in the Seventeenth Century. 25 



front can be found. It is obvious that the envelope of these 

 secondary waves, which constitutes the final wave-front, will be 

 a plane BN, which will be inclined to AB at the same angle as 

 AC. This gives the law of reflexion. 



The law of refraction is established by similar reasoning, 

 on the supposition that the velocity of light depends on the 

 medium in which it is propagated. Since a ray which passes 

 from air to glass is bent inwards towards the normal, it may be 

 inferred that light travels more slowly in glass than in air. 



Huygens offered a physical explanation of the variation in 

 velocity of light from one medium to another, by supposing 

 that transparent bodies consist of hard particles which interact 

 with the aethereal matter, modifying its elasticity. The 

 opacity of metals he explained by an extension of the same 

 idea, supposing that some of the particles of metals are hard 

 (these account for reflexion) and the rest soft : the latter destroy 

 the luminous motion by damping it. 



The second half of the Theorie de la lumiere is concerned with 

 a phenomenon which had been discovered a few years pre- 

 viously by a Danish philosopher, Erasmus Bartholin (b. 1625, 

 d. 1698). A sailor had brought from Iceland to Copenhagen a 

 number of beautiful crystals which he had collected in the Bay 

 of Eoerford. Bartholin, into whose hands they passed, noticed* 

 that any small object viewed through one of these crystals 

 appeared double, and found the immediate cause of this in the 

 fact that a ray of light entering the crystal gave rise in general 

 to two refracted rays. One of these rays was subject to the 

 ordinary law of refraction, while the other, which was called 

 the extraordinary ray, obeyed a different law, which Bartholin 

 did not succeed in determining. 



The matter had arrived at this stage when it was taken up 

 by Huygens. Since in his conception each ray of light corresponds 

 to the propagation of a wave-front, the two rays in Iceland 

 spar must correspond to two different wave-fronts propagated 



* Ejcperimenta cristatti Islandici disdiaclastici : 1669. 



