258 FRESNEL. 



testably one of the most important of his discoveries, 

 has shown how and under what circumstances this diver- 

 unanswerable, that on this principle there ought to be no darkness ; 

 light ought to spread equally into the shadow, and we ought to see 

 round a corner. 



It was the fertile principle of interference which was to supply the 

 answer, as indeed had been long before hinted generally by Huy- 

 ghens. The waves diverging from the different parts of a luminous 

 source of any sensible magnitude interfere with and neutralize each 

 other, except in the main direction, when alone they exactly concur; 

 a principle called " the mutual destruction of secondary waves." 

 Young dwelt much at first on this objection; and afterwards, in a 

 letter to Arago, he renews a similar expression of the difficulties he 

 felt in another point of view: " If light has so great a tendency to 

 diverge into the path of neighbouring rays, and to interfere with 

 them, as Huyghens supposed, I do not see how it escapes being to- 

 tally extinguished in a very short space, even in the most transparent 

 medium." Peacock's Life, p. 140. But the principle just adverted 

 to shows that the middle portion of the light coming from a point of 

 any physical magnitude is not subject to those mutual interferences, 

 and does not diverge, but is perpetually reinforced by the supply of 

 fresh waves incessantly propagated from the original source. In 

 these explanations Young at length expressed his full concurrence in 

 a letter to Fresnel. The actual divergence of light into a shadow is 

 demonstrated by the existence of the internal stripes. This, however, 

 is an effect only produced to a very limited extent; and the general 

 law of the " mutual destruction of secondary waves " in ordinary 

 cases applies to produce the effect of destroying all apparent lateral 

 divergence. There are, however, some cases where this cause operates 

 less extensively (such, at least, would seem to be the case, and is the 

 view upheld by some mathematicians); at all events, under certain 

 conditions, the divergence is rendered very much more conspicuous, 

 and reaches to a far greater distance from the edge. This appears to 

 have been the case in a remarkable experiment, mentioned both by 

 Newton and Hooke, and probably observed by each independently, 

 but described, especially by Newton, in somewhat obscure terms (see 

 Optics, book iii. part i. obs. 5, (Ed. 1721,) but more precisely by Hooke: 

 see Posthumous Works, pp. 186 and 190, and plate 11, fig. 8, p. 155, 

 Ed. 1705). Hooke ascribes it to a "deflexion of light differing both 

 from reflexion and refraction, and seeming to depend on the unequal 

 density of the constituent parts of the ray," &c. Newton enters on 



