IV] AND STRUCTURE OF THE CELL 319 



and Fa. The body if it be placed in the medium within either 

 branch of the inner curve, or outside the outer curve, will tend to 

 move into the neighbourhood of the adjacent pole. If it be placed 



Fb 



Fig. 97. 



Fig. 98. 



in the region intermediate to the two dotted curves, it will tend to 

 move towards regions of weaker field-strength. 



The locus Fly is therefore a locus of stable position, towards which 

 the body tends to move ; the locus F^ is a locus of unstable position, 

 from which it tends to move. If the body were placed across F^^, 



