September 21, 1900.] 



SCIENCE. 



433 



atom may itself involve a state of steady 

 disturbance extending through the medium, 

 instead of being only a local structure act- 

 ing by push and pull. The possibilities of 

 dynamical explanation are thus enlarged. 

 The most definite type of model yet imag- 

 ined of the physical interaction of atoms 

 through the sether is, perhaps, that which 

 takes the sether to be a rotationally elastic 

 medium after the manner of MacCullagh 

 and Eankine, and makes the ultimate atom 

 include the nucleus of a permanent ro- 

 tational strain-configuration, which as a 

 whole may be called an electron. The 

 question how far this is a legitimate and 

 efiective 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 pur- 

 poses of physical theory that problem can 

 be put aside, it may even be treated as in- 

 scrutable. All that is needed is a postulate 

 of free mobility of this nucleus through the 

 sether. This is definitely hypothetical, but 

 it ia not an unreasonable postulate because 

 a rotational sether has the properties of a 

 perfect fluid medium except where differen- 

 tially 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 iner- 

 tia. It thus seems possible to hold that 

 such a model forms an allowable represen- 

 tation of the dynamical activity of the 

 sether, 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 Maxwell's 

 hands toward the effective 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 con 

 tent has been explored and estimated, and 

 has become familiar through the introduc- 

 tion of new terms and ideas, then the lad- 

 der by which we have ascended may be 

 kicked awaj'', and the scheme of relations 

 which the model embodied can stand forth 

 in severely 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 essen- 

 tials of our working models while eliminat- 

 ing 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 at- 

 tained through the concrete ; and for pur- 

 poses 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 

 different 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 

 coloring of a physical illustration, or in the 

 tentiity and paleness of a symbolical ex- 

 pression.' The other side of the picture, 

 the necessary incompleteness of even our 

 legitimate images and modes of representa- 

 tion, comes out in the despairing opinion of 

 Young ('Chromatics,' 1817), at a time 

 when his faith in the undulatory theory of 

 light had been eclipsed by Malus's dis- 

 covery of the phenomena of polarization 

 by reflection, that this difSculty ' will 

 probably long remain, to mortify the van- 



