orion nebula, and the “queen of the nebulæ” in Andromeda. Both of these are distinctly visible to the naked eye, and are yet absolutely irresolvable even in those monster tubes which the Herschels directed towards the heavens. He dealt also with the peculiarities of the planetary nebulæ and of
nebulous stars. The former, shining as discs of bluish or greenish light, are very unlike star-clusters of equal apparent dimensions. In
many respects also they present a very different aspect from that which we should be disposed to assign to star-universes of abnormally large absolute dimensions, removed to a proportionately enormous distance. As regards the “nebulous stars,” there are equal difficulties to encounter. If the central brightness is really, as it appears to be, due to the presence of a single star, how largely must the dimensions of this Sun exceed those of the other members of the system it belongs to; and how different, therefore, must that system be from our own galaxy. On the other hand, if the central brightness affords evidence of a close aggregation of stars, how different is the variety of distribution indicated by this arrangement, from the uniformity attributed to our own sidereal system.
Influenced by considerations such as these, and by others on which I have not space now to dwell, Herschel propounded the hypothesis that many of the unresolved nebulæ are not sidereal systems, but are formed of a nebulous fluid resembling in some respects that which is assumed to form the substance of comets. He held, as a natural corollary to this view, the opinion that nebulæ thus composed are not necessarily far removed beyond the limits of our own galaxy, but may be situated amidst the interstellar spaces. He showed further how there may be traced among the different orders of vaporous or fluid nebulæ the stages of a process of development leading upwards, he held, to the formation of suns resembling
I have said that the former theory has been held to be the most probable one. This has arisen from the fact that under the amazing light-gathering power of Lord Rosse's great reflector, and under the exquisite defining power of the Harvard College refractor, the Orion and Andromeda nebulæ have been resolved, in part at any rate, into discrete stars.
But the marvellous revelations afforded by the spectroscope, in the able hands of Mr. Huggins, have shown that the one great mistake into which it had been assumed that Herschel had fallen, was in reality the most magnificent of his many anticipations of modern discoveries. We cannot, indeed, assert that Herschel's speculations respecting the genesis of stars have been confirmed. They have not, however, been disproved. And the great fact which he considered as the legitimate deduction from his observations, has been placed beyond a doubt. The spectroscope tells us in a manner which admits neither of doubt nor cavil, that many
the nebulæ are composed of luminous gas, and amongst these are to be included the Orion nebulæ and all the planetary nebulæ which have as yet been observed. The Andromeda nebulæ, about which Herschel expressed no decided opinion, is found to shine with stellar light. The same is the case with all the cluster-nebulæ which have yet been examined with the spectroscope.
In my paper on the distribution of the nebulæ* I have exhibited the very remarkable and significant relations which the nebulæ present when viewed as a system. The supposed nebular zone of Sir W. Herschel has been shown not to be a reality. On the contrary, the northern nebulæ are found to be gathered into a vast cluster covering about one-fourth of the surface of the northern celestial hemisphere, and situated nearly at the pole of the galactic circle. In the southern hemisphere there is a greater uniformity of distribution, save where, within the Magellanic Clouds, nebulæ are found clustered even more closely than in the richest parts of the northern nebular cluster. These mysterious clouds, however, are considered by our leading astronomers to possess a character which separates them as well from the true nebular system as from the sidereal system. The reasons on which this opinion is founded are sufficiently remarkable per se, though they are justified, I imagine, by the theories astronomers have been so long content to hold respecting the universe: The Magellanic Clouds, it would seem, are to be looked on as belonging neither to the nebular nor to the sidereal system, because they exhibit the characteristics of both systems. * “Notes on the Nebulæ.” See THE STUDENT for March last, and the illustrative maps.
HOW TO PHOTOGRAPH THE SOLAR SPECTRUM.
In his “Treatise on Light,” M. Becquerel gives the following directions for photographing the colours of the spectrum. Many observers have remarked that chloride of silver takes different tints according to the circumstances under which it is prepared, or according to the colour of the light which strikes upon it. Thus Seebeck noticed its affecting a red tint under the influence of red light, and Sir J. Herschel observed that paper made sensitive by chloride of silver, and exposed to the action of a powerful spectrum, received such an impression that the red was vivid though of a tint approaching brick red, the yellow was wanting, the green dark and passing into black. Mr. Hunt also obtained red tints upon paper prepared with chloride of silver.
It was also known that ordinary chloride of silver when heated after being coloured violet by light, assumed a reddish tint similar to that of the red portion of the spectrum. It might be asked whether this last effect was not due to the action of heat, for in all
common chloride of silver, when its coloration begins, is slightly violet, the impression being first manifested in the most refrangible part of the spectrum, and it takes a brick red tint in the least refrangible part, and it was a curious coincidence to see the two extremities of photographic impression of the spectrum made on chloride of silver, turning it violet in the violet rays, and red in the red ones. M. Becquerel having satisfied himself in 1838 or 1839 that the action was not calorific, proceeded to examine the precise conditions of the different effects. He says, “If chloride of silver is not obtained by double precipitation on the surface of the paper, but by means of reagents in a glass vessel, and deposited on glass, porcelain, cardboard, etc.; if this chloride has not been previously exposed to light, as soon as a solar spectrum is projected upon its surface, it begins by only receiving an impression in the part beyond the violet, and we only obtain a delicate violet tint, which grows gradually darker, and no action takes place in the visible portion of the prismatic image. But if we use perfectly pure chloride without any excess of silver, and it has been previously impressioned, the effect is very appreciable: in the violet the tint deepens until it resembles that produced in diffused light, but in the red portion we obtain a slight rose tint, and no effect is distinctly manifested in the yellow and green, although a faint coloration takes place.
If the chloride of silver is obtained on the surface of paper first