reach to the end of the stratum in which he is situated, so that he looks upon these patches as belonging to that system which to him seems to comprehend every celestial object. He now increases his power of vision, and applying himself to a close observation, finds the Milky Way is indeed no other than a collection of very small stars. He perceives that those objects which had been called nebulæ are evidently nothing but clusters of stars. He finds their number increase upon him, and when he resolves one nebula into stars he discovers ten new ones which he cannot resolve. He then forms the idea of immense strata of fixed stars, of clusters of stars, and of nebulæ, till going on with such interesting observations he now perceives that all these appearances must naturally arise from the confined situation in which he is now placed. Confined, it may justly be called, though in no less a space than what before appeared the whole region of the fixed stars; but which now has assumed the shape of a crookedly branching nebula, not one of the least, but perhaps very far from being the most considerable of those numberless clusters that enter into the construction of the heavens."

It cannot be denied that the passage just quoted is not very easy to understand. At one stage, or rather throughout the greater part of the passage, it seems abundantly clear that Herschel is describing our sidereal system as including multitudes of subordinate clusters and nebulæ. But then at the end, he describes it as itself a nebula, greater than some, but less than others, of numberless clusters, composing the sidereal heavens. And the per

plexity which the passage as a whole thus occasions, is accompanied by a perplexity arising from the variety of meaning which may be attributed to the different sentences. For instance, where he says that the observer "forms the idea of immense strata of fixed stars, of clusters, and of nebulæ," he might (so far as the grammatical interpretation of the sentence is concerned) mean either (1) the idea of immense strata, composed of fixed stars, clusters, and nebulæ, or (2) the idea of immense stellar strata, starclusters, and nebulæ. The latter has been the meaning usually adopted—if, at least, this particular sentence has been discussed at all. Such a meaning accords with the theory (the familiar Grindstone Theory) commonly attributed to Herschel. Nevertheless it should be manifest, from the passage just quoted (regarded as a whole), that Herschel not only recognized star-strata, including within their limits subordinate clusters and nebulæ, but that he regarded our sidereal system as a star-stratum of that kind. How, then, are we to remove the difficulties I have noted in the passage as a whole, and in its several parts? It must certainly be by taking a meaning which covers both the two views which appear contradictory, for no one will for a moment admit that Sir W. Herschel really held con-tradictory views. Accordingly, we must believe both that Herschel held our galaxy to be a stratum, including in its limits star-clusters and nebulæ, and that he regarded it as one among many systems of its own order, that is, one among many star-clusters and nebulæ, and of a higher order than those (spoken of under the same name; but

really) subordinate to, and included within, itself and its fellow systems.*

That this is Herschel's meaning we perceive clearly from a passage following almost immediately after the one just quoted. "It will appear," he says, "that many hundreds of nebulæ, of the first and second forms, are actually to be seen in the heavens, and their places will hereafter be pointed out; and many of the third form will be described." Thus, there can be seen in the heavens many hundreds of clusters and nebulæ of one kind (Forms I. and II), and also many clusters of a higher order (Form III.), within which the others exist as subordinate parts-or, in other words, we can see the clusters and nebula which form part of the architecture, as it were, of our own

* The case is one of those to which I have referred above, where we have to reason from the context in order to understand Herschel's true

meaning. And it would be unfair, I think, to blame the ordinary commentator for failing to apply such reasoning to Herschel's voluminous papers. What, however, does seem unfortunate, is the course adopted by our text-book writers, in selecting passages from Herschel's papers at random, notwithstanding these difficulties, and stringing them together as Herschel's matured views. It is as though a person not very familiar with a language were to pretend to analyse a book in that language by selecting from the book all the sentences he was able to understand. I may note, in passing, that the author of one of the best treatises on observational astronomy in existence, has been led into a most curious misapprehension. Herschel had expressed a belief that the stellar stratum extends one hundred times farther in the direction of its general level than at right angles to that level: but later (in the paper I am quoting from above) he assigned 5 to 1 as the proportion. Now the late Admiral Smyth, at p. 310 of his "Bedford Cycle, ' presents a picture of the sidereal system, showing that he had combined these two different results into one, thus giving to our system length, breadth, and thickness as 1, 54, and 100.

sidereal system; while we can see, but not in such great numbers, external nebulæ of the same order in the scale of creation as our own galaxy. Herschel, in fact, describes ten nebula of the latter order, speaking of them as external Milky Ways. Instances of the fourth order "will be related," he proceeds; "a few of the cavities mentioned in the fifth will be particularized, though many more have already been observed: so that upon the whole, I believe it will be found that the foregoing theoretical view, with all its consequential appearances, as seen by an eye enclosed in one of the nebulæ, is no more than a drawing from nature, wherein the features of the original have been closely copied; and I hope the resemblance will not be called a bad one, when it shall be considered how very limited must be the pencil of an inhabitant of so small and retired a portion of an indefinite system, in attempting the picture of so unbounded an extent."

In further confirmation of this interpretation of Herschel's views at this stage of his labours I will now quote a passage which is perfectly irreconcilable, I venture to affirm, with the simple theory of the sidereal system so commonly attributed to Sir W. Herschel.

"If," he says, "it were possible to distinguish between the parts of an indefinitely extended whole, the nebula we inhabit might be said to be one that has fewer marks of profound antiquity upon it than the rest. To explain this idea, perhaps, more clearly, we should recollect that the condensation of clusters of stars has been ascribed to a gradual approach; and whoever reflects upon the number

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of ages that must have passed before some of the clusters could be so far condensed as we find them at present, will not wonder if I ascribe a certain air of youth and vigour to many very regularly scattered regions of our sidereal stratum. There are, moreover, many places in the stratum where there is the greatest reason to believe that the stars, if we may judge from appearances, are now drawing towards various secondary centres, and will in time. separate into different clusters so as to occasion many subdivisions. Hence we may surmise that when a nebulous stratum consists chiefly of nebulæ of the first and second form, it probably owes its origin to what may be called the decay of a great compound nebula of the third form; and that the subdivisions which happened to it in the length of time occasioned all the small nebulæ which spring from it to lie in a certain range, according as they were detached from the primary one. In like manner our system, after numbers of ages, may very possibly become divided so as to give rise to a stratum of two or three hundred nebulæ ; for it would not be difficult to point out so many beginning or gathering clusters in it. This view of the subject throws a considerable light upon the appearance of that remarkable collection of many hundreds of nebula which are to be seen in what I have called the nebulous stratum of Coma Berenices. It appears from the extended and branching figure of our nebula, that there is room for the decomposed nebulæ of a large, reduced, former great one to approach nearer to us in the sides than in other parts. Nay, possibly, there