Astronomical and Nautical Collections. 441 



iv. Elementary View o/M^Undulatory Theory 0/ Light. 

 By Mr. Fresnel. 



[Continued from the last Number.] 



It is easy to convince ourselves here, as well as in 

 Dr. Young's experiments, that the fringes depend on the 

 mutual actions of the rays which meet each other ; for if we 

 intercept, by means of a screen placed near' to one of the mir- 

 rors, all the rays that it reflects to the place of observation, 

 either before or after their reflection, the fringes wholly dis- 

 appear, although the space continues to be enlightened by 

 the other mirror, and we can only see some paler and unequal 

 fringes which form a border to tlie shadow of the screen. If 

 we cover with the screen only one half of the mirror, so as 

 to annihilate the fringes for one half only of their entire 

 length, we may the more easily compare the remaining part 

 of the dark stripes with the neighbouring space which is 

 enlightened by one of the mirrors only, and we may thus 

 readily be assured that the neighbouring parts are much 

 brighter than the middle of the dark stripes, on which, how- 

 ever, the rays from both the mirrors fall. These rays must 

 therefore be neutralised in consequence of a certain action 

 which they exert on each other. 



This mutual influence of the rays of light, established by 

 the experiments which have been enumerated, is still more 

 fully confirmed by a great number of optical phenomena ; so 

 that it may now be considered as one of the best demon- 

 strated of physical truths. After having thus satisfied our- 

 selves of its direct foundation,' we may proceed to its con- 

 firmation by the most important collateral observations. But 

 we must first examine the circumstances under which the law 

 takes efiect. 



If we calculate the differences of the routes described by 

 the rays which concur in the production of each of the dark 

 and bright stripes, we first find that the bright stripe in the 

 centre corresponds to an equality in the length of the routes : 

 and that if we express by d the differences of the routes de- 

 scribed by the rays of the pencils which unite in the middle 



APRIL— JUNE, 1827. 2 G 



