56 KEPORT ON PHYSICAL OPTICS. 



of the fringes, or the alternations of light and darkness, were 

 shown to be cases of the more general principle of interference ; 

 and the connexion is now admitted by some of the warmest ad- 

 vocates of the Newtonian theory.* The bending of the light into 

 the shadow, or the fact of inflexion itself, was at first ascribed by 

 Young to the refraction of an ethereal atmosphere encompassing 

 bodies, and decreasing in density with the distance. He afterwards, 

 however, adopted the simpler doctrine of Huygens and Grimaldi, 

 and referred the phenomenon to the fundamental property of 

 waves. 



But perhaps the most important of the labours of Young on 

 this subject is that in which he descends into numerical details, and, 

 taking the observations of Newton, as well as his own, calculates 

 the differences of the lengths of the paths traversed by the two 

 pencils, when they destroy or reinforce one another by interference. 

 These intervals he found to constitute an arithmetical progression 

 for the successive bands, the first term of which was the same 

 in the same species of light, whatever be the distance at which the 

 fringes are received, or the other conditions of the experiment. 

 And, finally, comparing these constants with the similar intervals 

 of the two pencils reflected by the surfaces of a thin plate, as 

 deduced from the experiments of Newton, he found that their 

 difference was within the limits of error to which such observa- 

 tions are liable, and that we are warranted in concluding that 

 the two classes of phenomena are to be referred to one simple prin- 

 ciple.! It is true that, in these calculations, Young starts from an 

 erroneous principle respecting the lights which form the diffracted 

 fringes by their interference, and he has remarked some discord- 

 ances in his results which have, no doubt, their origin in that 

 circumstance ; but the results of the exact theory are not greatly 

 different from that which he adopted, and the more complete 

 analysis of Fresnel has only tended to confirm the conclusion ob- 

 tained by Young. 



The important experiment of Young, on the disappearance of 



the fringes in the shadow of a narrow opaque body, when the light 



ffifflg by one of its edges was intercepted, was that which first 



d him to the principle of interference. An instructive variation 



' Biot, Precit elementaire, vol. ii. p. 472, 3 Edit. 



'Experiments and Calculations relative to Physical Optics." Phil. Trans. 



