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 



