Dynamical Theory of Heat and Light. 7 



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 r 

 and carried out, with most searching care to secure a trust- 

 worthy 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, may be found in a brilliant suggestion made 

 independently by FitzGsrald f and by Lorentz \ of Ley den, 

 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 fol- 

 lowing information on the subject of these researches : 



11 The early (1888) Potsdam photographs of the spectra of 51 stars 

 brighter than 2.V 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 D ; 

 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 ex- 

 tending 20° on both sides of a point of Right Ascension* 275° and 

 Declination -\- 30 . The authentic mean of 13 values determined by 

 the methods of Argelander or Airy gives 274° and + 35° (Andre, 

 Traite (V Astronomic Stellaire)." 



* Phil. Mag., December 1887. 



f Public Lectures in Trinity College, Dublin. 



X Versuch einer Theorie der electrischen und optischen Erscheinungen 

 in bewegten Korpern. 



§ 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). 



