878 



SCIENCE. 



[N. S. Vol. II. No. 52. 



netic forces are either constant or slowly 

 varyiny is practically the same in the two 

 theories. The radical difference becomes ap- 

 parent when these variations are rapid, for it 

 is then only that the currents in the dielec- 

 tric both the electric and the magnetic show 

 their real power. Hertz was the first to 

 show us how to produce these rapid changes 

 by the disruptive discharge of a Leyden 

 jar. 



Maxwell's Electro-magnetic Theory of 

 Light can now be easily stated. Formally 

 it is the same as the Dj'namic Theory. 

 For this one starts from the hypothesis 

 that light is a vibratory motion of a sub- 

 stance which is a particular form of mat- 

 ter of very small density and very high 

 rigidity. The fundamental laws of the Dy- 

 namic Theory of Light are, therefore, N"ew- 

 ton's axioms, particularly the second law 

 of motion, and the law of elastic displace- 

 ment. Now these two laws bear a perfect 

 formal resemblance to the law of variation 

 of flux and the law of flux respectively; it 

 follows, therefore, that since these two theo- 

 ries start from the same formal laws they 

 ■will, formallj', account equally well for all 

 the simpler phenomena of light. 



It would lead us much beyond the already 

 extensive limits of this discussion to dwell 

 even briefly xipon the superiority of the 

 electro-magnetic theory over the other theo- 

 ries. I shall mention a few only of the 

 most striking features of this comparison. 

 First, it makes no hypothesis as to the 

 material constitution of ether; the Dy- 

 namic Theory does this and fails to recon- 

 • cile some of its hypotheses, as, for in- 

 stance, the very high rigiditj^, with well- 

 known physical facts. The only hypothesis 

 which the electro-magnetic theory makes is 

 that its two fundamental laws apply to 

 ether as well as to any other dielectric. In 

 fact, it defines the fundamental physical 

 l^roperties of ether bj' these two laws just 

 as Mechanics defines the fundamental phys- 



ical properties of matter by Newton's 

 axioms and the law of elastic deforma- 

 tion. It is in this sense that Maxwell's 

 electro-magnetic theory may be called the 

 D^mamics of Ether and treated distinctly 

 from Dynamics of ponderable matter. Sec- 

 ond, the hypotheses of the electro-magnetic 

 theory admit of a direct experimental test, 

 those of the Dynamic Theorj^ do not. The 

 Hertzian experiments furnished this test 

 for the electro-magnetic theory and verified 

 its hypotheses. Third, the beautiful pic- 

 ture of the phenomena of dispersion and ab- 

 sorption of light which Helmholtz gives us 

 in his extension of Maxwell's theory forms 

 by reason of its elegant simplicity a strik- 

 ing contrast to the mechanical model of 

 these phenomena which he gave us some 

 twenty years ago. Consider as Helmholtz 

 does the electro-magnetic forces that must 

 be acting between the luminous wave and 

 the definite electric charges which Faraday 

 detected long ago in every valency of the 

 atoms of ponderable matter, and the cloud 

 of uncertainty and of ignorance which for a 

 long time seemed to hang over the region 

 of these most interesting phenomena of 

 light clears away and leaves us rejoicing in 

 the possession of new knowledge, more 

 beautiful than anything that we have ever 

 known before. 



" One cannot study this wonderful the- 

 ory," says Hertz, "referring to Maxwell's 

 Electro-magnetic Theory of Light," with 

 out feeling from time to time that there 

 resides in its mathematical fomulse an in- 

 dependent life and an individual intel- 

 ligence; that they are wiser than we are, 

 wiser than their discoverer; that they 

 give us more than was formerlj' put into 

 them." 



Boltzmann expresses the same sentiment 

 as Hertz by placing the following verse 

 from Goethe's Faust as the motto of the 

 second volume of his lectures on Maxwell's 

 electro-magnetic theory : 



