Astronomical and Nautical Collections. 



435 



consideration, as well as from other reasoning, that the uni- 

 formity will be preserved throughout the progress of the 

 undulation, unless any part of it be intercepted or retarded; 

 because the result of the elementary motions, which have been 

 mentioned, will be the same for all the points. But if a por- 

 tion of the undulation be intercepted by the interposition of 

 an opaque body, then the intensity of each part will vary 

 according to the distance from the margin of the shadow, and 

 these variations will be particularly sensible in the neigh- 

 bourhood of the tangent rays. 



Let C be the luminous point, AG the screen, and A ME 

 the wave, arrived at A, ami partly intercepted by the opaque 

 body. We may suppose it to be divided into an infinite 

 number of small arcs, Am', mm, mM, Mtz", nn, nn^ and 

 so forth. In order to find its intensity at the point P, be- 

 longing to any subsequent situation of the undulation, BPD, 

 we must find the result of all the elementary agitations 

 which each of these portions of the primitive undulation 

 Would produce there if they acted separately. 



The impulse, which has been given to every part of the 



