620 KEPORT— 1900. 



in tlie free tetlier of space alone. It was possible, however, to determine the way in 

 which the characteristics of optical propagation are modified, but not wholly 

 transformed, when it takes place in a transparent material body instead of empty 

 space. The change in fact arises on account of the sether being entangled with 

 the network of ma^erial molecules ; but inasmuch as the length of a single wave of 

 radiation covers thousands of these molecules the wave-motion still remains uni- 

 form and does not lose its general type. A wider variation of the experimental 

 conditions has been provided for our examination in the case of those substances 

 in which the phenomenon of double refraction pointed to a change of the sethereal 

 properties which varied in different directions ; and minute study of this modifica- 

 tion has proved sufficient to guide to a consistent appreciation of the nature of 

 this change, and therefore of the mode of sethereai propagation that is thus altered. 

 In the same way, it was the study and development of the manner in which the 

 laws of electric phenomena in material bodies had been unravelled by Ampere 

 and Faraday, that guided Faraday himself and Maxwell — who were impressed 

 with the view that the asther was at the bottom of it all — in their progress towards 

 an application of similar laws to aether devoid of matter, such as would complete a 

 scheme of continuous action by consistently interconnecting the material bodies 

 and banishing all untraced interaction across empty space. Maxwell in fact chose 

 to finally expound the theory by ascribing to the iether of free space a dielectric 

 constant and a magnetic constant of the same types as had been found to express 

 the properties of material media, thus extending the seat of the phenomena to all 

 space on the plan of describing the activity of the asther in terms of the ordinary 

 electric ideas. The converse mode of development, starting with the free aether 

 under the directly dynamical form which has been usual in physical optics, and 

 introducing the influence of the material atoms through the electric charges which 

 are involved in their constitution,^ was hardly employed by him ; in part, perhaps, 

 because, owing to the necessity of correlating his theory with existing electric 

 knowledge and the mode of its expression, he seems never to have reached the 

 stage of moulding it into a completely deductive form. 



The dynamics of the sether, in fact the recognition of the existence of an aether, 

 has thus, as a matter of histor}^, been reached through study of the dynamical 

 phenomena of matter. When the dynamics of a material system is worked up to 

 its purest and most general form, it becomes a formulation of the relations between 

 the succession of the configurations and states of motion of the svstem, the assist- 

 ance of an independent idea offeree not being usually required. We can, however, 

 only attain to such a compact statement when the system is self-contained, when 

 its motion is not being dissipated by agencies of friciional type, and when its con- 

 nections can be directly specified by purely geometrical relations between the 

 coordinates, thus excluding such mechanisms as rolling contacts. The courseof the 

 system is then in all cases determined by some form or other of a single fundamental 

 property, that any alteration in any small portion of its actual course must produce 

 an increase in the total ' Action ' of the motion. It is to be observed that in 

 employing this law of minimum as regards the Action expressed as an integral 

 over the whole time of the motion, we no more introduce the future course as a 

 determining influence on the present state of motion than we do in drawing a 

 straight line from any point in any direction, although the length of the line is the 

 minimum distance between its ends. In drawing the line piece by piece we have to 

 make tentntive excursions into the immediate future in order to adjust each element 

 into straightness with the previous element ; so in tracing the next stage of the 

 motion of a material system we have similarly to secure that it is not given any 

 such directions as would unduly increase the Action. But whatever views may 

 be held as to the ultimate significance of this principle of Action, its importance, 



' In 1870 Maxwell, while admiring the breadth of the theory of Weber, which is 

 virtually based on atomic charges combined with action at a distance, still regarded 

 it as irreconcilable with his own theory, and left to the future the question as to 

 why ' theories apparently so fundamentally opposed should have so large a field o| 

 truth common to both.' — 'Scientific Papers,' ii. p. ?28. 



