WAVE THEORY. 261 
its shadow produce, by their reciprocal action, stripes 
analogous to the former, but differently distributed. 
I perceive that, without intending it, in following the 
theoretical speculations of Fresnel, I have mentioned the 
principal features of those curious phenomena of diffrac- 
tion, which I have before cited under another point of 
view, to which Newton devoted one entire book of his 
Optics. Newton believed that he could not give any ex- 
planation of these phenomena (so difficult did they seem 
to him), except by admitting that a ray of light cannot 
pass close to a body without there undergoing a sinuous 
movement like that of an eel. In the explanations of 
Fresnel this strange supposition is superfluous. 
The opaque body which seems to be the original cause 
of the diffracted bands does not act at all on the rays, 
either by attraction or by repulsion ; it simply intercepts 
a part of the principal wave. It stops in the ratio of 
their breadth a great number of oblique rays, which, but 
for this interruption, would have gone into certain parts 
of space to mix with other rays, and to interfere more or 
less with them. 
Thus it is no longer surprising that, as observation has 
proved, the resulting effect is independent of the nature 
and mass of the body. ‘The periods of maximum and 
minimum of the light, as well without as within the 
shadow, are directly deducible from the theory of Fresnel 
with a degree of precision of which hitherto, perhaps, no 
branch of physical science had afforded so striking an 
example. ‘Thus, whatever reserve it may be prudent to 
impose on ourselves when we run the risk of speaking 
of the labours of our successors, I would almost venture 
to affirm that, with regard to diffraction, they will add 
nothing essential to the discoveries with which Fresnel 
