324 ON LIGHT. 



at a certain distance a medial dark line makes its appear- 

 ance which, if the distance be still further increased, 

 changes to a bright one, and so on alternately till after a 

 certain distance is attained the alternations cease. If 

 the screen be stopped at the last position in which the 

 medial line is dark, and there fixed, and an opake strip, 

 exactly half the breadth of the slit, be held medially 

 along its whole length, so as to divide the slit and reduce 

 it to two parallel ones, each one quarter of the original 

 breadth (by which, of course, the total light traversing the 

 aperture will be reduced to half its amount), instead of 

 darkening still more the medial dark fringe on the screen, 

 as would naturally be expected on throwing a shadow up- 

 on it, the very reverse happens : the dark fringe in ques- 

 tion disappears altogether, and is replaced by a bright one. 

 (107.) If the shape of the body which casts the shadow 

 \>e angular, having salient and re-entering angles; the 

 fringes where they surround the re-entering angles cross 

 and pursue their courses up to the shadows of the sides 

 respectively opposite to them ; but those which surround 

 salient ones curve round them, preserving their continuity. 

 At an angle of the latter kind too, crested or plume- 

 shaped interior fringes are seen " Grimaldi's crested 

 fringes," as they are called, from the name of Father 

 Grimaldi of Bologna, who first described (in 1665) these 

 curious appearances. If a re-entering angle, however, be 

 very acute, the external fringes which border its sides on 

 approaching the angular point curve outwards, cross one 

 mother, and run out both ways into the shadow in ele- 

 gant curves of a hyperbolic form. Nothing can be ima- 



