476 Dr. G. J. Stoney on Texture in Media, and on the 



any stress that resists change of form, and let the alternate 

 vortex filaments of each row rotate in the same direction, 

 while the intermediate ones rotate in the opposite direction, 

 but let the vortex filaments he in other respects similar. 



Such would be the simplest case of a medium of the kind 

 that Professor FitzGrerald has conceived*. Let us next 

 imagine the whole space within the room to be divided into 

 large blocks of a cubic }^ard in size. One of these blocks will 

 include a great number of the vortex filaments, and all the 

 large blocks will closely resemble one another ; insomuch 

 that if an undulation consisting of shallow waves each a 

 quarter of a mile long were to traverse the medium, the 

 blocks would appear to act on one another in a certain definite 

 way. This corresponds to the way in which Professor 

 MacCullagh's elements of volume, his dx dy dz's, are in his 

 formulas assumed to act, and the waves correspond to his 

 waves of light. But if the whole space were divided into 

 much smaller blocks, suppose into cubes of half an inch, 

 great differences would be found to prevail between these 

 small blocks, and equally great differences in the way they 

 act on one another ; and the difference would become more 

 striking if the subdivision were carried so far as to render 

 the blocks small in comparison with the thickness of the wires 

 that represent vortex filaments. 



If, now, we further conceive small vortex tangles travelling 

 about in this medium, the long vortex filaments opening to 

 let them pass, and acting in front, sideways, and behind upon 

 them in such a way as to urge them equally forwards and 

 backwards, so long as their journey is along a straight path 

 with uniform speed — we shall have a first sketch of what con- 

 stitutes ponderable matter and the luminiferous aether, accord- 

 ing to these speculations. 



The particular hypotheses which are here described may 

 perhaps not have quite hit the mark ; but, though we have as 

 yet only a glimmering of this great subject, it is pretty certain 

 that either these hypotheses, or something like them, are the 

 true ultimate account of material Nature. 



We must, therefore, carefully distinguish between the 

 elemental and the luminiferous aethers. The elemental aether, 

 until motions create differences in it, is absolutely alike and 

 undistinguishable in all its parts ; and in the mathematical 

 investigation of motions in it, wherever in any of the 

 equations of dynamics an element of mass appears, we must 



* < Nature/ May 1889, xl. p. 32. 



