Then Herschel extended his researches to those strange objects called the nebulae. He showed that where astronomers had reckoned tens of these objects there were in reality thousands. And he found that a large proportion of the nebulae can be resolved into stars. He held that these, therefore, may be looked upon as external universes, resembling that great system of stars of which our sun is a member. We need not, at this point, dwell upon the distinction which Herschel drew between nebulae of this sort, and those objects which he held (and as we now know, justly) to be true clouds, formed of some vaporous substance, of the actual nature of which he forbore to express an opinion. Let it suffice to remark that in whatever mode those vaporous nebulae might be supposed to be formed, it was clear to Herschel that they cannot be held to lie necessarily beyond the system of the fixed stars, as he held to be certainly the case with the stellar nebulae.
Since Herschel's day a multitude of important discoveries have been made. His son, the present Sir John Herschel, carried the system of star-gaugings over the southern heavens, having first trained himself for the work by verifying Sir William's northern star-gaug-ings. The eminent astronomer Struve and others have applied a series of tests to the basis of Herschel's theory of the universe. Increased telescopic power has been applied to the examination of the nebulae. And lastly, a mode of research more wonderful than the boldest pioneers of science had ventured to hope for has been applied to determine what the stars and nebulae really are, nay even the very elements of which they are constituted.
Therefore we stand in a position so far in advance of that to which it was in Herschel's power to attain, that the attempt to modify his theories need no longer be thought to savour of undue boldness. Half a century does not pass without bringing a vast extension of knowledge, and certainly the last half-century has been no exception to this rule; insomuch that could the great astronomer take his place again among us, and become cognisant of the vast strides which his favourite science has made since he left us, he would be the first to point out that many of his views require to be modified or even to be wholly abandoned.
For instance, let us consider the meaning of the following observation made by the younger Herschel. While'sweeping' the southern heavens, this eminent astronomer noticed occasionally the existence of faint outlying streamers belonging to the Milky Way, not only irresolvable into stars, but so exceedingly distant that he could scarcely speak of them as really visible. He was sensible of their existence, but when the eye was turned directly upon them they vanished, insomuch that, he says, ' the idea of illusion has repeatedly arisen subsequently,' yet when he came to map down the places where these phantom star-streams had been detected, he found that they formed regular branches of the galactic system.
Now these outlying star-streams prove first of all that the star-system is not disc-shaped, but spiral in figure. Between the stars which form the ordinary streams of the Milky Way, and those which form the phantom streams, there must lie regions in which stars are either altogether wanting or strewn with much less profusion than in either the nearer or the farther stream.
But this is not the only nor the chief conclusion which may be drawn from the existence of the almost evanescent star-streams. According to Herschel's views the stars which compose those streams are only faint through enormity of distance. They may be as large as our sun, many of them perhaps far larger. And between them there may yawn distances as large as those which separate us from Arcturus or Aldebaran. Now, this being so - the outlying parts of our own sidereal system being removed so far from us as to be all but evanescent in Herschel's splendid reflector-how much greater ought to be the faintness of the sidereal systems which lie outside ours! If the nebulae are really such systems, and made up of suns like our own, then not only ought Herschel's great reflector to fail in rendering them visible, but even Lord Rosse's noble mirror would require to be increased a hundredfold in power before we could see them. For clearly the nebulae, which appear as mere tiny specks upon the vault of heaven, must be very much farther away than the confines of our system, if they are comparable with it in size.
Therefore we must have 'of two things one.' Either the confines of our sidereal system are constituted very differently from the parts in our neighbourhood; or the nebulae are constituted very differently from the sidereal system. We say, of two things one, meaning that one of the two views must be true; but it is plain that there is nothing to prevent both being true.
We may next come to the inquiry whether these views are severally supported by any special evidence.
Now as to the first, it happens that the southern heavens surveyed by the younger Herschel afford evidence such as Sir William Herschel was not possessed of. The former has seen places in the southern skies where the outline of the Milky Way is so sharply defined that even in the telescope the sudden change from a background of black sky to the sprinkled light of the galaxy is not lost. One half of the field of view will exhibit the former aspect, the other the latter. Now if we consider a cloud, or a dense flight of birds, or any cluster of objects exhibiting a well-defined outline, we see at once what that well-defined outline means. It signifies that the eye is directed along the edge or surface of a distinct cluster of objects-in one case globules of water, in another birds, and so on and the idea is at once precluded that the eye is within the cluster, of whatever nature that cluster may be. Therefore the theory that the sun forms one of a system of stars spread pretty uniformly over a disc-shaped space must he given up; for were it true, the approach to the Milky Way would always be gradual.