238 Royal Society : — 



The authors then attempted to answer the following questions : — 



(1) Is the umbra of a spot nearer the sun's centre than the penum- 

 bra, or, in other words, is it at a lower level ? 



(2) Is the photosphere of our luminary to be viewed as composed 

 of heavy solid, or liquid matter, or is it of the nature either of a gas 

 or cloud 1 



(3) Is a spot (including both umbra and penumbra) a pheno- 

 menon which takes place beneath thelevel of the sun's photosphere- 

 or above it ? 



In answering the first of these, it was shown that if the umbra 

 is appreciably at a lower level than the penumbra, we are entitled 

 to look for an apparent encroachment of the umbra upon the penum- 

 bra on that side which is nearest the visual centre of the disk. 

 This, in fact, was the phenomenon which Wilson observed, and which 

 led him to the belief that the umbra was nearer the sun's centre than 

 the penumbra. 



Two Tables are then given, showing the relative disposition of the 

 umbra and penumbra for each spot of the Kew pictures available for 

 this purpose. 



In the first of these, this disposition was estimated from left to 

 right, this being the direction in which spots advance across the 

 visible disk by rotation ; while in the second Table this disposition 

 was estimated in a direction parallel to circles of solar longitude, and 

 in this Table only spots having a high solar latitude were considered. 



From the first of these Tables it was shown that, taking all those 

 cases where an encroaching behaviour of the umbra in a right and 

 left direction has been perceptible, 86 per cent, are in favour of the 

 hypothesis that the umbra is nearer the centre than the penumbra, 

 while 14 per cent, are against it. It also appeared that, taking all 

 available spots and distributing them into zones according to their 

 distance from the centre, this encroaching behaviour is greatest when 

 spots are near the border, and least when they are near the centre. 



From the second Table, in which only spots of high latitude were 

 considered, it was shown that, taking all those cases where an 

 encroaching behaviour of the umbra in an up-and-down direction 

 has been perceptible, 80*9 per cent, are in favour of the hypothesis 

 that the umbra is nearer the centre than the penumbra, while 

 19*1 per cent, are against it. 



The result of these Tables is therefore favourable to this hypothesis. 



The authors next endeavoured to answer the following question : — 

 Is the photosphere of our luminary to be viewed as composed of 

 heavy solid, or liquid matter, or is it of the nature either of a gas or 

 cloud ? 



It was observed that the great relative brightness of faculee near 

 the limb leads to the belief that these masses exist at a high eleva- 

 tion in the solar atmosphere, thereby escaping a great part of the 

 absorptive influence, which is particularly strong near the border ; 

 and this conclusion was confirmed by certain stereoscopic pictures 

 produced by Mr. De la Rue, in which the faculse appear greatly 

 elevated. It was remarked that faculae often retain the same appear- 

 ance for several days, as if their matter were capable of remaining 

 suspended for some time. 



