6 Lord Kelvin on the 



undulatory theory of optics, is, How is it that the velocity 

 of light is smaller in transparent ponderable 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 mathe- 

 matical investigation of the subject worked out since that 

 time and communicated 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 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 intract- 

 able 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 neglect- 

 ing 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 imper- 

 fectly known motion of the solar system through space 

 towards the constellation Hercules, discovered by Herschel f r 



* 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. 



