CHAPTEE VII. 



DIFFRACTION. 



(113) IT has been shown to be a result of the wave-theory, 

 that the intensity of the light which encounters an obstacle 

 must diminish rapidly within the edge of the geometric 

 shadow. It now remains to consider the other phenomena 

 which arise under these circumstances ; and it will be found 

 that the same theory affords the most complete account, not 

 only of their general characters, but even of their most minute 

 details. 



In order to understand the theory of shadows, it is neces- 

 sary to investigate their laws in the simple case in which 

 the magnitude of the luminous body is reduced to a point. 

 The effects thus presented were first observed, and in some 

 degree explained, by Grimaldi ; and they have been since 

 studied, as a separate branch of Optical Science, under the 

 title of diffraction or inflexion. 



Grimaldi found, that when a small opaque body was 

 placed in the cone of light, admitted into a dark chamber 

 through a very small aperture, its shadow was much larger 

 than its geometric projection; so that the light suffered 

 some deviation from the rectilinear course in passing by the 

 edge. On observing these shadows more attentively, he 

 found that they were bordered with three iris-coloured 

 fringes, which decreased in breadth and intensity in the 

 order of their distances from the shadow, and which preserved 

 the same distance from the edge throughout its entire extent, 

 unless where the body terminated in a sharp angle. Similar 



