in the Seventeenth Century. 23 



culierement dans Vetrange refraction du cristal d'Islande. Par 

 C.ff.D.Z* 



The truth of Hooke's hypothesis, that light is essentially a 

 form of motion, seemed to Huygens to be proved ]}y the effects 

 observed with burning-glasses ; for in the combustion induced at 

 the focus of the glass, the molecules of bodies are dissociated ; 

 which, as he remarked, must be taken as a certain sign of motion, 

 if, in conformity to the Cartesian philosophy, we seek the cause 

 of all natural phenomena in purely mechanical actions. 



The question then arises as to whether the motion is that 

 of a medium, as is supposed in Hooke's theory, or whether it 

 may be compared rather to that of a flight of arrows, as in the 

 corpuscular theory. Huygens decided that the former alter- 

 native is the only tenable one, since beams of light proceeding 

 in directions inclined to each other do not interfere with each 

 other in any way. 



Moreover, it had previously been shown by Torricelli that 

 light is transmitted as readily through a vacuum as through 

 air ; and from this Huygens inferred that the medium or aether 

 in which the propagation takes place must penetrate all matter, 

 and be present even in all so-called vacua. 



The process of wave-propagation he discussed by aid of a 

 principle which was nowf introduced for the first time, and has 

 since been generally known by his name. It may be stated 

 thus : Consider a wave-front,* or locus of disturbance, as it 

 exists at a definite instant t : then each surface-element of the 

 wave-front may be regarded as the source of a secondary wave, 

 which in a homogeneous isotropic medium will be propagated 

 outwards from the surface-element in the form of a sphere 

 whose radius at any subsequent instant t is proportional to 

 (t-t ) ; and the wave-front which represents the whole distur- 



* i.e. Cbristiaan Huygens de Zuylichem. The custom of indicating names by 

 initials was not unusual in that age. 



t Traite de la lum., p. 17. 



I It maybe remarked that Huygens' " waves " are really what modern writers, 

 following Hooke, call " pulses "; Huygens never considered true wave-trains 

 having the property of periodicity. 



