Physical Constitution of the Sun, 319 



From Wullner' s researches* we may, I believe, conclude that 

 the pressure at the base of the chromosphere or at the outer edge 

 of the luminous solar disk is equal to that of a column of mer- 

 cury at the earth's surface from 50 millims. to 500 millims. in 

 height f. 



It is thus clear that it is not necessary, in order to explain the 

 presence of the dark lines in the solar spectrum, to assume that 

 the continuous spectrum is produced by the incandescence of 

 a solid or liquid body; for we may with equal right consider 

 that the continuous spectrum is produced by the glowing of a 

 powerfully compressed gas. 



This has indeed been experimentally proved by Wullner for 

 the sodium-lines, inasmuch as he remarks, " Under a pressure of 

 1230 millims. the maximum of light at H a becomes less distinct, 

 the whole spectrum is most dazzling, and the sodium-lines are 

 seen as beautiful dark lines J, so that the light of glowing hy- 

 drogen is intense enough to produce in an atmosphere of sodium- 

 vapour a Fraunhofer-line — a proof that the light of a glowing 

 solid is not requisite for this purpose." 



Hence it follows that the radius of the visible solar disk need 

 not be necessarily identical with that of the supposed layer of 

 separation, but that this latter may probably be assumed to lie 

 below the point at which the hydrogen gas under compression 

 evolves a continuous spectrum. The probability of these consi- 

 derations is much increased by the phenomena of sun-spots. 

 However different even now may be the theoretical speculations 

 as to the nature of the spots, almost all observers agree in admit- 

 ting that the umbra lies at a lower level than the surrounding 

 parts §. The depth at which the umbra lies has been ascertained, 

 parly by direct (De La Rue, Stewart, Loewy), partly by indirect 

 observations (Faye), to be about 8" ||. 



If, therefore, we consider the umbrse the products of a local 

 cooling floating on the surface of a glowing liquid like islands 

 on a, glowing ocean, and the penumbra? to be condensation- 

 clouds which surround these islands or cooler spaces at a certain 



* Pogg. Ann. vol. cxxxvii. pp. 336-361. 



t See Wullner, loc. c^.pp. 340-345. 



% In consequence of the high temperature, the sodium of the glass is vo- 

 latilized. At 1000 millims. pressure the sodium-lines are seen bright (loc. 

 cit. p. 345). 



§ Sporer, on the contrary, says, "we regard the spots as cloud-like 

 forms floating above the bright surface of the sun. The penumbra is no- 

 thing more than a collection of small spots, through the interstices of which 

 the bright surface above which the spot is situated can be seen" (Pogg. 

 Ann. vol. cxxviii. (1866), p. 270). 



|| Faye, by calculations from Carrington's observations, finds that this 

 depth corresponds to 0-005-0'009 of the sun's radius (Comptes Rendus, 

 vol. lxi. pp. 1082-1090). 



