24 The Theory of the Aether 



bance at the instant t is simply the envelope of the secondary 

 waves which arise from the various surface elements of the 

 original wave-front.* The introduction of this principle enabled 

 Huygens to succeed where Hooke and other contemporary 

 wave-theoristsf had failed, in achieving the explanation of 

 refraction and reflexion. His method was to combine his own 

 principle with Hooke's device of following separately the fortunes 

 of the right-hand and left-hand sides of a wave-front when it 

 reaches the interface between two media. The actual explana- 

 tion for the case of reflexion is as follows : 



Let AB represent the interface at which reflexion takes 

 place, AHC the incident wave-front at an instant , GMB the 

 position which the wave-front would occupy at a later instant t 

 if the propagation were not interrupted by reflexion. Then by 



"G 



Huygens' principle the secondary wave from A is at the instant 

 t a sphere ENS of radius equal to AG : the disturbance from H t 

 after meeting the interface at K, will generate a secondary 

 wave TV oi radius equal to KM, and similarly the secondary 

 wave corresponding to any other element of the original wave- 



* The justification for this was given long afterwards by Fresnel, Annales de 

 chimie, xxi. 



t e.g. Ignace Gaston Pardies and Pierre Ango, the latter of whom published 

 a work on Optics at Paris'in 1682. 



