the proximity of the two mirrors which would more than balance those of the long distances used by Foucault and Michelson.
The greatest difficulty which the undulatory theory of light has encountered is found in the attempted reconciliation between the requirements of the refraction of light and the aberration of light. To explain refraction, the density of the luminiferous ӕther must be greater when the index of refraction is greater. If a body moves, it must carry its inclosed ӕther with it, as its refractive power does not change. On the other hand, to explain the aberration of light, it must be supposed that the ӕther in the telescope does not move with the telescope; that the ӕther sifts through the telescope, the ӕther in front taking the place of the ӕther left behind; or, as Young expressed it, that the ӕther flows through the air and solid earth as easily as the wind blows through the trees of a forest.
The difficulty can be eluded by supposing that a refracting body carries along with it as much of the ӕther as it possesses in excess of what would exist in a vacuum of the same bulk. This, added to what is always sifting through it, would maintain its ӕther at a constant density. "What this fraction is which must travel with the body was calculated by Fresnel. But while the refracting power has been protected, how is it with aberration? That would be increased to a small extent. But as the aberration is very small, only about 201/2″ at its maximum, the required change in its value might be masked by ordinary errors of observation. Boscovich suggested to Lalande, in 1766, that a telescope filled with water instead of air would test the theory; but he made no experiment. Wilson, of Glasgow, also proposed a water telescope in 1782. In the course of time it appeared that not only was the effect of the earth’s motion on refraction and aberration under trial, but also the solar parallax, the motion of the solar system, and that of other stars.
The case is clearly stated by Lodge in this way: Sound travels quicker with the wind than against it. Is it the same with light? Does light travel quicker with the wind! Well, that depends altogether on whether the ӕther is blowing along as well as the air. If it is, then its motion must help the light on a little; but if the ӕther is at rest, no motion of the air, or of matter of any kind, can make any difference. According to Fresnel, the free ӕther is at rest, the bound is in motion. Therefore the speed of light will be changed by the motion of the medium; but only by a fraction, depending on its index of refraction, — infinitesimal for air, but sensible for water.
At an early day Arago investigated the effect which a change in the velocity of light would produce on aberration and refraction. He saw that a change of 5 per cent in the velocity of light would alter the aberration by only one second, whereas the refraction in a prism of 45° would be affected to the extent of two minutes. He observed the zenith distances of stars with and without the prism; and also the deviation of stars which passed the meridian at 6 a. m. and 6 p. m. The observa-
tions were made with a mural circle and a repeating circle. Arago expected to find a difference of ten or fifteen seconds, but found none. He thought that a difference no greater than one ten-thousandth would have been manifested by his observations had it existed. Arago attempted to explain his negative results by assumptions based upon the corpuscular theory of light. But Lloyd thought that the change in the length of the wave would balance the change in the direction of the ray. Arago’s observations were communicated to the Institute on December 10,1816, and excited great interest. They were quoted by Laplace and Biot. But the manuscript was mislaid and not found until 1853, when it was published. Mascart thinks that this experiment of Arago owes its reputation to Fresnel's explanation of it by his fraction.
In regard to the wave-motion involved in the transmission of light, Maxwell says: “It may be a displacement, or a rotation, or an electrical disturbance, or indeed any physical quantity which is capable o assuming negative as well as positive values. But the ӕther is loosely connected with the particles of gross matter; otherwise they would reflect more light.” Then he asks the question, “Does the ӕther pass through bodies as water through the meshes of a net which is towed by a boat?” It is difficult to obtain the relative motion of the earth and ӕther by experiment, as the light must move forward and then back again. One way is to compare the velocities of light obtained from the eclipses of Jupiter’s satellites when Jupiter is in opposite points of the ecliptic. Cornu referred, in 1883, to the difficulty of observing these eclipses, especially when Jupiter is in conjunction with the sun. On account of this difficulty observations have been neglected for the last fifty years. Observations must be made near quadratures. Cornu suggests a proper arrangement for this purpose.
At various times between 1864 and 1868, Maxwell repeated Arago's experiment in a more perfect form. A spectroscope was used, having three prisms of 60° each. A plane mirror was substituted for the slit of the collimator. The cross-wires of the observing telescope were illuminated by light reflected by a plate of thin glass placed at an angle of 45°. Light went to the mirror and was sent back to the wires from which it started after passing through six prisms. The experiment was tried when the light started in the direction of the earth’s motion, and when in the opposite; also, at different seasons of the year. In all cases the image of the wires coalesced with the wires.
Lodge states the case clearly thus: “If all the ӕther were free there would have been a displacement of the image of the wires. If all the ӕther were bound to the glass there would have been a difference on the other side. But, according to Fresnel's hypothesis there should be no difference either way. According to his hypothesis, the free ӕther, which is the portion in relative motion, has nothing to do with the refraction. It is the addition of the bound ӕther which causes the refraction, and this part is stationary relatively to the glass, and is not stream