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. ¥ - a 
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 transpavent 
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 
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