direction and velocity of the movement of the solar system through space. For finding the direction he used the method of Folie. For calculating the velocity he combined the observations on three groups of stars, the brightest belonging probably to the solar nebula. The resulting velocity was only about 10,000,000 miles a year. Homann, working on the spectroscopic observations at Greenwich, had obtained a velocity of 527,000,000 of miles. As late as 1887, Fizeau studied the nature of the phenomena when light was reflected from a mirror moving with a great velocity, and inferred that aberration was the same in this case as when the light was taken directly from a star.
Glaseuapp calculated the time taken by the light in travelling the mean distance of the earth's orbit as equal to 500″.85 ± 1.02. This time combined with Michelson's velocity of light makes the solar parallax 8″.76. Struve's constant of aberration with Michelson's velocity gives a parallax of 8″.81. From Gill's mean of the nine best modern determinations of aberration (=20″.496) the parallax comes out equal to 8″.78. If we regard the solar parallax as known, the eclipses give nearly the same velocity as aberration, though the former is a group-velocity and the latter a wave velocity. Gill’s parallax from observations of Mars (8″.78) agrees with Michelson’s velocity of light and the mean constant of aberration.
In 1877–’78, Lord Rayleigh, in his profound treatise on the Theory of Sound, discussed the distinction between wave-velocity and group-velocity. In 1881, he recognized the same difference in the case of luminous waves. In the experiments of Young and Forbes, the wavevelocity might be nearly three per cent, less than the group-velocity. With toothed wheels and the revolving mirror, group-velocity was the subject of observation. Aberration gave wave-velocity; Jupiter’s satellites, group-velocity; experiment however showed but little difference. Lord Rayleigh found formulae for the relation between these two kinds of velocity, which involved the wave-length and the index of refraction, and J. Willard Gibbs has compared them, and other formulae proposed by Schuster and Gouy, with the experimental velocities of light. Michelson’s experiment on the index of refraction of carbon disulphide agrees with the assumption that he was dealing with the group-velocity.
Although there is not a complete accordance between the results of
different methods of investigation, astronomers and physicists will be Blow to abandon the theory of undulations, and take up again the corpuscular theory of light. The latter theory has received fatal blows from which it cannot recover. The undulatory theory, which started with Huyghens more than two hundred years ago, and was elaborated by Fresnel sixty years ago, has survived many crises in its history, and is supported by a wonderful array of experiments. Some of the experiments of Mr. Michelson may require a modification in Fresnel's interpretation. Stokes and Challis have worked for many years upon it, and established it on mathematical principles differing from Fresnel's and from each other. Ketteler in his Theoretische Optik, published in 1885, builds upon the Sellmeier hypothesis, that ponderable particles are excited by the ӕtherial vibrations and then react upon them. There remains Maxwell’s electro-magnetic theory of light, which has been elaborated by Glazebrook and Fitzgerald, and is supported, to say the least of it, by remarkable numerical coincidences.
Discrepancies between theory and experiment are always to be welcomed, as they contain the germs of future discoveries. We have learned in astronomy not to be alarmed by them. More than once the law of gravitation has been put on trial, resulting in a new discovery or in improved mathematical analysis. We may not expect in light such a brilliant discovery as that of the planet Neptune. The luminiferous ӕther is a mysterious substance, enough of a fluid for the planets to pass easily through it, but at the same time enough of a solid to admit of transverse vibrations. Stokes suggests water with a little glue dissolved in it as a coarse representation of what is required of the ӕther.
Mr. G. A. Hirn has written recently on the constitution of celestial space. He decides against the existence of an all-pervading medium. He thinks that matter exists in space only in the condition of distinct bodies, such as stars, planets, satellites, and meteorites. In nebulӕ it is in a state of extreme diffusion; but elsewhere space is empty. But how would it be after the correction is applied for the equation of light? Humboldt said that the light of distant stars reaches us as a voice from the past. The astronomer is not seeing for the most part contemporaneous events. He is reading history; and often ancient history, and of very different dates. Stellar photography reveals millions of stars which cannot be seen in the largest telescopes, and new harvests of these blossoms of heaven (as they have been called) spring up like the grass in the night. Numbers fail to express their probable distances and the time taken by their light in coming to the earth. In the theogony of Hesiod, the brazen anvil took only nine days in falling from heaven to earth. On the other hand, the reduction of the sun’s distance by three per cent not only affects its mass and heat, but it changes the unit of measure for the universe. Such are the remote results of any change in the estimated velocity of light.