400 Scientific Proceedings, Royal Dublin Society. 



stresses characteristic of a solid body prevail between elements of 

 the volume hoioever small, and differ, according to the law laid down 

 as the law prevailing in the medium, at situations in the body how- 

 ever near. This is not true. 



On the other hand, if we proceed by summation, it is assumed 

 that the forces acting on each little block are distributed equally 

 and without any variation of direction to the several equal por- 

 tions into which its little mass may be conceived to be divided, 

 however minute this subdivision may be. If this were the case, 

 the internal stresses of a rigid body would be powerless to induce rota- 

 tion in any one of these blocks, or to alter any rotation that may have 

 pre-existed in it. Accordingly, each of these blocks would not rotate 

 round the instantaneous axis : it would merely revolve round it. («) 

 These, which are the real physical meanings of the assumptions 

 made in the two cases respectively, are specially instructive. 



About fifty years ago Professor MacCullagh announced his 

 great discovery that the phenomena of light could be accounted 

 for, if we suppose light to be an undulation in an incompressible 

 medium of uniform density, endowed with those dynamical pro- 

 perties which are embodied in his fundamental equations. These 

 properties are not very unlike the properties attributed to an 

 ordinary solid body ; and the question now arises, whether these 

 properties (or whatever are the real dynamical properties of the 

 medium in which are propagated light, radiant heat, and other 

 waves of electro-magnetic stress) are fundamental properties 

 of the medium, or whether, like the properties of solids, liquids, 

 or gases, they are the outcome of events of a wholly different 

 character happening at intervals so short that the elements of 

 volume (the dxdydz's of MacCullagh's formulae) contain vast 

 numbers of them. Now the dynamical properties of the lumi- 

 niferous medium — whether we use MacCullagh's or Cauchy's 

 fundamental equations — sufficiently resemble those of media which 

 we know to be "textured", to make the latter supposition the 

 more probable, after what we have found to be the real nature 

 of solid liquid and gaseous media. And this probability is 



(n) The proper inference from this is that our equations have only taken into 

 account a part of the forces that are really acting. This is true. 



