1868.] 



Constitution of the Sun and Stars. 



7 



at all events exists, if the space under the clouds be clear ; and the clouds 

 are in the position of a luminous and partially transparent body, with a 

 still brighter body beyond. If the average condition of the nearer body, 

 the screen of clouds, be such that it is in a considerable degree opaque, 

 then will a small spot in it which is thinner and consequently more trans- 

 parent than the neighbouring parts appear brighter than they ; whereas, 

 if the average condition of the nearer body allow rays to pass pretty freely 

 through it, then will a thin spot appear but little brighter than the parts 

 around, and the circumstances might even be such as to render it darker 

 than them. We find both these appearances on the sun's disk. In the 

 middle of the sun's disk we find it to be most luminous ; and here the clouds 

 would intercept least of the greater brightness beyond. In the marginal parts 

 of the disk the spectator looks obliquely through the stratum of clouds, 

 which is therefore more opaque to his view, so that, in approaching the 

 edge of the disk, the less intense light emitted by the clouds would be pro- 

 gressively less and less fortified by the splendour within. The brightness 

 of the disk would be accordingly shaded off towards the edge. At the 

 same time thinner parts of the film, if not too extensive, would be seen 

 conspicuously as faculse near the edge of the disk, while towards the centre 

 their brightness would be merged in the general illumination around. 



14. But, further, as the shell of clouds is at a much lower temperature 

 than the adjoining layer of the atmosphere beneath it, while it is subjected 

 to but little less pressure, it is evident that there must be a violent motion 

 of convection between the two, the chilled portions descending, while the 

 hot vapours from below boil upwards. Cloud will form in the rising 

 vapours, but it will be less dense than that of the parts more effectually 

 cooled. The appearance will be very much like what we see when looking 

 from above upon water in the act of boiling, the smooth tops of the columns 

 of ascending water being represented on the sun by the brighter patches 

 due to the thinness of that cloud which can maintain its existence in the 

 hotter vapours, while the turmoil which is seen in the water between these 

 columns corresponds to the darker interstitial spaces which give to the sun's 

 surface a minutely granulated appearance (rice-grains, willow-leaves, &c), 

 and in which the cloud at times becomes so opaque that those flakes which 

 by prolonged emission become the most dusky seem to show like black or, 

 at least, very dark pores. This honeycombed structure of the stratum of 

 clouds will modify the effect of obliquity in rendering the marginal parts 

 of the sun's disk less bright than the centre. It will cause the effect to be 

 perceived further from the border than it otherwise would be. 



15. So far, then, the hypothesis of a clear space between the clouds and 

 the ocean seems to square with the phenomena ; but upon a further scru- 

 tiny we are forced to resign it as untenable. For, as has been explained, 

 the light which has suffered but one reflection, or been but once scat- 

 tered, by the body of the sun, falls short of that which emanates di- 

 rectly from the clouds, and the greater brightness of the background is 



