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But this view, though countenanced by Ångström, and advocated by Hastings of Baltimore, and other authorities, is open to grave objections. In Faye's theory, sun-spots were regarded as simply breaks in the photospheric clouds, where the rising currents had strength to tear them asunder.
They indicated a marked deficiency of thermal radiations, implying for coronal light, in Professor Langley's opinion, an origin analogous to that of the electric glow-discharge, which, at low pressures, was found by K. Ångström in 1893 to have no invisible heat-spectrum. The corona was photographed by Professor Barnard, at Wadesborough, North Carolina, with a 61-1/2-foot horizontal "coelostat."
The following examples serve to illustrate the dispersion expressed in this way: Angstrom, Cornu, 10; Draper, photographer of normal solar spectrum, 3.1 and 5.2; Rowland, 23, 33, and 46; Draper, stellar spectra, 0.16; Huggins, 0.1. The most rapid plates are needed in this work, other considerations being generally of less importance.
M. Savélieff deduced for it a value of 3.47 from actinometrical observations made at Kieff in 1890; and Knut Ångström, taking account of the arrestive power of carbonic acid, inferred enormous atmospheric absorption, and a solar constant of four calories.
The main interest, however, of all these documents resides in the information afforded by them regarding the chemistry of the sun. The discovery that hydrogen exists in the atmosphere of the sun was made by Ångström in 1862.
Radiation strictly and necessarily corresponds with absorption only when the temperature is the same. In point of fact, Ångström was still, in 1853, divided between adsorption and interference as the mode of origin of the Fraunhofer dark rays.
To Ångström, indeed, belongs the great merit of having revived Euler's principle of the equivalence of emission and absorption; but he revived it in its original crude form, and without the qualifying proviso which alone gave it value as a clue to new truths.
Those that are invisible by reason of the quickness of their vibrations were mapped by Dr. Henry Draper, of New York, in 1873, and with superior accuracy by M. Cornu in 1881. The infra-red part of the spectrum, investigated by Langley, Abney, and Knut Ångström, reaches perhaps no definite end.
This mode of research promises to afford many new and useful data. The spectroscope has revealed the fact that, broadly speaking, the sun is composed of the same materials as the earth. Angstrom was the first to map out all of the lines to be found in the solar spectrum.
'But none of these distinguished men betrayed the least knowledge of the connexion between the bright bands of the metals and the dark lines of the solar spectrum; nor could spectrum analysis be said to be placed upon anything like a safe foundation prior to the researches of Bunsen and Kirchhoff. The man who, in a published paper, came nearest to the philosophy of the subject was Ångström.
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