570 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 whsn 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, bat this vector must be at right angles to the ray, and either in the plane of polarization or perpendicular to it. Fresuel 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 mag netic 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 im pression 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 heat ing 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 radia tion may be calculated. In the next place this energy is not transmitted instan taneously 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 por tions of the medium, and half in the form of kinetic energy, due to the motion of the medium. We must therefore re gard the aether as possessing elasticity similar to that of a solid body, and also as having a finite density. If we take Pouillefs estimate of 17633 as the number of gramme- centigrade units of heat produced by direct sunlight falling on a square centimetre in a minute, this is equivalent to 1 -234 x 10 6 ergs per second. Dividing this by 3 "00 4 x 10 10 , the velocity of light in centimetres per second, we get for the energy in a cubic centimetre 4 1 x 10~ 5 ergs. Near the sun the energy in a, cubic centimetre would be about 46,000 times this, or 1 SSG 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 = , or about yjr ; so that we have Energy per cubic centimetre = J-pY 2 A-j; 2 = 1-886 ero-s. Greatest tangential stress per square centimetre, = pV"Ap = 30 176 dynes. Coefficient of rigidity of ether, , . = p z =842-3 Density of aether, . = ^ 9-36 xlO~ 19 The coefficient of rigidity of steel is about 8 x 10 11 , and that of glass 2 4x 10 11 . If the temperature of the atmosphere were everywhere 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 x 10- 316 which is about 3 x 10 3 - 7 times less than the estimated density of the aether. In the regions of in terplanetary space the density of the tether 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 them selves. 1 The aether distinct from gross matter. When light travels through tho 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 some thing distinct from the transparent medium known to us, though it interpenetrates all transparent bodies and pro bably 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 par tides are vibrating as well as those of the aether, but the energy of the vibrations of the gross par tides must be very much smaller than that of the aether, for otherwise a much larger pro portion 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 cether. We must therefore con sider the aether within dense bodies as somewhat loosely connected with the dense bodies, and wo 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 millionth part of the whole time of transmission, and would therefore be quite in sensible. The theory of the motion of the tether is hanlly sufficiently developed to enable us to form a strict mathe matical 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
1 See Sir W. Thomson, Trans. R. S. Edin , vol. xxi. p. 60.