The result obtained by Arago suggested a wide field of experimental inquiry as to whether other optical phenomena as well as refraction were independent of the direction of the Earth's motion through space. In most cases the experimental test is very precise and delicate; for the apparatus exhibiting the optical effect has only to be installed in the most sensitive manner possible, and note taken as to whether the gradual change of absolute direction of the light passing through it, arising from the Earth's movement of rotation, causes any diurnal inequality in the results. The negative results of theory have gradually been extended, by special investigations, to other optical phenomena, such as dispersion and crystalline interference, as these were successively found by experiment to be uninfluenced by the Earth's motion in space. It will be seen that the modern or electric view of the aether supplies a succinct dynamical foundation for the whole matter.
8. Long before Arago's time it had occurred to Boscovich, reasoning from Bradley's original point of view, that inasmuch as the velocity of light in water is different from what it is in air, the aberration produced by the Earth's motion in the apparent path of a ray travelling through water should be different from the normal astronomical amount: he suggested the use of a telescope with its tube filled with water to find out by star observations whether this is the case, in the expectation that the line of collimation would be different, in order that the relative rays in the water should focus on the cross-wires, from what it would be if the interior of the tube contained only air. In recent times Sir George Airy has actually had such an instrument temporarily installed at the Greenwich Observatory: he has found that observations with it, continued over a considerable time, gave the ordinary value of the constant of aberration, the different aberration of the ray in water being thus compensated by a modification of the ordinary law of refraction on the passage of the light into that moving medium. This experiment had already been discussed by Fresnel in his letter to Arago, with the remark that there is no occasion to complicate the result by aberration, as a terrestrial object might equally well be focussed on the cross-wires of the instrument
