18G5.] Astronomy. 439 



centre the difference in brightness between them and the surrounding 

 parts of the disc is not so easily traced. Now the reason of their 

 great brightness is believed to be that they are portions of the sun's 

 photosphere thrown up into the higher regions of the atmosphere. 

 This enables them to escape a great portion of the absorbing effect of 

 this atmosphere, which, as before shown, is particularly strong'near the 

 border, and hence when there they appear much brighter than the 

 surface around them ; but near the centre the absorption is not great, 

 so that they do not gain much by escaping it. The idea that these 

 faculse are elevations, has been confirmed by a stereoscopic impression 

 of a sun spot and some faculaa taken by Mr. De la Rue, and in which 

 while the spot appears to be a hollow the faculse appear as elevated 

 ridges. It thus appears that the faculse are elevated, and further, that 

 they retain the same appearance often for a considerable time, some- 

 times even for days together, so thatfaculae are not composed of heavy 

 matter, otherwise they could not remain elevated ; and hence the 

 faculse, as well as the photosphere of which they are only the most 

 elevated parts, are not composed of heavy matter, such as a molten sea, 

 but are rather of the nature of a cloud. 



The phenomena of the third region, or that below the photo- 

 sphere, may be comprehended in one word — sun spots. These consist 

 of an umbra or central darkness, surrounded by a less dark penumbra. 

 Mr. Dawes has discovered in some spots even a deeper darkness in the 

 centre of the umbra. Now, if it be correct to suppose that spots are 

 cavities, of which the umbra forms the bottom, and the penumbra the 

 sloping sides, then the umbra ought to encroach on that side of the 

 penumbra which is next to the visual centre of the disc. Professor 

 Wilson, of Glasgow, was the first to remark that spots really behaved 

 in this manner ; and his remark has been abundantly confirmed by 

 the Kew photographs taken under the superintendence of Mr. De la 

 Rue. It, therefore, follows that the umbra of a spot is at a lower 

 level than the penumbra ; and since luminous ridges, and sometimes 

 detached portions of luminous matter, cross over spots, it must be con- 

 cluded that the whole phenomenon is below the surface. Again, spots 

 exhibit the rotation of our luminary. If we turn to the south and 

 view the sun, spots are always seen to pass from left to right. Besides 

 the proper motion due to rotation of the sun, spots have also a proper 

 motion of their own, first observed by Mr. Carrington ; this motion is 

 also from left to right ; those near the solar equator moving fastest. 

 Mr. Carrington also remarked that spots confine themselves to the 

 equatorial regions of the sun. H. Schwabe, of Dessau, has remarked 

 that spots have a period of maximum and minimum nearly every ten 

 years, and General Sabine has found that the year of maximum sun 

 spots is at the same time that of the greatest disturbance of the earth's 

 magnetism. 



A fitting supplement to the above lecture is to be found in the 

 second series of Researches on Solar Physics : On the Behaviour of 

 Sun Spots with regard to Increase and Diminution, which has recently 

 been presented to the Royal Society by Warren De la Rue, F.R.S., 



