156 PROGRESS OF SCIENCE IN THE CENTURY. 



perience of throwing a stone or shooting an arrow 

 into " empty space." The undulatory theory is 

 subtler, space is j&lled with a continuous elastic 

 medium, in which particles — no longer projectiles — 

 were supposed to oscillate in the direction of propa- 

 gation, like the particles of water in the ripples on 

 a pond. But this conception was insufficient and 

 gave place to FresneFs idea of waves of transverse 

 vibrations excited in an incompressible continuous 

 medium. 



Electro-magnetic Theory of Light. — The necessity 

 of admitting the existence of this medium was made 

 clearer by Faraday, and corroborated by his dis- 

 covery of induction, and Clerk Maxwell in his foot- 

 steps ventured to forecast, on theoretical grounds, 

 that light and electro-magnetic radiation are alike 

 due to rhythmical disturbances in the ether, differ- 

 ing only in their wave-lengths — one of the most uni- 

 fying ideas in modern science. 



Experiments of Hertz. — " But the abstract the- 

 ories of natural phenomena are nothing without the 

 control of experiment. The theory of Maxwell was 

 submitted to proof, and the success surpassed all 

 expectation. ... A young German physicist, 

 Heinrich Hertz, prematurely lost to science, starting 

 from the beautiful analysis of oscillatory discharges 

 by Von Helmholtz and Lord Kelvin, so perfectly 

 produced electric and electro-magnetic waves, that 

 these waves possess all the properties of luminous 

 waves; the only distinguishing peculiarity is that 

 their vibrations are less rapid than those of light. 

 It follows that one can reproduce with electric dis- 

 charges the most delicate experiments of modern 

 optics — reflection, refraction, diffraction, rectilinear, 

 circular, elliptic polarisation, etc." * 



* Cornu. Rede Lecture. Loc. cit., p. 296. 



