Energy in the Electromagnetic Field, 23 



if 86 be the difference in the ^-coordinates between two 

 successive points of intersection. Also if a be the arc 



Fi°\ 4. 



Dotted lines represent the reflected rays, the heary lines the loci of 

 minimum illumination, and the thin lines the mean lines of flow of 

 energy. 



measured along the curve r# = 0, we have from equation 

 (20 a) 



