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The discoveries of Grimaldi and of Fresnel in regard to interferences, those of Faraday, of Arago, of Foucault, of Fraunhofer, of Kirchoff, and of hundreds of others owed something to "fortune." It is said that the sight of a crab suggested to Watt the idea of an ingenious machine.
The brief memoir of two pages, in which this immortal discovery is recorded, was communicated to the Berlin Academy on October 27, 1859. Fraunhofer had remarked in the spectrum of a candle flame two bright lines, which coincide accurately, as to position, with the double dark line D of the solar spectrum.
A "reversing layer," or stratum of mixed vapours, glowing, but at a lower temperature than that of the actual solar surface, was an integral part of Kirchhoff's theory of the production of the Fraunhofer lines.
Either, like Garfield and Franklin, they served the State zealously in peace or war; or else, like Stephenson and Telford, they improved human life by their inventions and engineering works; or, again, like Herschel and Fraunhofer, they added to the wide field of scientific knowledge; or finally, like Millet and Gibson, they beautified the world with their noble and inspiring artistic productions.
This first discovery was quickly followed up by the identification of numerous bright rays in the spectra of other metallic bodies with others of the hitherto mysterious Fraunhofer lines. As to cobalt, he hesitated to pronounce, but its existence in the sun has since been established.
The substance emitting it was thus present, though in a low state of incandescence. The continuous spectrum was relatively strong; faint traces of the Fraunhofer lines attested for it an origin, in part by reflection; and polarisation was undoubted, increasing towards the limb, whereas in 1870 it reached a maximum at a considerable distance from it.
Traces of it may be found for a few years subsequent to 1859, but they are obviously survivals from an earlier order of ideas, doomed to speedy extinction. It needs only a moment's consideration of the meaning at last found for the Fraunhofer lines to see the incompatibility of the new facts with the old conceptions.
His list of solar elements published in that year, the result of an investigation separate from, though conducted on the same principle as Kirchhoff's, included the substance which we now know to be predominant among them. Dr. Plücker of Bonn had identified in 1859 the Fraunhofer line F with the green ray of hydrogen, but drew no inference from his observation.
Spectrum analysis has become of practical use, and photographs of the various Fraunhofer lines in the spectrum have been taken as permanent records of each experiment.
Chief among these was the D-line of sodium, the original index, it might be said, to solar chemistry. No proof could be afforded more decisive that this faint echoing back of the distinctive notes of the Fraunhofer spectrum, that the polariscope had spoken the truth in asserting a large part of the coronal radiance to be reflected sunlight.
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