The existence of these dark lines was first observed by Wollaston in 1802; but Fraunhofer, a celebrated optician of Munich, first studied and gave a detailed description of them. He mapped the lines, and denoted the most marked of them by the letters A, a, B, C, D, E, b, F, G, H; they are therefore generally known as Fraukhofer's lines.

The dark line A (see fig. 1 of the coloured plate) is towards the end, and B in the middle of the red ray; C is in the red but rather nearer the orange ray; D is in the orange ray, E in the yellow, F in the transition from green to blue, G in the indigo, H in the violet. There are certain other noticeable dark lines, such as a in the red, and b in the green. In the case of solar light the positions of the dark lines are fixed and definite; in the spectra of artificial lights and of the fixed stars the relative positions of the dark lines vary. For the electric light there are bright lines instead of dark arcs; and in coloured flames, that is to say, flames in which certain chemical substances undergo evaporation, the dark lines are replaced by very brilliant lines of light, which differ with different substances. 337. Spectrum analysis.—This property of coloured flames was first discovered by Sir John Herschell, who remarked that by volatilising substances in a flame a very delicate means is afforded of detecting certain ingredients by the colours they impart to certain of the dark lines of the spectrum; and Fox Talbot, in 1834, suggested optical analysis as probably the most delicate means of detecting minute portions of a substance. To Kirchhoff and Bunsen, however, is really due a method of basing on the observation of these lines a method of analysis. They ascertained that salts of the same metal, when introduced into a flame, always produce the lines identical in colour and position, but different in colour, position, or number, for different metals; and, finally, that an exceedingly small quantity of a metal suffices to disclose its existence. Hence has arisen a new method of analysis known as spectrum analysis.

338. Spectroscope.-The name spectroscope has been given to the apparatus used by Kirchhoff and Bunsen for the study of the spectrum. One of the forms of this apparatus is represented in fig. 261. It consists of three telescopes mounted on a common foot, and whose axes converge towards a prism, F, of flint glass. The telescope A is the one through which the spectrum is observed; it is focussed by means of the milled head-screw m. The telescope B has a slit the width of which can be regulated by the screw v.

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Spectroscope.

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K is what is known as a Bunsen's burner, in which coal gas is burned, mixed with air in such a manner that a flame of little or no

luminosity, but of intense heat, is produced. The substance to be examined is placed in this, either in the solid form, or in a state of solution on the platinum wire at the end of the support, c. It is thus volatilised by the intense heat, and the flame G is coloured. The rays emitted from this flame pass through the slit and through a system of lenses, so that on emerging they form a parallel pencil of rays which falls on the prism P. Here they are refracted and decomposed and form the prismatic spectrum. The spectator, on looking through the telescope A, sees a real and inverted image of the spectrum.

The telescope C has a different function; it contains a micrometric scale photographed on glass, so that it is white on a dark ground. The light from the candle passing through the scale and the lens in C falls in parallel rays on the face of the prism P, and is reflected from thence through the object glass of A, so that the observer seeing the spectrum and the scale simultaneously can exactly

measure the relative distances of the various spectral lines. M is a metal cap with three apertures which covers the prism so as to exclude the diffused light.

339. Experiments with the spectroscope. The adjacent coloured plate shows certain spectra observed by means of the spectroscope. Fig. I. represents the solar spectrum.

Fig. II. shows the spectrum of potassium. It is continuous; that is, it contains all the colours of the solar spectrum; moreover, it is marked by two brilliant lines, one in the extreme red, corresponding to Fraunhofer's dark line, A; the other in the extreme violet.

Fig. III. shows the spectrum of sodium. This spectrum contains neither red, orange, green, blue, nor violet. It is marked by a very brilliant yellow ray in exactly the same position as Fraunhofer's dark line D. Of all metals sodium is that which possesses the greatest spectral sensibility. In fact, it has been ascertained that one two hundred millionths of a grain of soda is enough to cause the appearance of the yellow line of sodium. Consequently it is very difficult to avoid the appearance of this line. A very little dust scattered in the apartment is enough to produce it,—a circumstance which shows how abundantly sodium is scattered throughout

nature.

Figs. IV. and V. show the spectra of cæsium and rubidium, metals discovered by MM. Bunsen and Kirchhoff by means of spectral analysis. The former is distinguished by two blue lines, the latter by two very brilliant red lines and by two less intense violet lines. A third metal, thallium, has been discovered by the same method by Mr. Crookes in England, and independently by M. Lamy in France. Thallium is characterised by a single green line.

Still more recently Richter and Reich have discovered a new metal associated, with zinc, and which they call indium, from a couple of characteristic lines which it forms in the indigo.

The extreme delicacy of the spectrum reactions, and the ease with which they are produced, constitute them a most valuable help in the quantitative analysis of the alkalies and alkaline earths. It is sufficient to place a small portion of the substance under examination on platinum wire as represented in fig. 261, and compare the spectrum thus obtained either directly with that of another substance, or with the charts in which the positions of the lines produced by the various metals are laid down.

With other metals the production of their spectrum is more difficult, especially in the case of some of their compounds. The

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Recomposition of White Light.

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heat of a Bunsen's burner is insufficient to vaporise the metals, and a more intense temperature must be used. This is effected by taking electric sparks between wires consisting of the metal whose spectrum is required, and the electric sparks are most conveniently obtained by means of Ruhmkorff's coil. Thus all the metals may be brought within the sphere of spectrum observations.

340. Recomposition of white light.-Not merely can white light be resolved into lights of various colours, but by combining the different pencils separated by the prism, white light can be reproduced. This may be effected in various ways:

I. A pencil of solar light is decomposed by a prism, as shown in fig. 262, and the spectrum is caught, not on a screen, but on a rather large double convex lens, in the focus of which is placed a small cardboard or ground glass and screen. The seven colours

of the spectrum coincide in the focus, and there is formed on the screen a perfectly white circular image, which shows that the union of the seven lights of the spectrum reproduces white light.

II. The same result is attained by replacing the double convex lens in the preceding experiment by a concave mirror. The seven coloured pencils being reflected from this mirror, there is formed in the focus the same white image as in the preceding experiment. III. By means of Newton's disc it may be shown that the seven colours of the spectrum form white. This is a cardboard disc of

about a foot in diameter, the centre and the edges are covered with black paper, while in the space between there are pasted strips or papers of the colours of the spectrum. They proceed from the centre to the circumference, and their relative dimensions and tints are such as to represent five spectra (fig. 263). When this disc is