440 Mr. J. Larmor. A Dynamical Theory of [Dec. 7, 



type, which I believe can be thoroughly established as the actual 

 type of elasticity transmitting all radiations, whether luminous and 

 thermal or electrical for they are all one and the same through 

 the ultimate medium of fluid character of which the vortices consti- 

 tute matter. 



It has always been the great puzzle of theories of radiation how 

 the medium which conveys it by transverse vibrations, such as we 

 know directly only in media of the elastic-solid type, could yet be so 

 yielding as to admit of the motion of the heavenly bodies through it 

 absolutely without resistance. According to the view of the consti- 

 tution of the a3ther which is developed in this paper, not only are 

 these different properties absolutely consistent with each other, but it 

 is, in fact, their absolute and rigorous coexistence which endows the 

 medium with the qualities necessary for the explanation of a further 

 very wide class of phenomena. The remark which is the key to 

 this matter has been already thrown out by Lord Kelvin, in con- 

 nexion with Sir George Stokes's suggested explanation of the astro- 

 nomical aberration of light. The motion of the ultimate homogeneous 

 frictionless fluid medium, conditioned by the motion of the vortices 

 existing in it, is, outside these vortices, of an absolutely irrotational 

 character. Now, suppose the medium is endowed with elasticity of a 

 purely rotational type, so that its elastic quality can be called into 

 play only by absolute rotational displacement of the elements of the 

 medium; just as motion of translation of a spinning gyrostat calls 

 into play no reaction, while any alteration of the absolute position of 

 its axis in space is resisted by an opposing couple. As regards the 

 motion of the medium involved in the movements of its vortices, this 

 rotational elasticity remains completely latent, as if it did not exist ; 

 and we can at once set down the whole theory of the vortical hydro- 

 dynamical constitution of matter as a part of the manifestations of an 

 ultimate medium of this kind. 



We have now to indicate some of the consequences of the assumed 

 constitution of the aether as regards the phenomena of radiation, 

 which depend on this elasticity : to do this it will be convenient to 

 make a fresh start, dealing more particularly with the first part of 

 the general question. 



The true nature of the phenomena of light had been brought to 

 view at the beginning of the present century by the intuition of 

 Thomas Young; and the secret of the exact quantitative mathe- 

 matical laws which govern the behaviour of light in all the various 

 circumstances attending its propagation, reflexion, and refraction 

 had been fathomed in a marvellous manner by the genius of Fresnel. 

 The nature of the mathematical reasoning by which Fresnel was led 

 to his results has for the most part never been understood ; and, as 

 presented by him in his writings, it certainly seems devoid of 



