108 DIFFRACTION. 



mauner, supposed to arise from the interference of the two 

 pencils reflected from the opposite edges ; while the interior 

 fringes, within the shadows of narrow bodies, were accounted 

 for by the interference of the pencils which passed on either 

 side of the body, and were bent into the shadow. The ob- 

 served facts closely correspond with the calculated results of 

 this theory ; and in the case last mentioned, Young proved 

 that the phenomenon admitted no other explanation. Plac- 

 ing a small opaque screen on either side of the diffracting 

 body, so as to intercept the portion of light which passed 

 by one of its edges, the whole system of bands immediately 

 disappeared, although the light passing by the other edge 

 was unmodified. 



The general laws of the fringes the dependence of their 

 magnitudes upon the length of the wave, and upon the distances 

 of the luminous origin and of the screen are fully explained 

 on these hypotheses. It is easy to infer from them that, as 

 the position of the screen is varied, the successive points of 

 the same fringe are not in a right line, but form an hyper- 

 bola ; and that, when the distance of the luminous origin is 

 lessened, the inclination of these hyperbolic branches (con- 

 sidered as coincident with their asymptots) augments, and 

 the fringes dilate. 



The theory of Young, however, did not bear a closer 

 comparison with facts. If th exterior fringes arose from the 

 interference of the direct light with that obliquely reflected 

 from the edge of the obstacle, it would follow that the inten- 

 sity of the light in them should depend on the extent and 

 curvature of the edge. Fresnel found, on the contrary, that 

 the fringes were wholly independent of the form of the dif- 

 fracting edge ; the fringes formed by the back and by the 

 edge of a razor, for example, being precisely alike in every 

 respect. In the other cases of diffraction also, he perceived 

 that the r#ys grazing the edge of the body were not the 



