Energy in the Electromagnetic Field. 



21 



energy is concentrated in the right half of the tube and 

 " rarefied " in the left half. If we draw the line of flow 

 passing through the point midway between A and B, we 

 see that we can conceive of the tube AB as made up of two 

 tubes each of which widens and contracts periodically, and 

 one of which is so shifted relatively to the other that the 



Fig. 6. 



1 ! 



V V V. v V 



Illustrating the "microscopic " structure of the Field. 



broadened part of one falls by the side of the narrow part 

 of the other. We see also from fig. 6 that the contracted 

 parts of successive tubes lie along the loci of maximum 

 illumination. 



We may now take into account the diffraction at the 

 " edge " of the cylinder 7 and obtain the correction to our 

 results necessary for points below the plane y = 0. Using 

 Sommerfeld's expression as before, the electric intensity at 



