Page:Popular Science Monthly Volume 3.djvu/585

Rh vindicated Herschel's claim to be looked upon as the most clear-sighted, as well as the boldest and most original, of astronomical theorizers.

Herschel had pointed out various circumstances which, in his opinion, justified a belief in the existence of a nebulous substance—fire-mist or star-mist, as it has been termed—throughout interstellar space. He had discovered and observed several thousand nebulæ, and he considered that among these he could detect traces of progressive development. Some nebulae were, he supposed, comparatively young; they showed no signs of systematic aggregation or of central condensation. In some nebulae he traced the approach toward the formation of subordinate centres of attraction; while in others, again, a single centre began to be noticeable. He showed the various steps by which aggregation of the former kind might be supposed to result in the formation of star-clusters, and condensation of the latter kind into the formation of conspicuous single stars.

But it was felt that the strongest part of Herschel's case lay in his reference to the great nebula of Orion. He pointed out that, among all the nebulas which might be reasonably assumed to be star-systems, a certain proportionality had always been found to exist between the telescope which first detected a nebula and that which effected its resolution into stars. And this was what might be expected to happen with star-systems lying beyond our galactic system. But how different is this from what was seen in the case of the Orion nebula! Here is an object so brilliant as to be visible to the naked eye, and which is found, on examination, to cover a large region of the heavens. And yet the most powerful telescopes had failed to show the slightest symptom of resolution. Were we to believe that we saw here a system of suns so far off that no telescope could exhibit the separate existence of the component luminaries, and therefore (considering merely the observed extent of the nebula, which is undoubtedly but a faint indication of its real dimensions) so inconceivably enormous in extent that the star-system of which our sun is a member shrinks into nothingness in comparison? Surely it seemed far more reasonable to recognize in the Orion nebula but a portion of our galaxy—a portion very vast in extent, but far inferior to that "limitless ocean of universes" presented to us by the other view.

And when Sir W. Herschel was able, as he thought, to point to changes taking place within the Orion nebula, it seemed yet more improbable that the object was a star-system.

But now telescopes more powerful than those with which the elder Herschel had scanned the great nebula were directed to its examination. Sir John Herschel, following in his father's footsteps, applied himself to the diligent survey of the marvellous nebula with a reflecting telescope 18 inches in aperture. He presented the nebula to us as an object of which "the revelation of the 10-feet telescope was but the