

DIFFRACTION. 113 



of the two portions of the wave which pass by the edges on 

 either side. Let PP' be the section of the opaque body, PC 



and PV the two portions of the diverging wave which has 

 just reached its edges, and E any point of the shadow. Then 

 if these portions be divided in the points #, 5, c, &c.,a', b\ c', &c., 

 in such a manner, that the difference of the distances of any 

 two consecutive points from the point E is equal to half 

 an undulation, the elementary wave sent from each portion 

 will be in complete discordance with those sent from the two 

 adjacent ; so that, if the several portions be equal, they 

 will neutralize one another's effects at the point E, with 

 the exception of the extreme portions, P#, PV, the halves of 

 which next the edges remain uncompensated. 



Now the arcs P#, ab, be, &c., are very nearly equal, when 

 the lines drawn from their extremities to the point E are 

 sufficiently inclined to the normal, or, in other words, when 

 this point is sufficiently removed from the edge of the geo- 

 metric shadow. In this case, then, the only efficacious parts 

 of the wave are the halves of the extreme portions, P# and 

 P'o/ ; and the intensity of the light at the point E will be de- 

 termined by the difference of their distances from that point, 

 or (which comes to the same thing) by the difference of the 

 lengths of the lines connecting it with the edges of the ob- 

 stacle. The phenomena of interference are therefore the same 

 as in the case of light emanating from two near origins, already 

 considered ; and we may transfer to the present case the con- 



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