626 EEPORT— 1900. 



the medium instead of teing concentrated in a nucleus ; and this remark seems to 

 go to the root of the question. On the other hand, the position here asserted is 

 that recent dynamical developments have permitted the extension of the principle 

 of Action to systems involving permanent motions, whether obvious or latent, as 

 part of their constitution ; that on this wider basis the atom may itself involve a 

 state of steady disturbance extending through the medium, instead of being only a 

 local structure acting by push and pull. The possibilities of dynamical explanation 

 are thus enlarged. The most definite type of model yet imagined of the physical 

 interaction of atoms through the aether is, perhaps, that which takes the aether to 

 be a rotationally elastic medium after the manner of MacCullagh and Rankine, 

 and makes the ultimate atom include the nucleus of a permanent rotational strain- 

 configuration, which as a whole may be called an electron. The question how far 

 this is a legitimate and eSective model stands by itself, apart from the dynamics 

 which it illustrates ; like all representations it can only cover a limited ground. 

 For instance, it cannot claim to include the internal structure of the nucleus of an 

 atom or even of an electron ; for purposes of physical theory that problem can be put 

 aside, it may even be treated as inscrutable. All that is needed is a postulate of 

 free mobilit}' of this nucleus through the aether. This is definitely hypothetical, but 

 it is not an unreasonable postulate because a rotational aether has the properties of 

 a perfect fluid medium except where diSerentially rotational motions are concerned, 

 and so would not react on the motion of any structure moving through it except 

 after the manner of an apparent change of inertia. It thus seems possible to hold 

 that such a model forms an allowable representation of the dynamical activity of 

 the aether, as distinguished from the complete constitution of the material nuclei 

 between which that medium establishes connection. 



At any rate, models of this nature have certainly been most helpful in Max- 

 well's hands towards the eifective intuitive grasp of a scheme of relations as a 

 whole, which might have proved too complex for abstract unravelment in detail. 

 When a physical model of concealed dynamical processes has served this kind of 

 purpose, when its content has been explored and estimated, and has become 

 familiar through the introduction of new terms and ideas, then the ladder by 

 which we have ascended may be kicked away, and the scheme of relations which 

 the model embodied can stand forth in sevei-ely abstract form. Indeed many of 

 the most fruitful branches of abstract mathematical analysis itself have owed their 

 start in this way to concrete physical conceptions. This gradual transition into 

 abstract statement of physical relations in fact amounts to retaining the essentials 

 of our working models while eliminating the accidental elements involved in 

 them; elements of the latter kind must always be present because otherwise the 

 model would be identical with the thing which it represents, whereas we cannot 

 expect to mentally grasp all aspects of the content of even the simplest phenomena. 

 Yet the abstract standpoint is always attained through the concrete; and for 

 purposes of instruction such models, properly guarded, do not perhaps ever lose 

 their value : they are just as legitimate aids as geometrical diagrams, and they 

 have the same kind of limitations. In Maxwell's words, ' for the sake of persons 

 of these diiferent types scientific truth should be presented in different forms, and 

 should be regarded as equally scientific whether it appear in the robust form and 

 the vivid colouring of a physical illustration, or in the tenuity and paleness of a 

 symbolical expression.' The other side of the picture, the necessai-y incomplete- 

 ness of even our legitimate images and modes of i-epresentation, comes out in the 

 despairing opinion of Young (' Chromatics,' 1S17), at a time when his faith in the 

 undulatoi-y theory of light had been eclipsed by Malus's discovery of the pheno- 

 mena of polarisation by retlection, that this difficulty ' will probably long remain, 

 to mortify the vanity of an ambitious philosophy, completely um-esolved by any 

 theory: ' not many years afterwards the mystery v.^as solved by Fresnel. 



This process of removing the intellectual scafiolding by which our knowledge 

 is reached, and preserving only the final formulae which express the corfelations 

 of the directly observable things, may moreover readily be pushed too far. It 

 asserts the conception that the universe is like an enclosed clock that is wound up 

 to go, and that accordingly we can observe that it is going, and can see some of 



