This result, be it noted, was not due to any real quenching of the bright lines of the flame, but to the augmentation of the intensity of the adjacent spectrum.  The experiment proved to demonstration, that when the white light sent through the flame was sufficiently intense, the quantity which the flame absorbed was far in excess of that which it radiated.

Here then is a result of the utmost significance.  Kirchhoff immediately inferred from it that the salt flame, which could intensify so remarkably the dark lines of Fraunhofer, ought also to be able to produce them.  The spectrum of the Drummond light is known to exhibit the two bright lines of sodium, which, however, gradually disappear as the modicum of sodium, contained as an impurity in the incandescent lime, is exhausted.  Kirchhoff formed a spectrum of the limelight, and after the two bright lines had vanished, he placed his salt flame in front of the slit.  The two dark lines immediately started forth.  Thus, in the continuous spectrum of the lime-light, he evoked, artificially, the lines D of Fraunhofer.

Kirchhoff knew that this was an action not peculiar to the sodium flame, and he immediately extended his generalisation to all coloured flames which yield sharply defined bright bands in their spectra.  White light, with all its constituents complete, sent through such flames, would, he inferred, have those precise constituents absorbed, whose refrangibilities are the same as those of the bright bands; so that after passing through such flames, the white light, if sufficiently intense, would have its spectrum furrowed by bands of darkness.  On the occasion here referred to Kirchhoff also succeeded in reversing a bright band of lithium.

The long-standing difficulty of Fraunhofer’s lines fell to pieces in the presence of facts and reflections like these, which also carried with them an immeasurable extension of the chemist’s power.  Kirchhoff saw that from the agreement of the lines in the spectra of terrestrial substances with Fraunhofer’s lines, the presence of these substances in the sun and fixed stars might be immediately inferred.  Thus the dark lines D in the solar spectrum proved the existence of sodium in the solar atmosphere; while the bright lines discovered by Brewster in a nitre flame, which had been proved to coincide exactly with certain dark lines between A and B in the solar spectrum, proved the existence of potassium in the sun.

All subsequent research verified the accuracy of these first daring conclusions.  In his second paper, communicated to the Berlin Academy before the close of 1859, Kirchhoff proved the existence of iron in the sun.  The bright lines of the spectrum of iron vapour are exceedingly numerous, and 65 of them were subsequently proved by Kirchhoff to be absolutely identical in position with 65 dark Fraunhofer’s lines.  Angstroem and Thalen pushed the coincidences to 450 for iron, while, according to the same excellent investigators, the following numbers express the coincidences, in the case of the respective metals to which they are attached:—­