The rapid changes in the forms of these solar cloud-summits are beautifully shown in the marvellous photographs taken by Janssen at Meudon, with exposures reduced at times to 1/100000 of a second! By their means, also, the curious phenomenon known as the _réseau photosphérique_ has been made evident.[509] This consists in the diffusion over the entire disc of fleeting blurred patches, separated by a reticulation of sharply-outlined and regularly-arranged granules. The imperfect definition in the smudged areas may be due to agitations in the solar or terrestrial atmosphere, unless it be--as Dr. Schemer thinks possible[510]--merely a photographic effect. M. Janssen considers that the photospheric cloudlets change their shape and character with the progress of the sun-spot period;[511] but this is as yet uncertain. The "grains," or more brilliant parts of the photosphere, are now generally held to represent the upper termination of ascending and condensing currents, while the darker interstices (Herschel's "pores") mark the positions of descending cooler ones. In the penumbræ of spots, the glowing streams rushing up from the tremendous sub-solar furnace are bent sideways by the powerful indraught, so as to change their vertical for a nearly horizontal motion, and are thus taken, as it were, in flank by the eye, instead of being seen end-on in mamelon-form. This gives a plausible explanation of the channelled structure of penumbræ which suggested the comparison to a rude thatch. Accepting this theory as in the main correct, we perceive that the very same circulatory process which, in its spasms of activity, gives rise to spots, produces in its regular course the singular "marbled" appearance, for the recording of which we are no longer at the mercy of the fugitive or delusive impressions of the human retina. And precisely this circulatory process it is which gives to our great luminary its permance as a _sun_, or warming and illuminating body. FOOTNOTES: [Footnote 405: _Mem. R. A. S._, vol. xxi., p. 157.] [Footnote 406: _Ibid._, p. 160.] [Footnote 407: _Month. Not._, vol. xxi., p. 144.] [Footnote 408: _Le Soleil_, t. i., pp. 87-90 (2nd ed., 1871).] [Footnote 409: See _ante_, p. 58.] [Footnote 410: _Observations at Redhill (1863)_, Introduction.] [Footnote 411: _Month. Not._, vol. xxxvi., p. 142.] [Footnote 412: _Cape Observations_, p. 435, _note_.] [Footnote 413: _Month. Not._, vol. x., p. 158.] [Footnote 414: _Rosa Ursina_, lib. iii., p. 348.] [Footnote 415: _Observations at Redhill_, p. 8.] [Footnote 416: _Op._, t. iii., p. 402.] [Footnote 417: _Rosa Ursina_, lib. iv., p. 601. Both Galileo and Scheiner spoke of the _apparent_ or "synodical" period, which is about one and a third days longer than the _true_ or "sidereal" one. The difference is caused by the revolution of the earth in its orbit in the same direction with the sun's rotation on its axis.] [Footnote 418: _Rosa Ursina_, lib. iii., p. 260.]