J = 



2(Po -^ Pl>^ 



[J cos^dv)^+/y sin ^ dvj (15.1) 



where A is a constant, and 



P T 1 1/2 

 V' = X' coscp Cx ^p * p ^^ ^ (15.2) 



V" = X" C0S9 [^ ^o * D ^^ ^ ^^^'^^ 



/A Pq Pi 



where X is the wave length of light and 



E = 1 - ^ (15.4) 



The significance of the symbols p^, p^, a, d and 6 is evident 

 from figure 25, and q) is the angle that p, makes with the z-axis. 



Note that the equations (15.1), (15.2) and (15.3) are the 

 same as those used by Drude (1922) exoept for the factor E in 

 equations (15.2) and (15,3). As d approaches zero, the factor E 

 approaches 1. 



Heirtzler obtained a geometrical optics solution for the 

 intensity of illumination along a plane of observation (15.2) and 

 made several graphs of J( intensity of illumination) against 

 D(the distance from the observer to the point on the normal to 

 the plane of observation through the source). 



When d = 0, that is the straight edges lie in the same 

 plane, it is seen from Heitzler's graph (fig. 26) that the theo- 

 retical diffraction pattern is symmetrical. This agrees with 

 several photographs which have just been shown (figs. 22, 23, 24), 

 As d is made larger, it is seen that the theoretical diffraction 

 curves become more and more dissymmetrical (figs. 27, 28). By 



56 



