Physical Constitution of the Sun, 2 ( J3 



the variations which the spectrum of a luminous gas at a constant 

 temperature undergoes through variations of pressure do not de- 

 pend on the value of the pressure or the density at a determined 

 point in the mass, but only on the quantity of the luminous paro- 

 tides which send their light to the eye and therefore (no matter 

 how distributed) are situated in the observer's line of vision. 



This principle of the spectroscopic equivalence of density and 

 thickness of a radiant gas stratum diminishes, as is readily per- 

 ceived, the previously assumed values of the pressure at the base 

 of the chromosphere, because at that time the modification of 

 the hydrogen-spectrum, as observed under terrestrial conditions, 

 was presupposed as a fuuetion of the pressure only, and not as a 

 function of the density and thickness of the luminous stratum. 



Hence, if we wish to infer, from the accordance of the regular 

 variations exhibited by the spectra of incandescent gases under ter- 

 restrial conditions with changes of pressure, the ratios of pressure 

 or density in the atmospheres of incandescent heavenly bodies, we 

 must always take into account the quantity of luminous particles 

 which, lying in the observer's line of sight, simultaneously emit 

 their light to his eye. 



Thus, for example, let the spectrum of the hydrogen in the 

 narrow channel of a Geissler' s tube of 1 millim. internal diameter 

 be compared with the hydrogen-spectrum of the chromosphere 

 simultaneously in the same field of view, in order as far as pos- 

 sible to eliminate all heterogeneous influences of the illumina- 

 tion upon the judgment of the spectra. 



If, now, we could by variations of the pressure produce in the 

 line F, for example, all those variations of breadth which the 

 same line has in the spectrum of the chromosphere at different 

 distances from the base of the latter, we should be justified in 

 assuming generally that like physical conditions are realized in 

 the narrow channel of the Geissler tube as in the chromosphere 

 — that is, that the same temperature prevails, and the same num- 

 ber of incandescent particles of gas lie in the observer's line of view. 

 Hence, were the immense stratum of gas which is cut through 

 by the visual line at the base of the chromosphere everywhere 

 of equal thickness, then the values of the pressure or the den- 

 sity in the Geissler tube and at the base of the chromosphere 

 would, under the condition mentioned, be in the inverse ratio of 

 the thicknesses of the radiating layers. An idea of the ratio of 

 the radiant gas-strata can also be obtained if we reflect that, 

 under the above-made hypotheses, the whole layer of gas at the 

 base of the chromosphere must be just as transparent as the thin 

 column in the narrow channel of the Geissler tube. It is hence 

 at the same time evident how slight must be the density already, 

 in such close proximity to the body of the sun, of the gaseous 

 matter surrounding it. 



