DARK LINES IN SPECTRUM. 49 These lines extend also over those parts which, until Mr. Stokes's inquiry, gave us no evidence of any luminous effect. By forming a spectrum on a Daguerreotype plate, M. E. Becquerel discovered the same lines in the chemical impression, and similar ones formed over the parts acted upon by the obscure rays. A very broad line is formed at I, by the union of many smaller lines. After it, at M Y, four other lines are formed; and at N, four others. There are at o two great lines; and another at г, which is very strong and black. A great number of others are formed, and many of them represented in the plate attached to M. E. Becquerel's memoir; but I have not thought it necessary to represent them. It is upon the strength of the fact of the existence of these inactive spaces in the "chemical," as well as in the "luminous," spectrum, that Becquerel inclines to regard the chemical and luminous rays as identical. (63.) Although the first edition of this Work was published in 1844, and then contained the Frontispiece which is affixed to this edition, in which is copied the fixed lines in the spectrum, as published by Becquerel in 1842*, it is curious to find Mr. Stokes adding a note, during the printing of his paper, to the effect that, until he forwarded it to the Royal Society, he was unacquainted with Becquerel's Map. "The only map of the fixed lines of the chemical spectrum, which I had for a good while after these researches were commenced, is Professor Draper's."† Mr. Stokes's lines, determined with all the accuracy of his new methods of observation, are shown in the following woodcut. Of these Mr. Stokes says, in his note :-" M. Bec querel's broad band 1 is ny l; his group of four lines M * Bibliothèque Universelle de Génève. † Philosophical Magazine, 1843. E with the preceding band, forms my m; his group of four lines N, forms the first of my group n; his line o is my n. It is only in the last group that there can be any doubt as to the identification; but I feel almost certain that M. Becquerel's P is my o; and the next two lines, the last in his map, are the two between o and p. It is difficult at first to believe that the strong line p should have been left out, while the two faint lines between o and p are represented; but the difficulty is, I think, removed by considering the feeble photographic action in that part of the spectrum." These fixed lines are so uniform that they are employed with the highest advantage in marking the spaces of the spectrum, instead of referring as formerly to the uncertain appreciation of the chromatic bands. The bright lines in the spectra of different flames, require no attention in this treatise, although they are phenomena of a highly remarkable character. The longitudinal lines observed by Zantedeschi have not yet been determined with the required accuracy; this reference to them will therefore be sufficient. (64.) Fraunhofer has given the world the first numerical estimate, on which any real dependence can be placed, of the illuminating power of the solar spectrum. He places the maximum at his line м, calling this 100; the Light at other parts being as follows: (65.) The experiments made by Dr. Herschel to determine the heating powers of the rays, consisted in passing each ray through an opening in a piece of pasteboard, and placing delicate thermometers with blackened * I cannot but express my doubts here, being quite disposed to believe the violet rays to have more illuminating power than the indigo. balls, so that they could be irradiated with each particular colour. The result of these investigations proved, in the first place, that the red rays possessed a greater amount of heating power than any other of the prismatic coloured rays; and, secondly, led to the discovery of "rays coming from the sun, which are less refrangible than any of those that affect the sight," and which have vested amongst them the maximum of the heating power. "A beam of radiant heat, emanating from the sun," says Dr. Herschel, "consists of rays that are differently refrangible. The range of their extent, when dispersed by a prism, begins at violet-coloured Light, where they are most refracted, and have the least efficacy. We have traced these calorific rays throughout the whole extent of the prismatic spectrum, and found their power increasing, while their refrangibility was lessened, as far as to the confines of red-coloured Light. But their diminishing refrangibility and increasing power did not stop here; for we have pursued them a considerable way beyond the prismatic spectrum into an invisible state, still exerting their increasing energy, with a decrease of refrangibility, up to the maximum of their power; and have also traced them to that state where, though still less refracted, their energy, on account, we may suppose, of their now failing density, decreased pretty fast; after which, the invisible. thermometrical spectrum, if I may so call it, soon vanished." Dr. Herschel determined that the invisible rays exerted a considerable heating power, at a point 1 inch distant from the extreme red ray, even though the thermometer was placed at a distance of 52 inches from the prism. (66.) These experiments were repeated by Sir Henry Englefield, with additional precautions against any source of error, and he found that the thermometer rose in the following order:— In the blue rays in 3′ from 55° to 56°, or 1°. Yellow in 3' from 56° to 62°, or 6°. Full red in 2 from 56° to 72° or 16°. Confines of the red in 21' from 58° to 734°, or 150. Quite out of visible Light in 2' from 61° to 79° or 18°. (67.) M. Berard obtained similar results, excepting that he placed the maximum of heat at the very extremity of the red rays, instead of beyond them. These experiments were afterwards repeated by Sir Humphry Davy at Geneva, who confirmed the correctness of Dr. Herschel's experiments; and still more recently by M. Seebeck, who has shown that the place of maximum heat varies with the substance of which the prism is made. Seebeck was assisted in his experiments by M. Wunsch; and they came to the following conclusions: Colour of Space in which Flint Glass Middle of the red. Middle of the red. (68.) Herschel having shown that the largest quantity of solar heat was manifested in the least refrangible rays, and particularly in rays which were not visible to us; and also having proved that the maximum of luminous power was found in the yellow ray- Ritter having demonstrated that invisible rays of great refrangibility had a large amount of chemical power, and Seebeck pointed out that this tendency to produce change was confined to these and the blue rays-it was long the custom to consider the prismatic spectrum as divisible into three classes of rays: the red or calorific rays; the yellow or luminous rays; and the blue or chemical rays. In the first place we are bound to regard all the coloured rays as luminous rays, but differing in the intensity of their effects. Calorific power is traced through all the luminous rays, and much below them, whereas chemical action extends, independent of Light at the other end of the spectrum. THERMOGRAPHIC SPECTRUM. 53 (69.) A brief notice of Sir John Herschel's experiments on the thermic spectrum must here have a place. To procure visible effects of the calorific power of the spectrum, the following method was adopted: "The thinnest post paper, such as is sold for foreign correspondence, was stretched on a frame. One side of this paper was blackened with Indian ink, or, which is better, smoked in the flame of oil of turpentine, or over a smoky candle, by drawing it often and quickly through the flame, giving it time to cool between each exposure, till it is coated on the under side with a film of deposited black, as nearly uniform as possible. The white side of this paper is exposed to the incident spectrum, properly adjusted, keeping the blackened side hollow to admit air and to avoid rubbing off the black coat. A fiducial dot being made on it, and this brought to coincide with the standard yellow ray, a flat brush, equal in breadth to the paper, dipped in good rectified spirits of wine, is to be passed over the white surface till the paper is completely saturated, which will be indicated by its acquiring a uniform blackness in place of the white it at first exhibited. "After a few minutes a whitish spot begins to appear considerably below the extreme red end of the luminous spectrum, which increases rapidly in breadth until it equals the breadth of the luminous spectrum, and even somewhat surpasses it; and in length, till it forms a long appendage exterior to the spectrum, and extends moreover within it, up to, and beyond the fiducial yellow. In this state, and just as the general drying of the paper begins by whitening the whole surface to confuse the appearances, a second, sudden, and copious wash of alcohol from above downwards must be applied, without disturbing the spectrum, or in any way shaking the apparatus. The superfluous alcohol will have hardly run off when the phenomena of the thermic spectrum will begin to appear in all their characters, at first faintly, and, as it were, sketched in by a dimness and dulness of the other |