The London, Edinburgh and Dublin philosophical magazine and journal of science . buted, since the researches ofProf. Kirchhoff, to the absorption produced by metallic sub-stances in a state of vapour on the surface of the sun. Theother, in consequence of their intensity varying with thethickness of the atmosphere traversed by the suns rays, areexplained by the selective absorption due to the cold gases or * Communicated by the Physical Society : read June 12, 1886. Spectral Lines of Solar and Terrestrial Origin. 459 vapours of the earths atmosphere. We ought then to distin-guish the solar from
The London, Edinburgh and Dublin philosophical magazine and journal of science . buted, since the researches ofProf. Kirchhoff, to the absorption produced by metallic sub-stances in a state of vapour on the surface of the sun. Theother, in consequence of their intensity varying with thethickness of the atmosphere traversed by the suns rays, areexplained by the selective absorption due to the cold gases or * Communicated by the Physical Society : read June 12, 1886. Spectral Lines of Solar and Terrestrial Origin. 459 vapours of the earths atmosphere. We ought then to distin-guish the solar from the telluric lines. Thus, of the eight principal Fraunhofer-lines, six are cha-racteristic of metallic elements and are of solar origin (C andF, hydrogen ; D, sodium : E, G, iron ; H, calcium) ; theother two (A and B) are telluric. Fraunhofer had besides distinguished two complex groups,namely a band a, very broad, in the extreme red, and a well-marked triple line, b, in the green ; b is solar (magnesium),and a of terrestrial origin (fig. 1). Brewster, on discovering Fig. Solar spectrum, with principal lines marked. new bands of variable intensity in the spectrum, added newdesignations; it is sufficient here to mention the band asituated in the orange, and the 0band 8 in the yellow. Thesesymbols have been adopted by Angstrom (Spectre normal duSoleil). Up to the present time it has been considered a difficult,and in any case a troublesome, matter to distinguish betweenthese two kinds of lines. It was necessary, in fact, to observethe solar spectrum at two very different altitudes of the sun,under various meteorological conditions, to be able to affirmthat the spectrum-lines do or do not change in intensity withthe thickness of the atmosphere or the quantity of water-vapour traversed by the solar rays. The improvement of spectroscopes, in respect of the sharp-ness and especially of the dispersion of the lines, has allowedme to arrive at a method which renders the distinction
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