Interference Phenomena. 511 



geneous." Here follows the well-known argument as to the 

 effects of want of homogeneousness in the interfering rays. 



Foucault and Fizeau * more definitely put it as a question 

 for experimental determination, whether interference pheno- 

 mena are ultimately stopped by want of homogeneousness or 

 want of " regularity ," Experimenting with white light, they 

 came to the conclusion that their experiments " reveal in the 

 emission of successive waves a persistent regularity which no 

 phenomenon has hitherto suggested. - ''' 



The opinion put forward by Fizeau and Foucault that 

 observations on interference of light with large difference of 

 path could teach us something as to the " regularity " of the 

 vibrations of a particle was generally accepted, until more 

 closely examined by Gi-ouy and Rayleigh in the papers already 

 alluded to. Their conclusions did not, however, receive the 

 attention they deserve. Thus, in the last part of the Encyclo- 

 pcedia der Naturwissenschaften, it is stated that " if natural 

 light really consists in a succession of different states of 

 vibration, these take place so slowly, that in a time which is 

 greater than 50,000 period, light must still be considered as 

 essentially of the same type of vibration."" 



3. Compare with this the following passage taken out of 

 Gouy's paper : — 



" Ainsi l'existence de franges d'interferences a grande dif- 

 ference de marche, dans le cas des sources de spectres continus 

 et de la lumiere blanche n'implique nullement la regularite 

 du mouvement lumineux incident. Cette regularite existe 

 dans le spectre, mais e'est l'appareil spectral qui la produit, 

 en separant plus ou moins completement les divers mouve- 

 ments simples, qui jusque la n'avaient qu'une existence 

 purement analytique." 



Lord Rayleigh, in his article on the " Wave Theory/ - ' ex- 

 presses the same idea as follows: — 



" Or, following Foucault and Fizeau, we may allow the 

 white light to pass, and subsequently analyse the mixture 

 transmitted by a narrow slit in the screen upon which the 

 interference bands are thrown. In the latter case we observe 

 a channelled spectrum, with maxima of brightness correspond- 

 ing to the wave-lengths bu/(nD). In either case the number 

 of bands observable is limited solely by the resolving power 

 of the spectroscope, and proves nothing with respect to the 

 regularity or otherwise of the vibrations of the original 

 light." 



Put shortly, the argument in favour of the above view is 

 this : — Any disturbance, however irregular, may by Fourier's 



* Academie des Sciences, 24th Nov. 1845 and 9th March 1846. 



2N2 



