102 DIFFRACTION. 



or fringes, analogous to those produced by the single edge 

 in the former experiment. These are the exterior fringes. 

 But we observe further that the whole space of the geometric 

 shadow itself is also occupied by parallel stripes, alternately 

 bright and dark. These are the interior fringes ; and they 

 are in general closer, and more finely marked than the ex- 

 terior. When the breadth of the obstacle is considerable, 

 the interior fringes disappear, and the phenomena fall under 

 the class already examined. 



The interior fringes are propagated, like the exterior, in 

 hyperbolic curves ; but their curvature is less considerable, 

 and the deviation from a right-lined course is scarcely per- 

 ceptible within the limits at which they are commonly ob- 

 served. They are also, like the exterior fringes, broader in 

 red than in violet light, and of intermediate breadths in the 

 light of intermediate refrangibility. Accordingly, in com- 

 pound or white light, the fringes of different dimensions are 

 superposed ; and the bands are no longer alternately bright 

 and black, but coloured with different tints, in the order of 

 the colours of the spectrum. 



(118) It still remains to examine the effects produced by 

 two edges turned inwards, so as to form an aperture of any 

 dimensions. 



For this purpose Fresnel employed an instrument consist- 

 ing of two metallic plates, one of which is fixed in the frame of 

 the apparatus, while the other is moveable by means of a fine 

 screw. The edges of these plates are right-lined and parallel, 

 so that they form always $> rectangular aperture ; and, by 

 means of the adjusting screw, the magnitude of this aperture 

 may be varied at pleasure. 



"When a narrow rectangular aperture, thus formed, is sub- 

 stituted for the wire in the last experiment, the resulting phe- 

 nomena are very remarkable. In the first place, the luminous 



