107 



bubble halfway between the surface and a horizontal rigid 

 bottom. The field between these two planes in the equivalent 

 electrostatic problem is that of the original charge e and 

 an itffinite set of images, as shown in Fig. 7. The two 

 nearest images give equal and opposite contributions to 

 the potential at the bubble, and thus cancel; however, the 

 next two Images both have charge -e , so give a negative 

 contribution. The potential at the bubble due to the infinite 

 set of images comes out to be - ^ l^S ^ » which differs 



