8 



Mr. G. J. Stoney on the~Physical 



[Recess, 



due to the additions made (1) by the rays emitted by the ocean in 

 virtue of its higher temperature, and (2) by the light which has suf- 

 fered more than one reflection or been more than once scattered at the 

 surface of the ocean. Now the umbrse of spots exhibit to us the body 

 of the sun so dark when compared with the luminous clouds, that the 

 great brightness of the facula? cannot be due to the light emitted by the 

 ocean. It must therefore be due to the second cause, which, as we 

 know, can only produce any considerable effect if the clouds are of such 

 a nature that they scatter light abundantly. But, again, we know from 

 the proximity in which the umbrae of spots have been seen to the edge of 

 the disk, that the interval between the clouds and the ocean is trifling as 

 compared with the superficial dimensions of many faculse. Hence, if the 

 illumination of the background be due to the second cause, to light reflected 

 or scattered from the body of the sun, the parts under extensive thin por- 

 tions of the clouds would be sensibly less illuminated, and would give rise to 

 an appearance more like that of penumbrae than of faculse. The hypothesis 

 would therefore fail to account for large faculse. Its rejection is also de- 

 manded by the appearance of the spectrum. For if the clouds had the 

 property of scattering light in the degree whieh would account for the gra- 

 nulated aspect of the photosphere, they would in the same proportion 

 emit light feebly ; and the whole light reaching us, whether from or through 

 them, would fall very perceptibly short of the maximum corresponding to 

 their temperature. And as, on the other hand, a gas is a perfect emitter of 

 the rays of which its spectrum consists, there could not fail to be conspi- 

 cuous bright lines from those gases which extend only to the hotter strata of 

 the solar atmosphere. Now it is certain that no such lines are conspicuous. 



16. The same objections lie with still more force against the hypothesis 

 that the clouds are in contact with a polished ocean. We may therefore 

 summarily dismiss this hypothesis. 



17. Let us then turn to the alternative of an interval with mist and 

 rain. The mist beneath the clouds, as it is found in a hotter region, 

 would emit more light, though the mist were no more dense than the 

 cloud. But the mist is probably much more dense ; and it is natural to 

 suppose that it is dense enough to be opake, in which case, if it be formed 

 of a material which is a good radiator, it will emit light of almost the max- 

 imum intensity which can be emitted by a body of its temperature. Indeed 

 this effect would be produced if the mist and rain were in a quantity much 

 less than that which would be opake, in consequence of the assistance ren- 

 dered by the body of the sun beneath, and that without any hypothesis as 

 to the state of the latter, except only that it is opake and at as high a tem- 

 perature as the mist. Now as it is likely that the average quantity of mist 

 and rain is much more than this, its density may undergo very considerable 

 fluctuations without its ceasing to pour forth its full torrent of light and 

 heat. Such, then, appears to be the brighter background which shines 

 through the clouds/ As in the last case, the currents of convection which 



