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 710  powers of different media, which necessitated a close examination of the spectrum, discovered that it was divided into several portions by extremely fine black lines, crossing it at right angles to its length, or apparently separating certain colours; little notice seems to have been taken of this beyond recording the observation, and designating the fine lines, five in number, by the letters A, B, C, D, and E. Fourteen years after, the German optician, Fraunhofer, put forth his observations—made independently and in ignorance of Wollaston’s discovery—and published a diagram of the spectrum, on which he had mapped down the positions and relative intensities of 354 lines; this number has since been multiplied tenfold by recent observers, using more delicate and powerful instruments: but they still do honour to their secondary discoverer by the name they bear, as “Fraunhofer’s lines.” Eight of these, more remarkable for their intensity, and consequent facility of observation, were called by Fraunhofer A, B, C, D, E, F, G, and H; they have since been used as points of reference by later observers, and are shown in their proper positions in our diagram. In the rough experiment we made with the hole in the shutter, the lines could not be seen; but they might be rendered visible by refining the experiment. To do this it would be necessary to limit the aperture through which the light falls upon the prism to an extremely narrow slit, and to view the spectrum with a telescope of small magnifying power; the lines would then make their appearance, and look like the finest spider-threads stretched across the spectrum. The number of lines seen would depend upon the angle and dispersive power of the prism. Messrs. Kirchoff and Bunsen used four prisms, in order to gain as much dispersion, and, consequently, as long a spectrum as possible. In the plates which accompany their memoir, the parts of the spectrum are represented on a scale which would give from sixteen to eighteen feet for its entire length, while the infinite variety of intensity of the lines, and the delicacy required to faithfully reproduce them, necessitated the employment of six different stones and as many shades of ink in the process of lithographing the drawing.

The remarkable feature in these lines is their immutability: they are always seen in the same positions under the same circumstances. Messrs. Brewster and Gladstone, and M. Janseen, have observed certain of them undergo a slight change according to the elevation of the sun; but, as a rule, they may be considered, as they are denominated, “fixed” lines.

The question naturally arises: What do these lines consist of, and how are they produced? To answer this we must leave them for a while, to seek materials for a reply from the labours of Messrs. Kirchoff and Bunsen. It has long been known to the chemist and the pyrotechnist, that the salts of certain metals and alkaline earths possess the property of imparting colours to the flames in which they may be consumed; for instance, if a piece of common salt (chloride of sodium) be held in the flame of a candle, it will cause it to emit a yellow light; if a salt of strontium be used, the colour will be red; and if barium, a green flame will be produced. Now the spectrum formed by the light of a candle, an oil-lamp, or a gas flame, is uniform and continuous, exhibiting the colours of the solar spectrum, but no traces of the dark lines; while the spectra formed by the light emanating from metals and alkaline earths in combustion are strikingly characteristic and peculiar, for instead of being uniform and continuous, they consist of a greater or less number of isolated coloured bands, each occupying a place in the spectrum coincident with the ray of the same colour in the continuous spectrum, by virtue of Newton’s law, which fixes the position of each coloured ray by its refrangibility. Every metal yields its own spectrum, completely different from all others, and, however it may be combined or confused with other matters, its presence can be infallibly detected by analysis of the light emitted during its combustion; this peculiar property of the prism, of sifting out a particular element, has led to the discovery of two new metals by Bunsen, who, while engaged in an analysis of the contents of the mineral waters of Baden and Durkheim, detected one by two bright blue lines unknown in any other spectrum, and the other by two equally strange red lines; the first of these he called cæsium, the second rubidium, by virtue of the coloured rays they emitted; a third member of the same family, discovered by Crookes, and yielding a green ray, has been christened thallium.

We have now paved the way for an experiment which may be regarded as a key to the researches of the German philosophers. If we take the prism and telescope used for observing Fraunhofer’s lines, and substitute for the beam of sunlight the light of a candle or gas-jet, we shall obtain a perfectly uniform or homogeneous spectrum. Now, if we introduce a bead of sodium or a portion of common salt into the flame, we shall perceive two very fine brilliant yellow lines, as it were laid close together upon the gas-flame spectrum; if a colourless flame be employed, such as that obtainable from a Bunsen’s gas burner, the yellow lines will be seen without the illuminated background; in this state they will appear as represented in the diagram; and now comes the most interesting and most important feature in the experiment. If a glass tube, containing the heated vapour given off from the volatilisation of a sodium salt be held in front of the incandescent sodium, the brilliant yellow lines will be found to disappear entirely, and, if the luminous spectrum has been used for a background, their places will be exactly filled by two black lines; if, instead of sodium, a salt of lithium be employed, two bright red bands will appear in the places shown in the diagram, which will also be replaced by black ones when the vapour of lithium is interposed between the burning metal and the prisms; if an iron salt be substituted, upwards of sixty bright lines will be seen scattered about the spectrum, all of which will be similarly transmuted when the light is passed through iron vapour: and the same results will be obtained whatever metal be used in the experiment. From observation of these facts, Kirchoff arrived at the important discovery, that