CHRISTIAN HUYGENS 8i 



balls must experience a certain retardation, in exact corres- 

 pondence with the finite velocity of light. In sources of 

 light, for example a candle flame, he assumes that each small 

 luminous particle produces waves independently, but that 

 all the single waves unite to form larger wave fronts, which 

 then combine and move forward as the beam of light -an 

 idea entirely in accordance with our present day view. 



Concerning the propagation of the wave-front, Huygens 

 then states a law, which has hitherto always been found to 

 hold for wave propagation, and is described as 'Huygens' 

 principle.' According to this, every part of the medium 

 of propagation which is disturbed by a wave, acts for its 

 part as a new centre of propagation for its own neighbour- 

 hood, whereby the elementary waves proceeding from a single 

 point of the medium, again join together to form new wave 

 fronts along their common surface of contact. By means of 

 this principle, Huygens was able to explain completely the 

 reflection and refraction of light, and the known laws re- 

 lating to these. In the case of refraction, we must assume 

 that the velocity of light in the refracting medium, for 

 example, glass or water, is smaller than in free ether, by an 

 amount proportional to the index of refraction. This was 

 shown by Fresnel a hundred and forty years later to be true, 

 and forty years later still, a further and more direct demon- 

 stration was given by Foucault. 



Huygens himself carried out another and special test of 

 these ideas, by applying them to the highly complicated 

 phenomenon of double refraction, which was known in the 

 case of Iceland Spar, but had remained completely incom- 

 prehensible. In the case of one of the two rays which result 

 from a single original ray upon refraction in the Spar, 

 the one called the 'extraordinary ray,' no law of refraction 

 exists; according to the circumstances it takes very various 

 directions, by no means to be expected from Snell's law 

 of refraction, which holds for the other, or 'ordinary,' ray. 



Gs 



