descent substance, such as charcoal. The band of colours is continuous from red to violet, and therefore can teach little or nothing of the constitution of A B FIG. 107. Arrangement of Charcoal silver. Contact is made ized. the body producing the light; such a spectrum could not be employed for analytical purposes. A spectrum of the second order differs essentially from the first, inasmuch as the colours are not continuous, but consist of distinct coloured bands; it can only be obtained from light emitted from incandescent gases; and any substance which can be converted into a gaseous state by intense heat without undergoing decomposition will afford distinct bands of colour, which are always the same. The metal silver placed in a cup-shaped charcoal pole and connected with the other pole, in the electric lantern figured in the frontispiece, is converted into silver gas, and produces on the disc two distinct green lines. (See Frontispiece.) Thallium-so cleverly discovered by Mr. Crookes, in 1861, in certain kinds of iron pyrites, and so called from the Greek because it produces a splendid green flamewould probably have been unknown but for this new method of analysis. The attention of Mr. Crookes was first directed to the splendid green line as obtained from certain specimens of pyrites, and it was by following up this simple fact-this slender clue-that he was at last enabled to isolate the body that produces the green lines, and confidently pronounce it to be a metal. The spectrum of thallium is shown in the frontispiece. In projecting metal spectra on to the disc, it must be understood that for exact purposes of research they cannot be so truthful as the spectra results obtained by the instruments described on p. 95. The optical arrangements required to show the spectra of incandescent metals to a large audience on the disc cannot be compared to the elaborate instruments already mentioned. Moreover, the charcoal crucible and points contain ash consisting of alkaline earths and salts, which must interfere with the spectrum, results. A spectrum of the third order is obtained when the regularity of the spectrum is interfered with by black fixed lines. Such a spectrum is always obtained from the rays of the sun. As Mr. Huggins remarks, "These dark spaces are not produced by the source of light." They tell us of vapours through which the light has passed on its way, and which have robbed the light by absorption of certain definite colours or rates of vibration. A very simple mode of showing such a spectrum crossed by dark lines is to interpose between the slit of the electric lantern and the double-convex lens a vessel containing some nitrous acid gas. Directly this is done, all the visible indigo, blue, and green colours vanish, and the remainder of the spectrum is crossed with numerous dark lines. In using the electric lantern it must always be borne in mind that if the aperture or slit is too widely opened the dark lines are very indistinct. The slit should be very narrow indeed to display the dark lines sharp and distinct. A more instructive mode is first to produce the two yellow lines representing the spectrum of sodium, and then with a peculiar-shaped crucible (Fig. 108). It was by this and kindred experiments that Kirchoff showed that if vapours of terrestrial substances come between the eye and an incandescent body, they cause groups of dark lines, and, further, that the group of dark lines produced by each vapour is identical in the number of lines and in their position in the spectrum with the group of lines of which the light of the vapour consists when it is luminous. The reversal of the spectrum of coloured flame, and the mode in which he obtained the proof of the identity between the terrestrial sodium line and the dark lines similarily placed in the solar spectrum, is thus described by Kirchoff : FIG. 108. "In order to test by direct experiment the truth of the frequently asserted fact of the coincidence of the sodium lines with the lines D (Frauenhofer), 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 The section of the Crucible to flame of a Bunsen's lamp threw the bright sodium lines upon the solar spectrum. 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 upon the slit, and, to my astonishment, I saw that the dark lines D appeared with an extraordinary degree of clearness." be used for showing the reversal of the bright sodium. lines, of which A is the central hole, and contains some chloride of sodium, and B B a ring or trench all round A, in which metallic sodium is placed; c, the upper charcoal pole. With respect to this important experiment, showing the reversal of the sodium lines, perhaps the most simple experiment is that of Roscoe, who seals up some of the metal sodium in a vacuum tube, and on volatilizing the metal the vapour is colourless by white light, but dark and opaque when the monochromatic or yellow light of sodium is shown behind it. It was by the exact reversal of the bright terrestrial lines, and the absolute identity in position of the bright terrestrial and dark solar lines, that Kirchoff discovered the elements that exist in the sun, viz., hydrogen, sodium, magnesium, iron, calcium, nickel, chromium, copper, zinc, barium, and probably strontium, cobalt, cadmium. At p. 92, and in Fig. 101, are shown the lines B, C, D, E, F, G, and H, which are called Frauenhofer's principal fixed dark lines in the solar spectrum. The labours of Kirchoff have now almost interpreted the whole of these lines, which are read as follows: C, F, and G are Hydrogen. D is Sodium. E is Iron. H, Aluminium. The limits of this work do not permit the consideration of stellar chemistry, and the extremely valuable researches of Mr. Huggins and Dr. Miller in this direction; but the reader is referred to Mr. Huggins's discourse "On the Results of Spectrum Analysis applied to the Heavenly Bodies," published by Ladd; or to Mr. Watt's "Dictionary of Chemistry," for a complete résumé of this subject. This much may be said, that spectrum analysis proves that the fixed stars are suns like our own-a fact which could only be assumed and taken for granted before the important experiments of Kirchoff, Huggins, and Miller. FIG. 109.-Star Spectroscope, with adjustible Reflecting Prism and Mirror. With finest object-glass micrometric apparatus for measuring the lines of the spectrum to 1-10,000th of an inch, extra eye-piece, and ivory tube to reader of vernier, as made for W. Huggins, Esq., F.R.S., and used during the observation of the red flames of the sun in India, August, 1868. Moreover, the spectroscope has discovered the real nature of the "red flames" or "prominences" of the sun, which are invisible under ordinary circumstances, being overpowered by the dazzling brilliancy of the rays which proceed from the sun; but visible during the few minutes that elapse during a total eclipse of the sun, as in the one which created so much interest in August of the present year, 1868, visible only in the line or path of the shadow, which fell in India. Four xpeditions went to India to observe the red flames; they were all armed with the spectroscopic apparatus, and their united statements all agree that the red flames belong to the sun, and that, as they give bright lines which belong only to spectra of the second order, they must be enormous gasheaps, intensely ignited or self-luminous. The bright lines chiefly observed appear to be those which belong to hydrogen gas and sodium, at least so far as we know at present (September, 1868); and this interesting statement was made through the telegrams from Major Tennant, Lieutenant Herschel, and M. Jannsen, which arrived in England, and were all sent independently of each other. As the red flames belong to the sun and show bright lines in the spectroscope, are they.great volumes of the photosphere thrust out (like the pips and juice of a squeezed gooseberry) beyond the last or gaseous atmosphere, which usually robs the light from the photosphere of its beautiful coloured bands, and changes them to dark lines? for where light is not, there can only be darkness. These and other facts are discoverable by another modification of the spectroscopic arrangement (Fig. 109), as constructed by Mr. Browning. SPHERICAL ABERRATION. In using an ordinary concave mirror the experimentalist cannot fail to notice that the rays reflected from the part near the circumference do not come to the same meeting-point or focus as the rays reflected from parts near the centre. (Fig. 110.) It is evident that the rays A B, A C, come to a focus at G, which is further off than the focus F from the parallel rays DD, D D. The distance between F and G, the two foci, is called the longitudinal spherical FIG. 110.-Concave Mirror, showing the Aberration of the Rays of Light. aberration. The natural consequence must be that an image projected by an ordinary concave mirror will be confused, because the eye has to look at a double image, the one superposed on the other. To get rid of the rays from the outer part of the mirror it is usual to employ a screen, so that the rays D D, DD, from the central part of the mirror only are used. Arising from this circumstance is the unequal illumination of a white ground on which rays are reflected to different foci, and the production of symmetrical curves, termed caustic lines or caustic curves, in the study of which mathematicians have been most industrious. Brewster lays claim to the following method of exhibiting caustic curves. He recommends the use of a piece of steel spring highly polished, or, better still, polished silver, which is to be bent into a concave figure and placed vertically on its edge upon a piece of card or white paper, and when exposed either to the rays of the sun or any good artificial light, the curves shown in Fig. 111 are well defined. In the same way, passing from reflecting to refracting bodies, the spherical figure of a convex lens causes the rays which fall near the outer edge to come to a focus nearer the lens than the rays which are refracted from the centre. The result, as might be expected, is just the reverse of the concave mirror. The rays A B, A B, Fig. 112, falling on the margin of the double-convex lens are refracted to a focus at F, whilst those rays, D D, D D, which fall near the axis of the lens come together at a more remote point, viz., at c. Here again a screen or diaphragm cutting off the rays refracted from the outer edge of the lens gives a better image; the picture produced by such a lens, provided with a screen, can be focused more distinctly; hence telescopes, microscopes, cameras, oxy-hydrogen lanterns, &c., &c., are usually fitted with diaphragms, which reduce the light, but cause the images to become more distinct. The lens of the eye would, from this cause, project on to the retina a confused or double picture, which might render vision extremely imperfect; this, however, is prevented by the iris, which acts as a diaphragm, thus the aberration of sphericity is corrected. THE DISPERSION OF LIGHT, OR CHROMATIC ABERRATION. If light consisted of a series of coloured rays, every one of which possessed the same index of refraction when they fall upon a glass lens, they would all come together in the same spot, and white light only would be obtained; but this is not the case, and it is known in practice that lenses, and especially condensing lenses, project coloured rings, and give images with coloured edges. And this is not remarkable when it is remembered that a double-convex lens may be regarded as a series of prisms united at their bases, and therefore capable of decomposing or dispersing light. It is a singular fact that Sir Isaac Newton considered, from the experiments he had tried with various prisms, that dispersion was proportioned to refraction, and he believed that all substances had the same chromatic aberrations when formed into lenses, and that any combination of a concave with a convex glass would produce colour with refraction. Newton reasoned only from the facts he had acquired on the dispersive powers of bodies, and pronounced the construction of achromatic telescopes which should not project images with coloured edges to be impossible. The fallibility even of his great mind is shown by the fact that, a few years after his death, Hall in 1733, and Dolland, the famous optician, in 1757, demonstrated that by using two media, viz., crown and flint glass, of different refractive and dispersive powers, a lens may be formed which is achromatic. The principle of the achromatic lens is not complicated or difficult to understand, provided the previous matter relating to compound and simple colours (p. 89) has been already studied. Given a lens made of a certain glass, and projecting, amongst other colours, a ring of red light, what colour, projected from another lens, is required, to neutralize it? The answer is obvious: any colour which together with the red light would form white light. That colour must be green, because it contains yellow and blue; and, as already shown, red, yellow, and blue form white light. In the adjustment of the two lenses |