94 THE WAVE THEORY OF LIGHT. 



is peculiarly instructive is the disproportion between the primitive 

 plienomenon and the greatness of the effects which industry has drawn 

 from it. Thus, those formidable engines, based on electricity or steam, 

 grew neither from lightning nor the volcano; they had their birth from 

 scarcely perceptible phenomena which would have remained forever 

 hidden from the vulgar eye, but that peneti'ating observers were able 

 to recognize and appreciate. This humble origin of most of the great 

 discoveries which are to-day a benefit to the whole human race, shows 

 us plainly that the scientific spirit is at present the mainspring of the 

 life of nations, and that it is in the onward march of pure science that 

 we are to look for the secret of the growing power of the modern 

 world. Whence a series of questions which demand more and more 

 the attention of all. How did this taste toward the study of natural 

 philosophy, so dear to the ancient philosophers, abandoned for cen- 

 turies, again revive and grow? What are the phases of its advance? 

 How appeared the new notions Avhich have so deeply modified our 

 ideas on the mechanism of nature's forces? What paths, rich in dis- 

 coveries, lead us gradually unawares to those admirable generalizations 

 in accordance with the vast plan forseen by the foundcM-s of modern 

 physics? These are the questions which as a physicist I intend to 

 inquire into bcfoi-c you. The subject is rather abstract, 1 might say 

 severe. But no other has seemed more worthy of your attention 

 during the fete which the University of Cambridge celebrates to-day 

 in honor of the Lucusian Professorship Jubilee of Sir Geo. Gabriel 

 Stokes, who in his fine career has laid a master hand on the very pro}»- 

 lems which seemed to me the most conducive to the ])r()gress of natural 

 philosophy. Th(> subject is all the more fitted here, as in citing the 

 names of those great minds to whom modcM'n science is most indebted, 

 we found amongst those who most honored the Universitj" of Cam- 

 bridge — ^its professors and fellows — Sir Isaac Newton, Thomas Young, 

 George Green, Sir George Airy. Lord Kelvin, Clerk Maxwell, Lord 

 Rayleigh, and the memory of that glory which links to-day back 

 through the centuries would add luster to the present ceremony. 



Let us then, in a rapid glance of the scientific revival, point out the 

 secret but mighty influence which has been the directing force of mod- 

 ern physics. I am inclined to attribute to the study of light, and to the 

 attraction it has for the highest minds, one of the most effective causes 

 of the return of ideas toward natural philosophy, and consider optics 

 as having exercised on the advance of science an influence it would be 

 diflBcult to exaggerate. This influence, already clear at the dawn of 

 the experimental philosophy under Galileo, grew so rapidly that 

 to-day it is easy to foresee a vast synthesis of natural forces founded 

 on the principles of the wave theory of light. This influence is easy 

 to understand if we reflect that light is the way by which knowledge 

 of the exterior world reaches our intelligence. It is, in fact, to sight 



