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speaks of one of his spectra giving the general impression of reversal of the solar spectrum. Foucault, Stokes, Thomson, and Stewart, have all been very close to the discovery; and for my own part the examination of the radiation and absorption of heat by gases and vapours would have led me in 1859 to the law on which all Kirchhoff's speculations are founded, had not an accident withdrawn me from the investigation.'

At the very moment, however, when the great secret was about to be revealed, an eminent physicist wrote thus: In quitting the mere phenomena of luminous spectra, and rising to the inquiry as to their causes, we enter a more arduous course. The phenomena defying, as we have seen, all attempts hitherto to reduce them within empirical laws, no complete explanation or theory of them is possible. All that theory can be expected to do is this-it may explain how dark lines of any sort may arise within the spectrum.'

But theory succeeded in doing very much more than was thus anticipated from her.

It had been noticed by Fraunhofer that the two orange-coloured lines close together which form the spectrum of the glowing vapour of sodium, coincide exactly in position with two dark lines in the orange-coloured part of the solar spectrum-the double D line in fig. 23. Kirchhoff, having a spectrosocope of great dispersive power, determined to inquire whether this coincidence

Its prismatic battery contained four flint-glass prisms, not fixed or clamped in any way, but standing freely on four little pedestals. The

was exact. . In order to test in the most direct manner possible,' he says, the frequently asserted fact of the coincidence of the sodium lines with the lines D, I obtained a tolerably bright solar spectrum, and brought a flame coloured by sodium vapour in front of the slit. I then saw the dark lines D change into bright ones. The flame of a Bunsen's lamp threw the bright sodium lines upon the solar spectrum with unexpected brilliancy. In order to find out the extent to which the intensity of the solar spectrum could be increased without impairing the distinctness of the sodium lines, I allowed the full sunlight to shine through the sodium flame, and, to my astonishment, I saw that the dark lines D appeared with an extraordinary degree of clearness.'

Let the full force of this result be recognised before we proceed to consider the method by which Kirchhoff certified himself as to its exactness. He had shining in through the slit two kinds of light-sunlight and the light of the sodium flame. The sunlight alone would have given the ordinary solar spectrum with the D lines of a certain degree of darkness. The sodium flame alone would have given two bright lines just where the dark D lines of the solar spectrum appear. What he expected was, naturally, that the bright lines of the sodium spectrum would at least partially reduce the whole arrangement seems singularly ineffective in comparison with the spectroscopes now in use; for example, if we compare Kirchhoff's battery with the battery of prisms in the fine spectroscope made by Browning for Mr. Gassiot, the former seems almost ridiculous in its imperfectness. A cough or a sneeze,' it has been well remarked, would set that whole battery in disarray, with which, nevertheless, Kirchhoff solved the secret of the solar spectrum.'

darkness of the coresponding D lines, even if they did not altogether cause these dark lines to disappear, or to be replaced by bright lines. But these lines actually appeared darker. It was precisely as though an experimenter were to cast a beam of light exactly upon a shadow, and to see the shadow actually intensified instead of the reverse.

Kirchhoff proceeded to test this astonishing result. I exchanged the sunlight for the Drummond's or oxy-hydrogen lime-light, which, like that of all incandescent solid or liquid bodies, gives a spectrum containing no dark lines. When this light was allowed to fall through a suitable flame coloured by common salt, dark lines were seen in the spectrum in the position of the sodium lines. The same phenomenon was observed if, instead of the incandescent lime, a platinum wire was used, which, being heated in a flame, was brought to a temperature near its melting point by passing an electric current through it.'

These experiments were, if possible, even more striking than the former; for now Kirchhoff had two lights which seemed to produce darkness. The electric light alone covered with its continuous spectrum the place where the sodium lines appear in the solar spectrum; the sodium light alone lit up this very part of the spectrum. When the two lights were both shining one would have expected the result to be an increased brightness in this part; instead of which there actually resulted darkness.

The observed fact is in itself important: its inter

pretation involves one of the most important facts ever discovered by man. It is well to distinguish between

the two.

The observed fact is that the sodium flame, which emits rays of a certain order of refrangibility—that is, rays which, if passed through a prism, will follow a certain path-has the power of absorbing rays of precisely the same order. The interpretation of the fact is founded on the existence of a law which is thus worded by Professor Roscoe- Every substance which emits at a given temperature certain kinds of light, must possess the power at that same temperature of absorbing the same kinds of light.'*

*This is merely a corollary from a more general law, according to which the same relation holds between the powers which substances possess of emitting and absorbing heat-waves as well as light-waves and actinic-waves. The law called the theory of exchanges was first enunciated for heat by Prevost of Geneva, and has since been established for heat and light by the researches of Prevostaye, Dessains, Stewart, and Kirchhoff. So far as the application of the theory to light is concerned, it must be admitted that there are still many difficulties in the way of its complete acceptance. These difficulties somewhat importantly affect our conclusions where we are considering the application of the theory in mode and measure to spectroscopic researches, though, so far as the general results here chiefly considered are concerned, they need not greatly trouble us.

I may refer my reader to Dr. Stewart's Elementary Treatise on Heat for a very interesting examination of the subject and the demonstration of the fundamental principles involved in the law. But I must caution the reader against one point in the course of Dr. Stewart's reasoning. which is very likely to mislead, and involves an error in an optical subject of considerable importance. It is necessary for the demonstration that the course of rays (light-rays or heat-rays) not falling quite perpendicularly, or rather not strictly parallel to each other, should be considered, and Dr. Stewart removes the difficulty for small beams of ight or heat, by considering the size of the source of light. He says, 'Just as a line is in reality always part of the boundary of a solid, so

Kirchhoff experimented on other elements. He found that a flame coloured by potassium causes dark lines to appear on the continuous spectrum of the limelight, and that these lines appear precisely where bright lines are seen when the spectrum of that coloured flame is viewed alone. The same was proved by Kirchhoff and Bunsen for the lines of lithium, calcium, strontium, and barium; while it has been shown to hold for other elements by Dr. Miller and others.

But now that the general law was established, important results respecting solar physics were established along with it. Since Kirchhoff had proved that when the electric light shines through a sodium flame, the sodium

a ray is always in reality part of the boundary of a beam or pencil of light. We may satisfy ourselves that this is the case in nature by considering the light which reaches the eye from a star or other object apparently very small; this would seem to be the nearest approach to a geometrical line of light, whereas since a star has a certain real, though very minute, angular diameter, the light from it is in reality a converging pencil, although no doubt the angle of convergence is very small.' On account of the importance of the optical considerations in question, I may be permitted to correct what is undoubtedly an erroneous statement. The difficulty must be got over by considering the size of the object on which light or heat rays fall, not by considering the size of the source of light. There is no such thing in nature as a converging pencil of light proceeding directly from a luminous object. All real pencils, whether of light or of heat, are originally diverging; they diverge from every point of the selfluminous or heat-giving body, and the angle of their divergence is real-however minute it may be-so long as the object which receives light or heat has real dimensions, however minute. This consideration does not affect the conclusions deduced by Dr. Stewart, since undoubtedly the diverging pencils of light or heat, from a source of considerable dimensions, form a converging beam, when we consider them with reference to a smaller object on which they fall.

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