68 Prof. Phillips on a Zone of Spots on the Sun. [Mar. 22, 



Luminosity of the Sun. 



Attentive observation shows the light of the sun to be feeblest toward the 

 edges of the disk, strongest about the centre. By M. Chacoruac's measure 

 the ratio of the central to the marginal light is 100 to 45. Looking 

 directly on the central regions, the light is found to be much the brightest 

 on the apparent summits of the undulations of the photosphere ("rice- 

 grains'^) ; and looking to the hmb, it is the faculse which are the brightest 

 parts. In each case it is the outermost parts of the photosphere which 

 are the brightest, and it is the innermost parts which are the darkest. 

 The depth of shade appears to be in direct proportion to the depth 

 below the outer surface of the photosphere. It appears to me that such 

 appearances would follow naturally from the hypothesis mentioned in my 

 first communication on this subject *. The hypothesis is that the lowest 

 parts of the umbral spaces yield the least refrangible and least luminous 

 rays, such as belong to the space about the red end of the spectrum. 

 These spaces may not be black, not even very dark, except by comparison 

 with the brilliant spaces around ; where the rays from them pass into the 

 photosphere, they heat it, as the dark rays separated by sulphuret of 

 carbon in Tyndall's experiment heat the platinum foil, and other solid 

 bodies. Bodies thus heated emit new rays according to their nature ; the 

 solar photosphere is of such a nature as to send to us the mingled pencils 

 which we receive ; the principal effect of this kind being at a maximum on 

 the outermost layer. The effect of this will be to cause streams of the 

 most luminous rays from the most elevated parts of the photosphere, which 

 will be seen directly in front, about the sun's centre, while toward the edges 

 the sides of the undulations alone will be seen, and the rays which, they 

 yield will be less luminous — the only parts which are there very bright 

 being the high ridges of the faculse. 



Another view has presented itself to me. If we admit the depressed 

 part to be the body of the sun disclosed from below the luminous envelope, 

 we may suppose its darkness to be due simply to radiation. For however 

 hot the sun may be, if its composition be like that of the earth, the fused 

 parts would be cooled and darkened at the surface where it may be 

 uncovered — very much darkened where the exposure is complete (the 

 umbra), partially so where the envelope is not wholly removed (the pe- 

 numbra). It might be some test of this view, if the hourly changes of the 

 umbra, immediately after its first appearance, could be accurately noted 

 in respect of the degree of darkness as well as of the change of form. 

 The umbra ought to grow darker and darker, never lighter and lighter, 

 except by the overspreading of photosphere, which would be indicated 

 independently by changes on the penumbral area. 



From what I have seen in the course of these observations, I infer that 

 the study of the physical condition of the solar spots cannot be regarded 

 as likely to yield data of sufficient weight, if it do not include determina- 

 * Proc. Roy. Soc. January 1865. 



