rays for nearly all preparations; yet, that is not found at the same point for any two, however slight the difference between them may be. - (602.) Melloni has endeavoured to explain this. The hypothesis which this philosopher has given, supposes all bodies to have, what he terms "a chemical colouration." To render his meaning clear, we must diverge from our subject slightly. If we paint a board, of the seven prismatic colours or take, which is still better, seven pieces of cloth of the same colours as the rays; and receive the spectrum upon them, we shall find when all the rays fall upon their own colour that the colour of each is considerably exalted. If we receive the spectral image upon a red ground, all the rays will suffer diminution in intensity, except the red, which will be increased. If, on the contrary, it is received on a blue ground, the blue ray will become more intense, whereas all the others will, in different degrees, be diminished. Melloni, therefore, reasoning by analogy from these differently reflective powers of various colours, supposes all bodies, even a white sheet of paper, to have an invisible "chemical colouration ;" and hence as the colour of the body presented to the spectrum belongs to that of one or other of the rays, so is the maximum chemical power of that ray exalted, whilst all the others are depressed. We find that coloured media allow the passage of a larger quantity of the rays of their own particular colour than of any other. We also find that colourless fluids admit the permeation of the chemical influences of the solar beam in very different degrees. Hence M. Melloni argues, that according to the "chemical colouration" of the fluid, so is its permeability to the different rays which produce chemical change. This is certainly an exceedingly ingenious hypothesis, and we may by it explain a great number of phenomena; but it appears to me an unnecessary refinement in speculative philosophy, to build so elaborate and complicated a structure upon a foundation so purely imaginary. Any speculation from THE CONSTITUTION OF THE SPECTRUM. 351 so eminent a philosopher as Melloni, to whose researches on Heat we are indebted for the elucidation of some of the most complicated phenomena of this element, must be received with all the respect due to the efforts of an original mind; but at the same time we must not be led too widely astray by an entire reliance upon authority. It would appear that M. Moser has borrowed his ideas of the colours of the "latent Light" of vapours from this philo sopher. (603.) It is necessary for the fair and full consideration of the question, that the views of another experimental philosopher should be examined-those of the younger Becquerel. I shall, therefore, extract a few passages from the translation of E. Becquerel's Memoir on the Constitution of the Solar Spectrum in the Scientific Memoirs, Part XII. August, 1843.* "It has been generally admitted that these radiations which accompany Light are different from each other, and that according to such or such a sensible substance, the active rays were also different; but I do not suppose that the question is so complex. In fact, the luminous phenomena, according to the theory of undulations, depend on the vibrations of the molecules of the illuminating body, which are transmitted to the retina by the intermediation of the ether, the molecules of which are themselves in vibration. Fresnel, whose beautiful investigations have contributed to the triumph of this theory, had stated that the chemical effects produced by the influence of Light, are owing to a mechanical action exerted by the molecules of ether on the atoms of bodies, so as to cause them to assume new states of equilibrium, dependent on the nature and on the velocity of the vibrations to which they are subjected. This idea had been suggested to him by a remarkable experiment of M. Arago, the result of which was to show, that the chemical * See also Annales de Chimie, November, 1843, for an extended memoir by the same author, and Appendix. rays which influence the chloride of silver, interfere in the same manner as the luminous rays. (See Frontispiece, last figure.) I think that the hypothesis of Fresnel is accurate, and even that it may be extended further, espe cially if we consider that the chemical and phosphorogenic rays possess the same physical properties as the luminous rays; thus they are subjected to the physical laws of reflection, of double refraction, of polarisation, and of interference, in the same manner as are these rays; and, moreover, the spectra of these different radiations have the same lines. Thus, it would be more simple to sup pose "1st. That a pencil of solar rays is the union of an infinite number of rays of different refrangibility, each ray arising from undulations of ether, not having the same velocity. "2dly. That, by refracting a pencil of solar rays through a prism, we have the solar spectrum, which possesses different properties, on account of its different action on external bodies. "3dly. That, with respect to certain substances, the molecules of which are united by weak affinities, such as salts of silver, of gold, of mercury, &c.; the solar rays act according to the velocities of undulation, which may be transmitted to the molecules of matter, and consequently between certain limits of refrangibility. I have called the whole of the rays which affect a substance a chemical spectrum. "4thly. That phosphorescent bodies becoming luminous by means of the molecular movement impressed on their molecules, a movement which gives rise to the separation of the two electricities, necessary for maintaining the molecular equilibrium, and the neutralisation of which forms the flashes we observe, we may consider the action of the solar rays on these bodies, as analogous to that of these rays on bodies chemically sensible, with this difference only, that the mechanical action of the molecules of BECQUEREL'S HYPOTHESIS. 353 ether is transmitted to these bodies without chemical decomposition. According to their nature, therefore, these phosphorescent substances are sensible between certain limits of refrangibility, in the solar spectrum. "5thly. Besides, if we consider the retina as an organ which perceives the vibrations of the ether, it is only sensible to rays contained between certain limits of refrangibility, and the active rays form a spectrum, which in this case, is found to be the luminous spectrum. "According to this hypothesis, we shall bring back all the effects produced under the influence of Light, to the action of one same radiation upon different bodies, and there will be as many spectra, as there are sensible substances. This mode of viewing the subject is verified on all the phosphorescent bodies, and on those whose molecular state changes under the action of the solar rays. As to the luminous rays, or those which act upon the retina, we can only judge of them by our own sensations; but it is probable, that the retina of the different beings which exist on the surface of the globe, are not all sensible between the same limits of refrangibility. We have some examples of this; amongst others of fish which live in the depths of the sea; and which see how to find their way which would not be perceptible by our organs." (604.) This last assumption is purely gratuitous, we know perfectly well, that as we descend in the ocean zone after zone in depth, the colour of fishes gradually and regularly declines, and they degenerate in their order of organisation. That at the depth of about fifty fathoms around our own shores, all vegetable life ceases, and that in the clearest seas and that under the brightest skies all animal life is extinct at three hundred fathoms from the surface, consequently no fishes exist in spaces dark to the human eye. (605.) Much stress has been laid upon the fixed lines of the chemical spectrum, as proving the identity of the actinic with the luminous force. In the frontispiece will A A be seen the fixed lines obtained by M. E. Becquerel upon a Daguerréotype plate, and those of Mr. Stokes' fluorescent rays are described, page 49., par. 63. M. Arago, it appears, first suggested to M. E. Becquerel a set of inquiries, connected with the action of the rays of the spectrum on the same body placed in different media, say air and water. It was not found that any difference was produced by either of these media. The fixed lines were the same in each, although the velocity of Light is much quicker in water than in air. On this M. Arago remarks:-"The velocity with which a luminous ray passes through a given body, depends exclusively on the refringency of this body, and on the velocity of emission of the ray, on the velocity it had in vacuo. The ray which reached the surface of the stratum of iodine through the water at the point where it meets this surface, possesses a velocity superior to that which the ray that moved through the air had at the same point; but in the interior of the stratum, at a sufficient depth, the two rays possess exactly the same velocity. Let us make the photogenic phenomena depend not upon an action exerted at the surface, but upon an action originating in the interior of the stratum, and every difficulty disappears; only single result we are compelled to establish an essential distinction between the interior and the surface of a stratum, the thickness of which is incredibly small." (606.) All this is so purely hypothetical that it scarcely requires a remark. We know that the action always commences on the very surface of the sensitive film. We know that the chemical action is retarded by water,— that of Light, according to the undulatory view, is quickened. Indeed, Arago himself subsequently wrote:-" It is by no means proved that the photogenic modifications of sensitive substances result from the action of solar Light itself. These modifications are perhaps engendered by invisible radiations, mixed with Light, properly so called, proceeding with it and being similarly refracted. In this case the experiment |