38 De la Rue, Stewart, and Loewy on Solar Physics. [Jan. 26, 



The authors then attempted to answer the following questions: 



(1) Is the umbra of a spot nearer the sun's centre than the penumbra, 

 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 ? 



(3) Is a spot (including both umbra and penumbra) a phenomenon 

 which takes place beneath the level of the sun's photosphere or above it ? 



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

 preciably at a lower level than the penumbra, we are entitled to look for 

 an apparent encroachment of the umbra upon the penumbra 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 distri- 

 buting 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 con- 

 sidered, 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 faculse near the 

 limb leads to the belief that these masses exist at a high elevation in the 

 solar atmosphere, thereby escaping a great part of the absorptive in- 

 fluence, 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 faculae appear greatly elevated. It was remarked that 

 faculse often retain the same appearance for several days, as if their matter 

 were dbpable of remaining suspended for some time. 



A Table was then given, showing the relative posit on of sun-spots and 



