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| A further insight into the physical nature of the process is obtained from the fact that if the two rays are polarized, and if the plane of polarization of one of them be made to turn round the axis of the ray, then when the two planes of polarization are parallel the phenomena of interference appear as above described. As the plane turns round, the dark and light bands become less distinct, and when the planes of polarization are at right angles, the illumination of the screen becomes uniform, and no trace of interference can be discovered. Hence the physical process involved in the propagation of light must not only be a directed quantity or vector capable of having its direction reversed, but this vector must be at right angles to the ray, and either in the plane of polarization or perpendicular to it. Fresnel supposed it to be a displacement of the medium perpendicular to the plane of polarization. Maccullagh and Neumann supposed it to be a displacement in the plane of polarization. The comparison of these two theories must be deferred till we come to the phenomena of dense media. The process may, however, be an electromagnetic one, and as in this case the electric displacement and the magnetic disturbance are perpendicular to each other, either of these may be supposed to be in the plane of polarization. All that has been said with respect to the radiations which affect our eyes, and which we call light, applies also to those radiations which do not produce a luminous impression on our eyes, for the phenomena of interference have been observed, and the wave-lengths measured, in the case of radiations, which can be detected only by their heating or by their chemical effects. Elasticity, tenacity, and density of the aether. Having so far determined the geometrical character of the process, we must now turn our attention to the medium in which it takes place. We may use the term aether to denote this medium, whatever it may be. In the first place, it is capable of transmitting energy. The radiations which it transmits are able not only to act on our senses, which of itself is evidence of work done, but to heat bodies which absorb them ; and by measuring the heat communicated to such bodies, the energy of the radiation may be calculated. In the next place this energy is not transmitted instantaneously from the radiating body to the absorbing body, but exists for a certain time in the medium. If we adopt either Fresnel's or Maccullagh's form of the undulatory theory, half of this energy is in the form of potential energy, due to the distortion of elementary portions of the medium, and half in the form of kinetic energy, due to the motion of the medium. We must therefore regard the aether as possessing elasticity similar to that of a solid body, and also as having a finite density. If we take Pouillet's estimate of 1.7633 as the number of grammecentigrade units of heat produced by direct sunlight falling on a square centimetre in a minute, this is equivalent to 1.234×106 ergs per second. Dividing this by 3.004×1010, the velocity of light in centimetres per second, we get for the energy in a cubic centimetre 4.1×10–5 ergs. Near the sun the energy in a cubic centimetre would be about 46,000 times this, or 1.886 ergs. If we further assume, with Sir W. Thomson, that the amplitude is not more than one hundredth of the wave-length, we have Ap = 2π/100, or about 1/16; so that we have — Energy per cubic centimetre = (1/2)ρV2A2p2 = 1.886 ergs. Greatest tangential stress per square centimetre = ρV2Ap = 30.176 dynes. Coefficient of rigidity of ether = ρV2 = 842.8 Density of aether = ρ = 9.36 ×10–19 The coefficient of rigidity of steel is about 8 ×1011, and that of glass 2.4 ×1011. If the temperature of the atmosphere were everywhere 0°C, and if it were in equilibrium about the earth supposed at rest, its density at an infinite distance from the earth would be 3 ×10–346 which is about 1.8 ×10327 times less than the estimated density of the aether. In the regions of interplanetary space the density of the aether is therefore very great compared with that of the attenuated atmosphere of interplanetary space, but the whole mass of aether within a sphere whose radius is that of the most distant planet is very small compared with that of the planets themselves.[1] The aether distinct from gross matter. When light travels through the atmosphere it is manifest that the medium through which the light is propagated is not the air itself, for in the first place the air cannot transmit transverse vibrations, and the normal vibrations which the air does transmit travel about a million times slower than light. Solid transparent bodies, such as glass and crystals, are no doubt capable of transmitting transverse vibrations, but the velocity of transmission is still hundreds of thousand times less than that with which light is transmitted through these bodies. We are therefore obliged to suppose that the medium through which light is propagated is something distinct from the transparent medium known to us, though it interpenetrates all transparent bodies and probably opaque bodies too. The velocity of light, however, is different in different transparent media, and we must therefore suppose that these media take some part in the process, and that their particles are vibrating as well as those of the aether, but the energy of the vibrations of the gross particles must be very much smaller than that of the aether, for otherwise a much larger proportion of the incident light would be reflected when a ray passes from vacuum to glass or from glass to vacuum than we find to be the case. Relative motion of the aether. We must therefore consider the aether within dense bodies as somewhat loosely connected with the dense bodies, and we have next to inquire whether, when these dense bodies are in motion through the great ocean of aether, they carry along with them the aether they contain, or whether the aether passes through them as the water of the sea passes through the meshes of a net when it is towed along by a boat. If it were possible to determine the velocity of light by observing the time it takes to travel between one station and another on the earth's surface, we might, by comparing the observed velocities in opposite directions, determine the velocity of the aether with respect to these terrestrial stations. All methods, however, by which it is practicable to determine the velocity of light from terrestrial experiments depend on the measurement of the time required for the double journey from one station to the other and back, again, and the increase of this time on account of a relative velocity of the aether equal to that of the earth in its orbit would be only about one hundred milliontth part of the whole time of transmission, and would therefore be quite insensible. The theory of the motion of the aether is hardly sufficiently developed to enable us to form a strict mathematical theory of the aberration of light, taking into account the motion of the aether. Professor Stokes, however, has shown that, on a very probable hypothesis with respect to the motion of the aether, the amount of aberration would not be sensibly affected by that motion. The only practicable method of determining directly the relative velocity of the aether with respect to the solar system is to compare the values of the velocity of light {571} deduced from the observation of the eclipses of Jupiter's satellites when Jupiter is seen from the earth at nearly opposite points of the ecliptic. [1] See Sir W. Thomson, Trans. R. S. Edin. Vol. xxi. p. 60. | The only practicable method of determining directly the relative velocity of the aether with respect to the solar system is to compare the values of the velocity of light {571} deduced from the observation of the eclipses of Jupiter's satellites when Jupiter is seen from the earth at nearly opposite points of the ecliptic. Arago proposed to compare the deviation produced in the light of a star after passing through an achromatic prism when the direction of the ray within the prism formed different angles with the direction of motion of the earth in its orbit. If the aether were moving swiftly through the prism, the deviation might be expected to be different when the direction of the light was the same as that of the aether, and when these directions were opposite. The present writer[2] arranged the experiment in a more practicable manner by using an ordinary spectroscope, in which a plane mirror was substituted for the slit of the collimator. The cross wires of the observing telescope were illuminated. The light from any point of the wire passed through the objectglass and then through the prisms as a parallel pencil till it fell on the object-glass of the collimator, and came to a focus at the mirror, where it was reflected, and after passing again through the object-glass it formed a pencil passing through each of the prisms parallel to its original direction, so that the object-glass of the observing telescope brought it to a focus coinciding with the point of the cross wires from which it originally proceeded. Since the image coincided with the object, it could not be observed directly, but by diverting the pencil by partial reflection at a plane surface of glass, it was found that the image of the finest spider line could be distinctly seen, though the light which formed the image had passed twice through three prisms of 60°. The apparatus was first turned so that the direction of the light in first passing through the second prism was that of the earth's motion in its orbit. The apparatus was afterwards placed so that the direction of the light was opposite to that of the earth's motion. If the deviation of the ray by the prisms was increased or diminished for this reason in the first journey, it would be diminished or increased in the return journey, and the image would appear on one side of the object. When the apparatus was turned round it would appear on the other side. The experiment was tried at different times of the year, but only negative results were obtained. We cannot, however, conclude absolutely from this experiment that the aether near the surface of the earth is carried along with the earth in its orbit, for it has been shown by Professor Stokes[3] that according to Fresnel's hypothesis the relative velocity of the aether within the prism would be to that of the aether outside inversely as the square of the index of refraction, and that in this case the deviation would not be sensibly altered on account of the motion of the prism through the aether. Fizeau[4], however, by observing the change of the plane of polarization of light transmitted obliquely through a series of glass plates, obtained what he supposed to be evidence of a difference in the result when the direction of the ray in space was different, and Angstrom obtained analogous results by diffraction. The writer is not aware that either of these very difficult experiments has been verified by repetition. In another experiment of M. Fizeau, which seems entitled to greater confidence, he has observed that the propagation of light in a stream of water takes place with greater velocity in the direction in which the water moves than in the opposite direction, but that the change of velocity is less than that which would be due to the actual velocity of the water, and that the phenomenon does not occur when air is substituted for water. This experiment seems rather to verify Fresnel's theory of the aether; but the whole question of the state of the luminiferous medium near the earth, and of its connexion with gross matter, is very far as yet from being settled by experiment. Function of the aether in electromagnetic phenomena. — Faraday conjectured that, the same medium which is concerned in the propagation of light might also be the agent in electromagnetic phenomena. “For my own part,” he says, “considering the relation of a vacuum to the magnetic force, and the general character of magnetic phenomena external to the magnet, I am much more inclined to the notion that in the transmission of the force there is such an action, external to the magnet, than that the effects are merely attraction and repulsion at a distance. Such an action may be a function of the aether; for it is not unlikely that, if there be an aether, it should have other uses than simply the conveyance of radiation[5].” This conjecture has only been strengthened by subsequent investigations. Electrical energy is of two kinds, electrostatic and electrokinetic. We have reason to believe that the former depends on a property of the medium, in virtue of which an electric displacement elicits an electromotive force in the opposite direction, the electromotive force for unit displacement being inversely as the specific inductive capacity of the medium. The electrokinetic energy, on the other hand, is simply the energy of the motion set up in the medium by electric currents and magnets, this motion not being confined to the wires which carry the currents, or to the magnet, but existing in every place where magnetic force can be found. Electromagnetic Theory of Light. The properties of the electromagnetic medium are therefore as far as we have gone similar to those of the luminiferous medium, but the best way to compare them is to determine the velocity with which an electromagnetic disturbance would be propagated through the medium. If this should be equal to the velocity of light, we would have strong reason to believe that the two media, occupying as they do the same space, are really identical. The data for making the calculation are furnished by the experiments made in order to compare the electromagnetic with the electrostatic system of units. The velocity of propagation of an electromagnetic disturbance in air, as calculated from different sets of data, does not differ more from the velocity of light in air, as determined by different observers, than the several calculated values of these quantities differ among each other. If the velocity of propagation of an electromagnetic disturbance is equal to that of fight in other transparent media, then in non-magnetic media the specific inductive capacity should be equal to the square of the index of refraction. Boltzmann[6] has found that this is very accurately true for the gases which he has examined. Liquids and solids exhibit a greater divergence from this relation, but we can hardly expect even an approximate verification when we have to compare the results of our sluggish electrical experiments with the alternations of light, which take place billions of times in a second. The undulatory theory, in the form which treats the phenomena of light as the motion of an elastic solid, is still encumbered with several difficulties.[7] The first and most important of these is that the theory indicates the possibility of undulations consisting of vibrations normal to the surface of the wave. The only way of {572} accounting for the fact that the optical phenomena which would arise from these waves do not take place is to assume that the aether is incompressible. [2] Phil. Trans. clviii. (1868), p. 532. [3] Phil. Mag. 1846, p. 53. [4] Ann. de Chimie et de Physique, Feb. 1860. [5] Experimental Researches, 3075. [6] Wiener Sitzb., 23 April, 1874. [7] See Prof. Stokes, “Report on Double Refraction,” British Ass. Report, 1862, p. 253. |