In the preceding hypothesis, the comets do not belong to the solar system. If they be considered, as we have done, as small nebula, wandering from one solar system to another, and formed by the condensation of the nebulous matter, which is diffused so profusely throughout the universe, we may conceive that when they arrive in that part of space where the attraction of the Sun predominates, it should force them to describe elliptic or hyperbolic orbits. But as their velocities are equally possible in every direction, they must move indifferently in all directions, and at every possible inclination to the ecliptic; which is conformable to observation. Thus the condensation of the nebulous matter, which explains the motions of rotation and revolution of the planets and satellites in the same direction, and in orbits very little inclined to each other, likewise explains why the motions of the comets deviate from this general law.

FOUCAULT, STOKES, BUNSEN
AND KIRCHHOFF

THE old distinction in kind between the celestial and the terrestrial spheres was broken down by Galileo and Newton. The mechanical laws which describe the movement of falling bodies on the surface of the earth were found to hold good in the farthest limits of the solar system. Experiments and calculations made in a laboratory, could elucidate the majestic motion of planets in their orbits.

To complete the demonstration of identity, however, it is necessary to show similarity of structure and composition as well as of motion. Are the chemical elements, from which all terrestrial things are made, the basis also of sun, planets and stars? It may well have seemed a question hopeless to ask. Yet a solution was found along a road opened up by Newton himself. He it was who discovered the coloured band of light produced by passing the sun's rays through a prism. This solar spectrum is crossed by a number of dark lines, while the light from colourless flames tinged with metals or salts shows bright lines on a dark field. These bright and dark lines were found to coincide-the spectrum of sodium for example giving two bright lines coincident with two dark lines in the sun's light.

The explanation of these facts was given independently by M. Léon Foucault, by Professor Gustav Robert Kirchhoff of Heidelberg and by Sir George Gabriel Stokes, who held Newton's old professorial chair at Cambridge. They showed that this spectrum analysis gave a proof that terrestrial chemical elements existed in the sun and the luminous stars. The heavenly bodies were as much and no more incorruptible, divine, than the familiar though mysterious matter of this our globe. The boundless depths of space lay open yet further to our investigation.

The principle underlying this method of spectrum analysis is that of resonance. If a note be sung near a piano, the particular wires tuned to that note will be set in vibration. That is to say, they will absorb vibrational energy of the same period as they themselves emit. If a complex sound passed through a grove of piano wires, and were analysed on the further side, it would be found to be wanting in those particular notes which the wires, if more vigorously excited, would send forth. And so the light of the sun, passing through his cooler envelope, is deprived of those rays the constituents of that envelope would emit, and the coloured band of the solar spectrum is crossed by dark lines indicating the elements present in the sun's atmosphere. This explanation, modestly given in his lectures by Stokes, made the name of other men.

EXTRACTS RELATING TO THE EARLY HISTORY OF SPECTRUM ANALYSIS

SAME

ON THE SIMULTANEOUS EMISSION AND ABSORPTION OF RAYS OF THE DEFINITE REFRANGIBILITY; being a translation of a portion of a paper by M. LÉON FOUCAULT, and of a paper by Professor KIRCHHOFF.

(From the Philosophical Magazine, XIX. (1860), pp. 196–7.)

Gentlemen,

Some years ago M. Foucault mentioned to me in conversation a most remarkable phænomenon which he had observed in the course of some researches on the voltaic arc, but which, though published in L'Institut, does not seem to have attracted the attention which it deserves. Having recently received from Prof. Kirchhoff a copy of a very important communication to

the Academy of Sciences at Berlin, I take the liberty of sending you translations of the two, which I doubt not will prove highly interesting to many of your readers.

I am, Gentlemen, Yours sincerely,

G. G. STOKES.

M. Foucault's discovery is mentioned in the course of a paper published in L'Institut of Feb. 7, 1849, having been brought forward at a meeting of the Philomathic Society on the 20th of January preceding. In describing the result of a prismatic analysis of the voltaic arc formed between charcoal poles, M. Foucault writes as follows (p. 45):—

"Its spectrum is marked, as is known, in its whole extent by a multitude of irregularly grouped luminous lines: but among these may be remarked a double line situated at the boundary of the yellow and orange. As this double line recalled by its form and situation the line D of the solar spectrum, I wished to try if it corresponded to it; and in default of instruments for measuring the angles, I had recourse to a particular process.

"I caused an image of the sun, formed by a converging lens, to fall on the arc itself, which allowed me to observe at the same time the electric and the solar spectrum superposed; I convinced myself in this way that the double bright line of the arc coincides exactly with the double dark line of the solar spectrum.

"This process of investigation furnished me matter for some unexpected observations. It proved to me in the first instance the extreme transparency of the arc, which occasions only a faint shadow in the solar light. It showed me that this arc, placed in the path of a beam of solar light, absorbs the rays D, so that the above mentioned line D of the solar light is considerably strengthened when the two spectra are exactly superposed. When, on the contrary, they jut out one beyond the other, the line D appears darker than usual in the solar light, and stands out bright in the electric spectrum, which allows one easily to judge of their perfect coincidence. Thus the arc presents us with a medium which emits the rays D on its own account,

and which at the same time absorbs them when they come from another quarter.

"To make the experiment in a manner still more decisive, I projected on the arc the reflected image of one of the charcoal points, which, like all solid bodies in ignition, gives no lines; and under these circumstances the line D appeared to me as in the solar spectrum."

Prof. Kirchhoff's communication "On Fraunhofer's Lines," dated Heidelberg, 20th of October, 1859, was brought before the Berlin Academy on the 27th of that month, and is printed in the Monatsbericht, p. 662.

"On the occasion of an examination of the spectra of coloured flames not yet published, conducted by Bunsen and myself in common, by which it has become possible for us to recognise the qualitative composition of complicated mixtures from the appearance of the spectrum of their blowpipe-flame, I made some observations which disclose an unexpected explanation of the origin of Fraunhofer's lines, and authorise conclusions therefrom respecting the material constitution of the atmosphere of the sun, and perhaps also of the brighter fixed stars.

"Fraunhofer had remarked that in the spectrum of the flame of a candle there appear two bright lines, which coincide with the two dark lines D of the solar spectrum. The same bright lines are obtained of greater intensity from a flame in which some common salt is put. I formed a solar spectrum by projection, and allowed the solar rays concerned, before they fell on the slit, to pass through a powerful salt-flame. If the sunlight were sufficiently reduced, there appeared in place of the two dark lines D two bright lines: if, on the other hand, its intensity surpassed a certain limit, the two dark lines D showed themselves in much greater distinctness than without the employment of the salt-flame.

"The spectrum of the Drummond light contains, as a general rule, the two bright lines of sodium, if the luminous spot of the cylinder of lime has not long been exposed to the white heat; if the cylinder remains unmoved these lines become weaker, and finally vanish altogether. If they have vanished, or only faintly

appear, an alcohol flame into which salt has been put, and which is placed between the cylinder of lime and the slit, causes two dark lines of remarkable sharpness and fineness, which in that respect agree with the lines D of the solar spectrum, to show themselves in their stead. Thus the lines D of the solar spectrum are artificially evoked in a spectrum in which naturally they

are not present.

"If chloride of lithium is brought into the flame of Bunsen's gas-lamp, the spectrum of the flame shows a very bright sharply defined line, which lies midway between Fraunhofer's lines B and C. If, now, solar rays of moderate intensity are allowed to fall through the flame on the slit, the line at the place pointed out is seen bright on a darker ground; but with greater strength of sunlight there appears in its place a dark line, which has quite the same character as Fraunhofer's lines. If the flame be taken away, the line disappears, as far as I have been able to see, completely.

"I conclude from these observations, that coloured flames, in the spectra of which bright sharp lines present themselves, so weaken rays of the colour of these lines, when such rays pass through the flames, that in place of the bright lines dark ones appear as soon as there is brought behind the flame a source of light of sufficient intensity, in the spectrum of which these lines are otherwise wanting. I conclude further, that the dark lines of the solar spectrum which are not evoked by the atmosphere of the earth, exist in consequence of the presence, in the incandescent atmosphere of the sun, of those substances which in the spectrum of a flame produce bright lines at the same place. We may assume that the bright lines agreeing with D in the spectrum of a flame always arise from sodium contained in it; the dark line D in the solar spectrum allows us, therefore, to conclude that there exists sodium in the sun's atmosphere. Brewster has found bright lines in the spectrum of the flame of saltpeter at the place of Fraunhofer's lines A, a, B; these lines point to the existence of potassium in the sun's atmosphere. From my observation, according to which no dark line in the solar spectrum answers to the red line of lithium, it would follow with probability that in the atmosphere of the sun lithium is either absent, or is present in comparatively small quantity.