1900.] on the Dynamical Theory of Heat and Light. 867 



of the very general question pointed out in § 3 as needing answer. 

 Another part, fundamental in the undulatory theory of optics, is, 

 How is it that the velocity of light is smaller in transparent ponder- 

 able matter than 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 communi- 

 cated under the title, '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,' to the Royal Society 

 of Edinburgh on July 17, 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 of the undulatory theory of light, and the 

 full explanation of them all is brought much nearer if we have a satis- 

 factory 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,* there would be at all points of the earth's surface a flow 



* 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. My Glasgow colleague, Professor Becker, has 

 kindly given me the following information on the subject of these researches : 



" The early (1888) Potsdam photographs of the spectra of 51 stars brighter than 

 2J 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 (Astronomicaljournal, 1893) finds practically the samo 



