“Let us,” says a French writer, “to these immortal labours add the ingenious ideas that we owe to Herschel on the nebulae, on the constitution of the Milky Way, on the Universe as a whole,—­ideas which almost by themselves constitute the actual history of the formation of the worlds,—­and we cannot but have a deep reverence for that powerful genius that scarcely ever erred, notwithstanding the ardour of its imagination.”

The ordinary spectator, looking upon the face of the heavens through a telescope, had, prior to Herschel’s time, felt his curiosity excited by the appearance here and there of filmy patches, vague in structure and irregular in shape, which, from their resemblance to clouds, received the name of nebulae.  What these were, no astronomer had succeeded in defining.  It was left for Herschel, with his rare powers of patient and discriminating observation, assisted by the more powerful instruments which his ingenuity succeeded in constructing, to discern in them innumerable groups of worlds, in various stages of formation!  A new light was thrown upon the history of the Universe.  Man was able to assist, as it were, at the process of creation, and to watch the development of a mass of incoherent matter into a perfect star.  This alone was a discovery which might well have immortalised the name of Herschel.

But we owe to him the elements of our knowledge of the Sun’s physical constitution.  He swept aside the erroneous theories and conjectures which had previously prevailed, and guided the astronomical inquirer into the right path.  He convinced himself, by long and patient researches, that the luminous envelope of the great “orb of day” was neither a liquid nor an elastic fluid; that it was in certain respects analogous to the clouds which wreathe our mountain-summits and fertilize our plains; that it floated in the solar atmosphere.  Thence he came to the conclusion that the Sun has two atmospheres, endowed with motions quite independent of each other.  An elastic fluid, now known as the photosphere, is in course of continual formation on the dark rugged surface of the solar mass; and rising, on account of its specific lightness, it forms the pores in the stratum of reflecting clouds; then, combining with other gases, it produces the irregularities or furrows in the luminous cloud-region.  When the ascending currents are powerful, they create those appearances which astronomers designate the nuclei, the penumbrae, the faculae.

Such was Herschel’s explanation of the mode of formation of the solar spots; and allowing it to be well-founded, we must expect to find—­what is, indeed, the case—­that the Sun does not always and regularly pour forth equal quantities of light and heat.  It is true that Herschel’s hypothesis has been modified by later astronomers; but his is the credit of having directed them into the right course of inquiry and observation.