Looking, however, below the surface of things, we find the semi-fabulous quintessence to be unobtrusively doing all the world's work. It embodies the energies of motion; is, perhaps, in a very real sense, the true _primum mobile_; the potencies of matter are rooted in it; the substance of matter is latent in it; universal intercourse is maintained by means of the ether; cosmic influences can be exerted only through its aid; unfelt, it is the source of solidity; unseen, it is the vehicle of light; itself non-phenomenal, it is the indispensable originator of phenomena. A contradiction in terms, it points the perennial moral that what eludes the senses is likely to be more permanently and intensely actual than what strikes them. FOOTNOTES: [Footnote 74: _Theory of Light_, second edition, p. 28.] [Footnote 75: _Encyclopædia Britannica_, vol. xxv., p. 106.] [Footnote 76: _Report of the British Association_, 1900, p. 626.] [Footnote 77: _Æther and Matter_, p. 26.] [Footnote 78: _Nature_, vol. xlii., p. 453.] [Footnote 79: Larmor, _Report British Association_, 1900, p. 625.] [Footnote 80: _Electricity and Matter_, p. 51.] [Footnote 81: _The Structure of the Universe_, Rede Lecture, June 10, 1902, p. 14.] [Footnote 82: _Nature_, June, vol. lxii., p. 451.] [Footnote 83: _Proceedings of the Royal Institution_, vol. xvii., p. 177.] [Footnote 84: _Nature_, vol. lxx., p. 101.] CHAPTER XII THE FORMS OF NEBULÆ Sir William Herschel's celestial surveys first made the classification of nebulæ practicable. Until he began grinding specula at Bath very few such objects were known, and those too imperfectly for the effectual discrimination of their differences. Arrangement presupposes comparison, and comparison some variety of specimens to be compared, which became available only through Herschel's scrutiny. The rapidity and penetrative power of his observations in this field almost passes belief. He detected with discernment. Discovery and enrolment did not satisfy him; he was, besides, keen to note analogies and contrasts, likenesses and dissimilitudes. He could not see without at the same time setting in order what he saw; and the law of order that commended itself to him was founded on an evolutionary principle. The contents of the heavens seemed to fall spontaneously, as he regarded them, into genetic sequences; and the nebulæ with particular facility. The criterion adopted was that of progressive condensation. Development must clearly, he judged, be attended by contraction and local brightening. Diffused milky tracts represented cosmic formations in their most rudimentary form; they assumed, through the unremitting action of gravity in drawing their particles together, a more compact texture, more definite shapes, and a heightened lustre.