100 DIFFRACTION. 



shadow, at QR, the paper is not uniformly illuminated by the 

 diverging beam, but is observed to be covered with a series 

 of alternate bright and dark bands, which are parallel to the 

 edge of the shadow. The distances of these fringes inter se, 

 and from the edge of the shadow, vary with the position of 

 the screen, and diminish indefinitely as the screen is brought 

 near the obstacle. These fringes succeed one another for 

 many alternations, becoming, however, less marked as the 

 distance from the edge of the geometric shadow increases, 

 until at length they are wholly obliterated and lost. They 

 preserve the same distances from the shadow in all parts, 

 except only where the edge of the body forms a sharp 

 angle.* 



IY. The dimensions of the fringes vary with the colour 

 of the light ; being broadest in red light, narrowest in violet 

 light, and of intermediate magnitude in the light of mean re- 

 frangibility. Hence, when white or compound light is em- 

 ployed, the fringes of different colours will not be accurately 

 superposed ; and there will be formed a succession of iris-co- 

 loured fringes, the colours following the order which they 

 have in the spectrum. 



(115) If we follow the course of the fringes from their 

 origin, we shall observe that they are propagated in lines sen- 

 sibly curved, whose concave side is turned towards the sha- 

 dow. In order to obtain accurate measures of the distances 

 of the fringes from the edge of the shadow, at different dis- 

 tances from the obstacle, Fresnel viewed them directly with 

 an eye-piece, furnished with a micrometer. He thus ascer- 

 tained that the curved path of each fringe was an hyperbola, 



* If this angle be salient, the fringes, instead of forming a similar angle, 

 are observed to curve round the shadow. "When the angle is re-entrant, they 

 cross, and enter on the shadow at each side, without interfering with one 

 another. 



