The elements of astronomy; a textbook . ctrum which shows dark lines inplace of the characteristic bright lines. This principle of reversal is illustrated by Fig. 53. Supposethat in front of the slit of the spectroscope we place a spiritlamp with a little carbonate of soda and some salt of thalliumupon the wick. We shall then get a spectrum showing thetwo yellow lines of sodium and the green line of thallium, allbright. If now the lime light be started behind the flame, we 195] PRINCIPLES OF SPECTRUM ANALYSIS. 137 shall at once have the effect shown in the lower figure, — a con-tinuous spectru
The elements of astronomy; a textbook . ctrum which shows dark lines inplace of the characteristic bright lines. This principle of reversal is illustrated by Fig. 53. Supposethat in front of the slit of the spectroscope we place a spiritlamp with a little carbonate of soda and some salt of thalliumupon the wick. We shall then get a spectrum showing thetwo yellow lines of sodium and the green line of thallium, allbright. If now the lime light be started behind the flame, we 195] PRINCIPLES OF SPECTRUM ANALYSIS. 137 shall at once have the effect shown in the lower figure, — a con-tinuous spectrum crossed by blackx lines which exactly replacethe bright lines. Insert a screen between the lamp flame andthe lime, and the dark lines instantly turn bright again. Theexplanation of the Fraunhofer lines, therefore, is that they aremainly due to the action of the gases and vapors of the solaratmosphere upon the light that comes from the liquid or solidparticles composing thephotospherie clouds. Some of them, how- Reversal of Spectrum. Fig. 53. — Reversal of the Spectrum. ever, known as Telluric lines, are due to the gases and vapors of the earths atmosphere, — to water-vapor and oxygen especially. 196. Chemical Constituents of the Sun. — Numerous lines of the solar spectrum can be identified as due to the pres- 1 Their darkening, however, when the light from the lime is trans-mitted through the flame, is only relative and apparent, not real. Theirbrightness is actually a little increased; but that of the background is in-creased immensely, making it so much brighter than the lines that, con-trasted with it, they look black. 138 CHEMICAL CONSTITUENTS OF THE SUN. [§196
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