Thus, then, the structure of the sun as now known is very different from the conception of it which would be given by mere naked-eye, or even telescopic, observation. We have first the visible bright surface, or photosphere, with its spots, faculæ, and mottling, and surrounded by a kind of atmosphere which absorbs much of its light, as is evidenced by the fact that the solar limb is much darker than the centre of the disc (Plate V.); next the reversing layer, consisting of an envelope of incandescent vapours, which by their absorption of the solar rays corresponding to themselves give rise to the dark lines in the spectrum. Beyond these again lies the chromosphere, rising into gigantic eruptive or cloud-like forms in the prominences; and yet further out the strange enigmatic corona. It must be confessed that the reversing layer, the chromosphere, and the corona lie somewhat beyond the bounds and purpose of this volume; but without mention of them any account of the sun is hopelessly incomplete, and it is not at all improbable that a few years may see the spectroscope so brought within the reach of ordinary observers as to enable them in great measure to realize for themselves the facts connected with the complex structure of the sun. In any case, the mere recital of these facts is fitted to convey to the mind a sense of the utter inadequacy of our ordinary conceptions of that great body which governs the motions of our earth, and supplies to it and to the other planets of our system life and heat, light and guidance. With the unaided eye we view the sun as a small tranquil white disc; the telescope reveals to us that it is a vast globe convulsed by storms which involve the upheaval or submersion, within a few hours, of areas far greater than our own world; the spectroscope or the total eclipse adds to this revelation the further conception of a sweltering ocean of flame surrounding the whole solar surface, and rising in great jets of fire which would dissolve our whole earth as a drop of wax is melted in the flame of a candle; while beyond that again the mysterious corona stretches through unknown millions of miles its streamers of silvery light--the great enigma of solar physics. Other bodies in the universe present us with pictures of beautiful symmetry and vast size: some even within our own system suggest by their appearance the presence within their frame of tremendous forces which are still actively moulding them; but the sun gives us the most stupendous demonstration of living force that the mind of man can apprehend. Of course there are many stars which are known to be suns on which processes similar to those we have been considering are being carried on on a yet vaster scale; but the nearness of our sun brings the tremendous energy of these processes home to us in a way that impresses the mind with a sense almost of fear.