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undulatory theory of optics, is, How is it that the velocity of light is smaller in transparent ponderable matter thau in pure ether ? Attention was called to this particular question in my address, to the Royal Institution, of last April ; and a slight explanation of my proposal for answering it was given, and illustrated by a diagram. The validity of this proposal is confirmed by a somewhat elaborate discussion and mathematical investigation of the subject worked out since that time and communicated under the title, u On the Motion produced in an infinite Elastic Solid by the Motion through the Space occupied by it of a Body acting on it only by Attraction or Repulsion,5’ to the Royal Society of Edinburgh on July 16, and to the Congres International de Physique for its meeting at Paris in the beginning of August *.
§ 10. The other phenomena referred to in § 3 come naturally under the general dynamics ol: the undulatory theory of light, and the full explanation of them all is brought much nearer if we have a satisfactory fundamental relation between ether and matter, instead of the old intractable idea that atoms of matter displace ether from the space before them, when they are in motion relatively to the ether around them. May we then suppose that the hypothesis which I have suggested clears away the first of our two clouds ? It certainly would explain the “ aberration of light ” connected with the earth’s motion through ether in a thoroughly satisfactory manner. It would allow the earth to move with perfect freedom through space occupied by ether without displacing it. In passing through the earth the ether, an elastic solid, would not be lacerated as it would be according to Fresnel’s idea of porosity and ether moving through the pores as if it were a fluid. Ether would move relatively to ponderables with the perfect freedom wanted for what we know of aberration, instead of the imperfect freedom of air moving through a grove of trees suggested by Thomas Young. According to it, and for simplicity neglecting the comparatively very small component due to the earth’s rotation (only *46 of a kilometre per second at the equator where it is a maximum), and neglecting the imperfectly known motion of the solar system through space towards the constellation Hercules, discovered by Herschel f,
* Phil. Mag., Aug. 1900.
t The splendid spectroscopic method originated by Huggins thirty-three years ago, for measuring the component in the line of vision of the relative motion of the earth, and any visible star, has been carried on since that time with admirable perseverance and skill by other observers, who have from their results made estimates of the velocity and direction of the motion through space of the centre of inertia of the solar system.
there would be at all points of the earth’s surface a flow of ether at the rate of 30 kilometres per second in lines all parallel to the tangent to the earth’s orbit round the sun. There is nothing inconsistent with this in all we know of the ordinary phenomena of terrestrial optics ; but, alas! there is inconsistency with a conclusion that ether in the earth’s atmosphere is motionless relatively to the earth, seemingly proved by an admirable experiment designed by Michelsen, and carried out, with most searching care to secure a trustworthy result, by himself and Morley *. I cannot see any flaw either in the idea or in the execution of this experiment.. But a possibility of escaping from the conclusion which it seemed to prove, maybe found in a brilliant suggestion made independently by FitzGerald t and by Lorentz J of Leyden, to the effect that the motion of ether through matter may slightly alter its linear dimensions, according to which if the stone slab constituting the sole plate of Michelsen and Morley *s apparatus has, in virtue of its motion through space occupied by ether, its lineal dimensions shortened one one-hundred-millionth § in the direction of motion, the result of the experiment would not disprove the free motion of ether through space occupied by the earth.
§ 11. I am afraid we must still regard Cloud No. I. as very dense.
§ 12. Cloud II.—Waterston (in a communication to the Royal Society, now famous; which, after lying forty-five years buried and almost forgotten in the archives, was
My Glasgow colleague, Professor Becker, has kindly given me the following information on the subject of these researches:
u The early (1888) Potsdam photographs of the spectra of 51 stars brighter than 2^ magnitude have been employed for the determination of the apex and velocity of the solar system. Kempf (Astronomische Nachrichten, vol. 132) finds for the apex : right ascension, 206° + 12°; declination, 46° + 9°; velocity, 19 kilometres per second; and Risteen (Astronomical Journal, 1893) finds practically the same quantities. The proper motions of the fixed stars assign to the apex a position which may be anywhere in a narrow zone parallel to the Milky-way, and extending 20° on both sides of a point of Right Ascension 275° and Declination 4* SO3. The authentic mean of 13 values determined by the methods of Argelander or Airy gives 274° and + 35° (Andrl, TraitS dy Astronomic Stellaire
* Phil. Mag., December 1887.
f Public Lectures in Trinity College, Dublin.
J Versuch einer Theorie der electrischm tmd optischen Er&cheinungen in bewegten Korpem.
§ This being the square of the ratio of the earth’s velocity round the sun (30 kilometres per sec.) to the velocity of light (300,000 kilometres per sec.).