34 



Mr. G. J. Stoney on the Physical 



[Recess, 



lower temperature has but just begun to glow, we know that this situation 

 of minimum difference of brightness is found in the orange ; at tempera- 

 tures approaching that of the photosphere it has removed at all events as 

 far as the line A, that is nearly to the extreme verge of the visible spectrum, 

 and it has, perhaps, advanced beyond it. This, as we shall find further 

 on, explains how some solitary stars can attain a depth of colour that ap- 

 proaches crimson. 



54. It appears from the analysis which has-been made that none other 

 of the gases in the solar atmosphere that extend as far as the stratum 

 from which iron lines come, can compare in quantity with hydrogen and 

 iron ; and from what has been stated in § 36, we may be sure that there is 

 no very abundant gas which comes to its limit in the hot regions that in- 

 tervene between this stratum and the photosphere. Hydrogen and iron are 

 accordingly the principal ingredients of the parts of the sun's atmosphere 

 which extend beyond the photosphere. 



Section IV. — Of the Photosphere and the subjacent parts. 



55. In interpreting phenomena of solar spots we should never forget the 

 disadvantages under which we attempt the enterprise. Our theory may 

 be true, but it is incomparably more meagre than our knowledge of the 

 causes of terrestrial weather. Our observations may be correct, but they 

 give us only outside glimpses, and from such a distance that France or 

 Spain would be specks too small to make out whether they are round or 

 square. We must not imitate the peasant who saw from afar the smoke of 

 a great city, and persuaded himself he had a very good idea of the kind of 

 place a city is. If our explanations of the phenomena of terrestrial weather 

 are dim and unsatisfying, we cannot reasonably ask from a theory of the cor- 

 responding phenomena of the sun, even though it were beyond a doubt the 

 true theory, more than the first hazy and rude sketch of an interpretation. 



56. Many fixed gases which are too heavy to extend at all, or in any abun- 

 dance, through the stratum of minimum temperature, must wax in density 

 very rapidly within it. Hence the density of the solar atmosphere becomes 

 almost suddenly greater at the shell of luminous clouds. This may be 

 the cause of an appearance not unfrequent in spots near the margin of the 

 sun's disk, in which situation the further side of the umbra of a spot is 

 often bordered by a bright crescent, giving to the umbra the appearance of 

 a hole punched through a plate. This appears to be because there is, in 

 these cases, in reality a depression of the dense strata at the umbra, shal- 

 low, perhaps, but yet with sides sufficiently inclined to enable light coming 

 so obliquely as to suffer total reflection* against the flatter surface of the 

 penumbra, to escape through it. A similar cause may, perhaps, and pro- 

 bably does, enable light to escape from patches of the penumbra when 

 the surface of the penumbra is irregularly undulating in a sufficient degree. 



* Such as that which produces Fata Morgana. 



