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ance of the same principles which have enabled us to calculate the tints produced by common polarised light.
Before I discovered these modifications, produced in polarised light by total reflection, I had studied those which are produced by partial reflection on the external surface of transparent bodies, and I had observed that the light is in this case never depolarised, even in part, and that whatever may be the inclination and the relative situation of the plane of incidence, it produces no change but a simple alteration of the plane of polarisation. The new hypothesis that I have adopted, respecting the constitution of the luminous undulations, has pointed out to me the law of these deviations, which I had before in vain attempted to represent by empirical formulas. These formulas agreed well enough with the phenomena in the three principal cases, of rays parallel to the surface, a perpendicular incidence, and a complete polarisation : but they did not agree correctly with the facts at intermediate incidences. The formala to which I have at last been led by theoretical considerations, and which will be found in an addition to the note which I have already mentioned, (Ann. Chim. xvii. p. 312,) seems to express correctly the law of the phenomenon, if we may judge by its agreement with observations. I have deduced it from the general formulas for the intensity of reflected light, which :I have derived from the same considerations, and which I have also published in the same note.
I shall here terminate this extract from my memoirs, and I shall not enter into the theoretical and experimental researches which I have undertaken, in examining the phenomena of polarisation, discovered by M. Bior in certain homogeneous liquids, such as the essence of turpentine, and the essence of lemon. I have thought it necessary to confine myself to the explanation of the more general properties of light, and to the elementary facts, if I may so call them, or those which occur the most frequently, and of which others are in some sense only more or less complicated combinations. I have shown in what manner the theory of undulations might be employed for explaining them, and for furnishing the means of representing, by analytical expressions, the laws which govern them. In order to calculate the phenomena of diffraction, which are so
diversified, those of the coloured rings produced by a thin plate of air or of any other refractive medium; the law of refraction itself, in which the proportion of the sines of incidence and refraction is exactly that of the velocities in the two mediums; the colours and the singular modes of polarisation, which are obtained from crystallized plates, it is sufficient to know the different lengths of the undulations of light, in the mediums which transmit it, and this is the only quantity that we are obliged to borrow from experiment, as the basis of all our formulas. If we attend to the intimate and multiplied relations «which the theory of undulations establishes between the most different phenomena, we must be struck at once with its simplicity and its fecundity; and we must allow, that even if it had not the advantage over the system of emanation, that it explains many facts wholly unaccountable in this latter system, it would amply deserve a preference from the means which it affords us of connecting together all the phenomena of optics I in its general formulas. ut Without doubt there are still many obscurities to be enlightened, especially such as relate to the absorption of light, for instance, in the reflection of metallic surfaces, and in black 1 bodies; the passage of light through bodies imperfectly transparent, and the proper colours of bodies. It is probable, that in these different cases, a part of the light changes its nature, and affords calorific vibrations, which are no longer sensible to our eyes, because they cannot penetrate their substance, or cannot make the optic nerve vibrate in unison with them, on account of the modifications which they have undergone. But the total quantity of the living force must remain the same, unless the action of light have produced a chemical orla calorific effect, sufficiently powerful to change the state of equilibrium of the particles of the bodies, and with it the identity of the forces to which they are subjected : for it is easily understood, that if these forces were suddenly weakened, there would be a -sudden diminution of the energy of the oscillations in the particles of the heated body, and consequently an absorption of heat, to use the common expression. And this is perhaps what happens in the melting of a solid, or in the vaporisation of a liquid. JAN.-MARCH, 1829.
If light is only a certain mode of vibration in a universal fluid, as the phenomena of diffraction show, we must no longer suppose that its chemical action on material substapces consists in a combination of its particles with theirs ; but in a mechanical action, which the particles of this fluid exercise on the ponderable particles, and which causes them to enter into new arrangements, into more stable forms of equilibrium, from the peculiar kind or energy of the vibrations to which they are exposed. It is obvious, that the hypothesis adopted respecting the nature of light and heat, may materially influence our conception of the nature of chemical actions, and that it is very important not to be deceived in our fundamental notions of this theory, in order that we may hope to arrive at last at the discovery of the principles of atomic mechanics, the knowledge of which would throw so much light on the whole of chemistry. If any thing can contribute very essentially to advance this great discovery, and to assist us in penetrating the secrets of the internal constitution of bodies, it must be the minute and indefatigable study of the phenomena of light.
PostSCRIPT.- On the Chemical Action of Light, Mr. Arago has lately confirmed, by a very interesting experiment, the opinion of Mr. Fresnel, respecting the chemical action of light, and has demonstrated that it cannot be attributed to the combination of its particles with those of material bodies.
Experiment. When the fringes produced by the interference of two pencils, reflected by mirrors slightly inclined to each other, were thrown on some muriate of silver, recently prepared, Mr. Arago has found that they marked on it black lines, placed at equal intervals, and separated by white spaces: and this shows that the chemical influence of the rays of light is modified by their interference, as well as their optical properties, and that it varies in intensity, according to the difference of the paths described by them. When this difference is equal to a whole number of undulations, the two systems of undulations are in perfect agreement, and their oscillations have the greatest possible energy; it is then that their chemical effects become a maximum ; when the difference of the paths, on the contrary, is an uneven number of undulations, the discordance being complete, the chemical effects ought to disappear, as well as the sensation of light arising from the same points in the eye: and this is equally confirmed by experiment. It must only be remarked, that the extreme violet rays being those which have the greatest chemical action, the black lines traced on the muriate of silver ought not to correspond to the brightest parts of the fringes produced by white light, which answer nearly to the points of perfect coincidence for the yellow rays: and this experiment affords a simple and very accurate mode of determining the mean length of the luminous undulations which have the most chemical influence; for it is sufficient for this purpose to measure the intervals between the middle points of the black lines marked on the muriate; and the formula given in this essay will determine the length of the undulations which produce them.
Dr. Young had long ago shown, by throwing light modified by the coloured rings, on the muriate of silver, that the same modifications affected its chemical action : but the experiment of M. Arago has the further “ advantage" of proving directly, that the unequal action of the light on the different points in which the two pencils meet, depends on their mutual influence; because, when we intercept either of the pencils, we see the muriate of silver assume
a uniform tint in the same space which was marked by black and white lines, when both the pencils fell on it; while in Dr. Young's experiment, made with the coloured rings, it was impossible to separate the two systems of undulations. It is also demonstrable by the experiment of Mr. Arago, that in all the points which answer to differences of paths, equal to an odd number of semi-undulations, the chemical action of the light is insensible, when the two pencils arrive there together, and that it reappears when one of the pencils is withdrawn. This fact alone, independently of all theory, reverses the supposition adopted by several philosophers, according to which the chemical effects of light depend on its combination with material substances; for if this were the case, there would always be a greater effect when the number of luminous particles become more consider
able, and it would be impossible to augment the chemical action of light, by withdrawing a part of the incident rays.
Mr. ARAGO's experiment contains also another remarkable circumstance, which does not exist in that of Dr. YOUNG; in this latter, the rays are parallel, and do not again separate after their combination; while the rays reflected by the mirrors form a sensible angle, and allow us to observe, that having once lost their luminous and chemical properties, by their complete discordance with each other, they recover them again under other circumstances a little further off; which shows, aś Mr. Arago observes, that they were not actually destroyed, but only momentarily neutralised, as long as there were motions in opposite directions to counteract their oscillations. This restoration may easily be understood by an inspection of the first diagram in this abstract.
Mr. Arago's experiment requires several precautions in order to be repeated with success.
The solar rays, thrown into the dark chamber, must be kept in a constant direction by a good heliostate, in order that the fringes thrown on the surface of the muriate should undergo no sensible change of place, for ten minutes at least: and in order that the very small changes, which are unavoidable, should not injure the neatness of the black lines which they form, it is proper to give the fringes the greatest possible breadth, by placing the mirrors very nearly in the same plane: and instead of a common lens in the window shutter, to form a luminous point, which would afford by much too feeble a light, it is necessary to employ a cylindrical lens, which gives us a very valuable command of the intensity of light; but since this affords a linear image instead of a point, we must be very careful to turn it into a direction perfectly parallel to the fringes : and the distinctness of the fringes will serve as a sufficiently correct test of the performance of this condition. The cylindrical lens, which was employed, was about four tenths of an inch in focal length; the two metallic mirrors were at the distance of about two feet from it, and the plate, covered with the muriate of silver, was about the same distance from the mirrors; and it was necessary that the distances should be no greater, in order to preserve a sufficient intensity in the light. It must be remarked,