1919] on Ether and Matter 457 



not only simulates the properties of matter by carrying momentum 

 — as we know it does from fclie investi'^ations of Xichols and Hull. 

 Poynting and Barlow, and others — but that it is even subject to 

 gravity. For this would be the angle between the asymptotes of a 

 cometary orbit when the comet is moving with the speed of hghtand 

 passing close to the sun."' But the principle of relativity — through 

 the refractive or converging influence of a strong divergent gravita- 

 tional field — demands a greater deflexion than this, more than twice 

 as orreat. So there are three alternate deflexions before us, to be 

 settled by observation : — 



1*75 sec. ; 0*75 sec. ; and zero. 



Let us hope that the result of this or of some other eclipse-opportunity 

 may be definite enough to discriminate clearly and quantitatively 

 between these three alternative values ; any one of which should be 

 equally welcome to any lover of truth. 



If the first answer is given decisively, it will be a conspicuous 

 triumph for the theory of relativity, and will for a time be hailed 

 as a death-blow to the ether. I claim beforehand that such a con- 

 tention is illegitimate, that the reality of the ether of space depends 

 on other things, and that the establishment of the principle of 

 relativity leaves it as real as before ; though truly it becomes even 

 less accessible, less amenable to experiment, than we might have 

 hoped. Xevertheless the ether is needed for any clear conception 

 of potential energy, for any exphmation of elasticity, for any physical 

 idea of the forces which unite and hold together the discrete particles 

 of matter whether by gravitation or cohesion or electric or magnetic 

 attraction, as well as for any reasonable understanding of what is 

 meant by the velocity of light. Let us try to realise the position 

 beforehand ; for we shall be handicapped in the progress of our 

 knowledge of the relation between matter and ether until these 

 fundamental things are settled, and untd everyone agrees that the 

 ether has a real existence. I want people generally to admit that 

 the ether is itself stationary as regards locomotion, and that it is the 

 seat of all potential energy ; and further, at least as a surmise, that 

 it is the medium out of which matter is probably made, and in 

 which matter is perpetually moving by reason of its fundamental 

 property called inertia — a property the full explanation of which 

 must, I expect, ultimately be relegated to and considered as a 

 property derived from the ether itself. 



I call this lecture Ether and Matter, but I might equally well 

 have called it Inertia, for that is the main theme with which I have 

 to deal — at least in this first part. 



Is there anything else, besides matter, which possesses or seems 



* See, for instauce, my paper in the Philosophical Magazine for August, 

 1917, page 93. 



2 I 2 



