58 Mr. R, Meldola on a Cause for the Appearance 



souroe of white Light, with the mixture of two vapours of dif- 

 ferent specific absorptive powers in front. The oxygen unci 

 hydrogen of the zone of combustion represent the second layer 

 of incandescent gases of paragraph 7, supposed to Lave been 

 placed in "front of the first system, the total radiation of which 

 is imagined to have been weakened by general absorption or 

 by removal to a distance. It has been shown in paragraph 8 

 that the total radiation of the photosphere has probably under- 

 gone a great amount of weakening from both these causes. 

 Thus the spectrum of a ray which reaches the zone of com- 

 bustion would exhibit (supposing the zone of combustion and 

 all exterior to it to be stripped off) the lines of oxygen and 

 hydrogen dark, but those of the former much fainter than those 

 of the latter. The action of the incandescent gases of the 

 zone of combustion upon such a spectrum would be to reverse 

 the oxygen lines and to weaken those of hydrogen. 



The temperature of the region outside the zone of combus- 

 tion must fall off, so that any oxygen which might there exist* 

 would be in the state of molecular aggregation corresponding 

 to the compound spectrum, and would thus be without action 

 on the bright-line spectrum of this gas, but would give rise to 

 the dark lines of its compound spectrum. The hydrogen of the 

 region now under consideration by further absorption inten- 

 sities the lines of this gas. Thus the solar spectrum as now 

 known is shown to be in complete accordance with the hypo- 

 thesis here advanced. 



The hypothesis of a zone of combustion in the higher regions 

 of the sun's atmosphere, as already stated, furnishes sugges- 

 gestions for the explanation of many observed facts in solar 

 physics hitherto unaccounted for. 



I will first call attention to the intense brilliancy of the line 

 D 3 in the spectrum of the chromosphere, and the extreme 

 faintness of the corresponding dark line in the solar spectrumt. 

 If we consider to what an enormous height this element ex- 

 tends, bearing also in mind that it must consequently reach 

 into comparatively cool regions, and that its radiative (and 

 therefore absorptive) powers are very great, it seems impro- 



* It may be supposed that the oxygen atmosphere terminates with the 

 zone of combustiou, in which case Dr. Schuster's new oxygen-spectrum 

 must be produced by the absorptive action of the gas in the upper regions 

 of the chromosphere (see also note to paragraph 7). 



t This line was seen in July 1877 by H. 0. Russell at Sydney. The 

 observe* states that " it is a difficult line to see, and only to be made out 

 with high powers." The greatest dispersion of the spectroscope em- 

 ployed was equal to eighteen 64°-prisms (Moaib. Not. Roy. Astr. Soc. 

 Nov. 9, 1877, pp. 30-82). 



