38 



SCIENCE. 



[N. S. Vol. II. No. 28. 



its selective absorption produces the Fraun- 

 hofer lines in the solar spectrum. It prob- 

 ably is somewhat less than 1,000 miles in 

 thickness. The reversing layer contains a 

 considerable quantity of those vapors which 

 have given rise to the brilliant clouds of 

 the photosphere, just as the terrestrial at- 

 mosphere adjacent to clouds is itself satu- 

 rated with the vapor of water. 



(d) The chromosphere and prominences 

 are permanent gases, mainly hydrogen and 

 heUum, mingled with the vapors of the 

 reversing layer but I'ising to far greater 

 elevations than the vapors do. Jets of in- 

 candescent hydrogen appear to ascend be- 

 tween the photospheric clouds, much like 

 flames plaj'ing over a coal fire. Calcium vapor 

 is the most intenselj^ marked of all the metals 

 in the solar spectrum, even more so than of 

 iron, which has over 2,000 line-coincidences, 

 while calcium has only about 80. 



(e) Still above photosphere and promi- 

 nences is the corona, hitherto observable 

 only during total eclipses, and extending 

 to elevations far greater than any truly so- 

 lar atmosphere possiblj- could. The char- 

 acteristic gTeen line of its sj)ectrum, due to 

 a substance not yet discovered on the Earth, 

 and hence called ' coi'onium,' is brightest 

 close to the Sun's limb, and during the 

 eclipse of 1st Januarj^, 1889, it was traced 

 outward by Professor Keeler to a distance 

 of 32.5,000 miles. But much of the coronal 

 light is known to originate in something- 

 other than the gaseous incandescence of 

 hydrogen and coronium, because of the dark 

 lines seen to cross its spectrum. These indi- 

 cate solar light, reflected probably from 

 small meteoric particles, possibly the debris 

 of comets, circulating about the Sun in or- 

 bits of their own. 



Dr. Huggins and Dr. Schuster maintain 

 the view that the coronal streamers are in 

 part due to electric discharges. The cor- 

 ona appears to be a very complex phenome- 

 non, and as yet it is onlj' in part under- 



stood. Two rival theories are now promi- 

 nent ; Mr. Schaeberle's mechanical theory 

 (Lick Observatory Reports on the Total Eclipse, 

 22d December, 1889), and Professor Bige- 

 low's theory ( The Solar Corona discussed by 

 Spherical IIarmonics,Wsishmgton, 1889), that 

 the coronal light is merelj' a phenomenon 

 of the Sun's magnetism. But neither of 

 these theories has yet been shown compe- 

 tent to undergo the ultimate test — that of 

 predicting the general configuration of the 

 coronal streamers at future eclipses. 



Among modern solar theories may be 

 mentioned that of Schmidt (1891), an opti- 

 cal theory of the solar disk, making the 

 Sun wholly gaseous, in fact, a planet- 

 ary nebula, existing in space without a 

 definite outline anj'where, as we see it ; so 

 that the photosphere would be an apparent 

 or optical surface merelj', and not a real or 

 natural one, such as the Sun's disk and 

 limb seem actually to be in the telescope. 

 The best English exposition of Schmidt's 

 theorj' is that of Herr Wilczjmski of Ber- 

 lia, in the February (1895) number of the 

 Astrophysical Journal ; followed iu the same 

 issue by Professor Keeler's clear statement 

 of certain practical objections to this the- 

 ory. If Schmidt's theory were true, it is 

 exceedingly improbable that the Sun's ap- 

 parent or angular diameter would remain 

 practically a constant quantity', as we know 

 it does. Also maj' be mentioned the theoiy 

 of the Sun bj' Dr. Brester of Delffc, pub- 

 lished in 1892. and characterized bj' much 

 novelty. Eejecting the hypothesis of erup- 

 tional translation of solar matter, he con- 

 ceives the Sun to be a relatively tranquil 

 gaseous bodj', of essentiallj' the same ele- 

 mentary composition as our Earth ; and he 

 attempts to show, in accordance with well 

 known properties of matter, that the same 

 cause ^\'hich would keep the mass in repose 

 must produce also ' chemical luminescence,' 

 as he terms it. Great material eruptions, 

 then, are merely deceptive appearances, be- 



