214 FRESNEL. 



destroy each other, that darkness may result from the 

 superposition of two portions of light. But when this 



Since, if we take the partial differentials in respect to t and to x, 

 ==n cos (nt kx) -j- Jc cos (nt lex) 



d?u d^n 



dfi = dx* = 



Whence, d?u n^ d?u 



dft ~ ~ WdHfr 



And since that wave-function goes through all its changes while t, 

 increases to and the velocity v = -|- the time of the undulation 



_ 2?T _ A _ A 



&quot;&quot; ~n ~ r ~~ 27T 



27TU 27T 



Whence, n = and k = - 



A A 



Or the fon-nula becomes (adopting an arbitrary coefficient, a, for 

 the amplitude of vibration which is wholly independent of the other 

 quantities) 



27T 



u = a sin -r- (i i %) 



A 



Here it is to be observed, all depends on the coefficient-^ being 



constant. To obtain a similar equation with a variable, velocity or 

 refraction is the object of the researches of M. Cauchy. 



The more extended views of M. Cauchy have led to the deduction 

 of analogous, but more complex, equations, exhibiting resulting ex 

 pressions for the displacement, in three rectangular directions; besides 

 including in the analysis a coefficient which expresses the variable 

 relation of the velocity which gives the theoretical explanation of un 

 equal refrangibility. These forms thus include the deduction of 

 transverse vibrations, as a direct consequence of the first assump 

 tions, as to the constitution of an sethereal medium. But, with refer 

 ence to light, considered as homogeneous, the conditions admit of 

 great simplification; which is best shown in that form of the investi 

 gation which was pursued by Sir J. Lubbock (Philo*. Mag. Nov. 

 1837), where, if the fourth powers of the disturbed distances of the 

 molecules are neglected, the equations are at once reduced to the 

 form above. 



The object of M. Cauchy s researches here alluded to was to ex- 



