110 



KNOWLEDGE. 



[Mat 1, 1895. 



spot approaches the limb. The penumbra on the side 

 of the spot remote from the limb would, therefore, tend 

 to be hidden, as Prof. Wilson pointed out. The nucleus 

 ■woidd not only be made darker, but would suffer a 

 fictitious extension in the other direction. In short, the 

 actual effect to be expected on this hypothesis is not 

 the mere hiding of the penumbra and nucleus on the side 

 furthest from the limb. It would be a threefold effect ; 

 the spot, as a whole, would be darker, the umbra would be 

 relatively enlarged, and the contrast between the photo- 

 sphere and the spot on the side furthest from the limb 

 would be increased. 



So far as my own observations go, only one of these 

 three effects is perceived, and that not very markedly — the 

 sharper contrast at the inner edge of the spot. It seems 

 to me clear, then, that we cannot look upon a sunspot as 

 merely an absorption effect. 



That it is partly so, the spectroscope proves beyond a 

 doubt. I have been quite unable to follow those observers 

 who can find no broadening of the Fraunhofer lines in a 

 spot spectrum. There are, indeed, wide belts of the 

 spectrum where the lines broadened are few, and the 

 broadening shght. But from C to D, and near the h lines, 

 I cannot understand how the effect can be overlooked. 



Still, when the entire additional broadening seen 

 throughout the whole sjiectrum is allowed for, it accounts 

 for but a very small fraction of the loss of light in the sjjot. 

 Gaseous absorption there unquestionably is, but it fails to 

 account for the darkness of the spot. 



We have, however, come to recognize the existence 

 round the sun of a vast quantity of dark solid particles — 

 dark, at all events, as compared with the photosphere. It 

 is to this solar " dust " or " smoke " that the chief part of 

 the rapid falling off of the sun's light near the limb is due. 

 If the darkening of the spot were due to this dust filhng 

 up the hollow of the spot, then we should have a general 

 darkening throughout the spectrum, as is most certainly 

 the case, but should also have the threefold effects described 

 above, but which are much less obvious than they should 

 be. 



It seems conclusive, then, that the main cause of the 

 darkness of a spot is not increased absorption — either 

 selective, as from gases, or general, as from solid particles. 

 It is chiefly due to diminished radiation. 



The light of the photosphere has been ascribed, since 

 Mr. .Johnstone Stoney's paper of nearly thirty years ago, 

 to the condensation of carbon. There is a certain level 

 where the uprushing streams of glowing gases from the 

 centre of the sun become so far cooled by expansion 

 and radiation as to condense carbon, and possibly silicon, 

 into the form of clouds of dazzling brilliance. For the 

 most part, no doubt that level remains pretty constant. 

 It must do so, indeed, for the greatness of the sun's 

 attractive force will render the diminution of atmospheric 

 pressure for every mile of ascent very rapid. 



The exception is seen in such a luminous region as we 

 observe in the midst of the northern spot-group. Here 

 there has evidently been an uprush of gases, so strongly 

 heated, and rising with such speed, that condensation does 

 not take place until a much higher level than usual is 

 attained, and there it is both sudden and complete. These 

 brilliant clouds rise above much of the dust layer, no 

 doubt, and appear the more brilliant on that account, 

 as they escape some of the dimming which the dust layer 

 causes. 



With the uprush there is necessarily associated a region 

 of diminished pressure, a region of downrush into which 

 no small amount of this darker, colder solid matter is 

 drawn. Sometimes we have the uprush coming up nearly 



straight from below. In this case the corresponding 



depressions tend to be pretty evenly balanced, one in 

 advance of, the other following the uprush. We have 

 then a group of spots like the northern one, essentially a 

 pair of spots. In this case, as the strength of the outburst 

 grows, the two depressions at either end are forced apart as 

 they widen. In the case of the western spot-group the 

 outburst seems itself to have had a strong forward motion. 

 Hence the accompanying depressions were mostly confined 

 to a single spot, which moved rapidly forward as it grew. 



We may look, then, upon the nucleus of a spot as being 

 relatively dark, partly because seen through an increased 

 depth of absorbing gases, partly and to a much greater 

 extent because the spot cavity is filled with comparatively 

 cool solid particles, and partly, and perhaps chiefly, because 

 the floor of the spot hes below the level at which gaseous 

 carbon suffers condensation. A spot is thus a region of 

 diminished radiation, as well as of increased absorption. 

 It is a hollow, but so shallow relatively to the vast bulk of the 

 sun as almost to justify us in speaking of it as a surface 

 stain. It is a hollow, but not an empty one, nor filled 

 merely with gases. It is fiUed with matter of the same 

 nature as that which causes the limb of the sun to look so 

 much less bright than the centre — that is to say, in all 

 probabihty, with finely divided solid particles that are far 

 advanced in cooling. 



ANOTHER SPECTROSCOPIC BINARY STAR. 



By Miss A. M. Clehke, Authoress of " Tlu System of the 



Stars " (171(1 " A Popular History of Astronomy during the 



Nineteenth Century," <i'-c., li'v. 



IT seems that 8 Cephei is to be added to the list of 

 these wonderful objects. The result is not altogether 

 unexpected. Successive discoveries, and especially 

 that of the duplicity of /3 Lyrse, led up to it. Never- 

 theless, the actual revelation of the fact is of profound 

 interest. It throws open new views, modifies old ones, and 

 must powerfully stimulate research into the conditions of 

 stellar variability. 



The light-changes of S Cephei were detected by Goodricke, 

 of York, in 1784, and have since been attentively watched. 

 They are accomplished, on the whole, with exactness and 

 punctuality. Argelander stated in 1848 (Astr. Xac/t., No. 

 624) that, during seven years of constant observation, he 

 had perceived no break in their uniformity. Schonfeld 

 concluded in 1876 (Zweitir Kataloy, p. 49) against the 

 presence of any genuine irregularities. The light-curve, 

 however (ns the accompanying figure shows), is by no 

 means smooth or symmetrical. The rise from 4-9 to 3-7 

 magnitude is executed in Id. li{-6h., or in just one-third 

 of the entire period of 5d. 8'8h., the decline being, 

 moreover, interrupted by a stationary interval beginning 

 about sixteen hours after maximum. This curious halt, 

 which almost suggests an attempt at a second maximum, 

 and is common to many variable stars, is vouched for, in 

 the case of 8 Cephei, by both Schonfeld and Argelander, 

 but does not appear in Prof. Gudemans' curve representative 

 of its ijhases. Perhaps it is less emphasized at some times 

 than at others. The range of variability is sensibly 

 constant. Schmidt recorded, it is true, one completely 

 abortive maximum. The increase of brightness due on 

 May 6th, 1868, was barely indicated [Astr. Xach., No. 

 1745). But this alleged failure, since it rests on the 

 testimony of one, albeit a very competent observer, suggests 

 illusion. The character of the star for accuracy in 

 variation is otherwise unimpeached. 



It was given the first place on M. Belopolsky's list of 



