THE GROWTH OF PHYSICAL IDEAS 99 



Another observation of the greatest importance in under- 

 standing the nature of light was the discovery, as a result of 

 observations of the eclipses of the satellites of Jupiter by the 

 body of the planet, that light did not travel with infinite 

 speed. Indeed, in 1676 Olaus Romer calculated from these 

 observations that the velocity of light was about one hun- 

 dred and ninety thousand miles a second, a value little dif- 

 ferent from the value used today. 



The great opponent of Newton's theory of the emission of 

 light as particles was Christiaan Huygens, the Dutch astrono- 

 mer who first made accurate clocks by the use of the pendu- 

 lum and discovered the double refraction of Iceland spar and 

 the refraction of the light of the stars by the atmosphere. 

 Huygens regarded light as being non-material because of its 

 great velocity of propagation and because two rays traversing 

 the same path in contrary directions do not hinder each other. 

 He therefore adopted the theory that light consists of wave 

 motions in a hypothetical medium that is called the ether. 

 The properties of the ether are deduced from the properties 

 of light. Huygens considered each point of a luminous body 

 to be the origin of elementary spherical waves, of which the 

 envelope corresponds at any instant to the position of the 

 wave front. Thus, as the wave front travels forward ^vith 

 the velocity of light, it could always be considered as the 

 envelope of an infinite number of elementary waves. The 

 perpendicular to the wave front corresponds to what is termed 

 a ray. 



Newton's corpuscular theory and Huygens' wave theory 

 are equally adapted to describe the phenomena of reflection 

 and refraction. The literature of the eighteenth century is 

 full of discussion of the two theories, but in 1827 W. B. Ham- 

 ilton proved that they are only different aspects of the same 

 mathematical laws which can be derived from de Fermat's 

 principle. The wave surfaces can be considered as the poten- 

 tial surfaces of the light rays, and the light rays as the normals 

 of the wave surfaces. None of these theories alone, however, 

 can explain the phenomena of diffraction and interference. 



