the vapour of every substance has the power of absorbing just those rays that its light emits.
Following the course of the German philosophers, we will now revert to the solar spectrum, and see how it can be connected with the above experiment, with the view of tracing the origin of Fraunhofer’s lines. By a slight modification of the instrument used in the preceding observations, amounting to the use of the same prism for a double purpose, the solar spectrum may be reduced to half its breadth, and so fill only one half of the field of view of the telescope, leaving the other half to be occupied by the spectrum from any independent source of light; this is done by merely covering half the slit through which the sunlight passes with a small prism or reflector, so placed as to conduct the beam of light under analysis through the apparatus unconfused with the solar beam. Now, with the solar spectrum occupying its half of the field, if we place a gas or other hydrocarbon flame in front of the little prism, we shall see its spectrum in juxtaposition to that of the sunlight, and they will appear as one spectrum, half of which contains the dark lines, the other half being homogeneous; now, if we introduce sodium in any form into the gas-flame, we shall see our yellow line as before produced, and its position will exactly correspond with one of the most prominent of Fraunhofer’s lines, namely, that called D; following up our previous experiment, if we interpose the sodium vapour between the burning sodium and the little prism, our bright line will become reversed, and a fine and continuous black line will stretch across both spectra. The relation between the two sections of this line in the different spectra is absolutely identical; the higher the power employed to observe them, the more perfect is their coincidence. If potassium, with its absorbing vapour, be substituted for the sodium, we shall obtain a dark line, corresponding exactly with Fraunhofer’s line A; while, if iron with its vapour be employed, about sixty lines will be seen, all in perfect coincidence with some of the more or less distinct of the solar lines.
In this way Kirchoff and Bunsen have compared the spectra of nearly the whole of the known metals and earths with that of the sun, and have thence discovered that its spectrum contains lines identical with those of iron, magnesium, sodium, potassium, calcium, chromium, nickel, and possibly with those of barium, copper, manganese and zinc, while, to the present time, they have discovered no identity between the solar lines and those of gold, silver, lead, tin, antimony, arsenic, mercury, lithium, or strontium.
We are thus put in possession of the facts necessary to answer the question relative to the nature of these fixed lines, and with it to partially solve the problem of the physical constitution of the sun; we cannot do this otherwise so well as by quoting the words of Kirchoff himself: “The sun consists of a glowing gaseous atmosphere surrounding a solid nucleus possessing a still higher temperature. If we could see the spectrum of the solar atmosphere without that of the solid nucleus, we should observe in it the bright lines which are characteristic of the metals it contains. The more intense luminosity of the internal nucleus does not, however, permit the spectrum of the solar atmosphere to become apparent; it is reversed (as in our experiment we reversed the soda line), so that instead of the bright lines which the luminous atmosphere itself would have shown, dark ones appear. We do not see the spectrum of the solar atmosphere, but a negative image of it. This can, however, with an equal degree of certainty, serve to detect the metals present in the sun’s atmosphere; all that we require for the purpose is a very accurate knowledge of the solar spectrum, and the spectra of the individual metals.”
Thus have the solar rays revealed the history of their birth, and thus has the minute sunbeam admitted through the chink in the window-shutter, borne on its silent course and in occult language telegraphed its secret message: by successive steps the philosopher, first creating an alphabet, next a lexicon, then a grammar of the mystic language, has at last deciphered the cunning telegram by which the solar beam betrays the secrets of its prison-house.
Much as we have already seen accomplished, the work is yet far from complete; for a vast number of solar lines remain unmatched and unclaimed by terrestrial partners. For a time, however, we must rest content, at least so far as the German philosophers are concerned; for Kirchoff tells us his eyes have become so weakened by continual observation, that he is compelled to suspend his labours till they shall have repaired their exhausted power. Meanwhile, stimulated by the results of these researches, astronomers are actively engaged in mapping the spectra of the heavenly bodies. Fraunhofer, in his time, observed the lines given by the planets and a few of the fixed stars: in our time, Donati (of comet celebrity) and Padre Secchi, of Rome, have each observed and published a few stellar spectra, while the magnificent equatorial of our national Observatory, equipped with a spectrometric apparatus, has already furnished accurate micrometric comparisons between the solar spectrum and the spectra of about forty stars.
The spectra of the moon and planets show lines coinciding with those of the sun. This, though curious, is only natural, when we consider that these bodies are merely reflectors, giving off the light they, in common with the earth, receive from the sun. The spectra of stars, however, differ widely, not only from the sun, but from each other. In our diagram we have shown two of quite different characters, the first of Betelgeux, the bright star in Orion, the second of Sirius, or the Dog star. In the spectrum of Betelgeux, the most remarkable coincidence is that of a line with the sodium line D of the solar spectrum, and it is curious that several other stars show this line. This is primâ facie evidence of the existence of sodium in the atmosphere of these stars. But the most remarkable circumstance is connected with the solar line F, for out of forty stars, twenty-eight exhibit a line perfectly coincident with it: Sirius is one of them. This line has not yet been found to correspond with any metallic line, but the group of which it forms the nucleus contains several strongly-marked iron lines. At first sight