IV] 



STRUCTURE OF THE CELL 



183 



medium, and let the curved line in Fig. 55 represent generally 

 its permeability at other field-strengths; and let the outer and 

 inner dotted curves in Fig. 54 represent respectively the loci of 

 the field-strengths F^ and F^- 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 in the region intermediate to the 

 two dotted curves, it will tend to move towards regions of weaker 

 field-strength. 



The locus 2^6 is therefore a locus of stable position, towards 

 which the body tends to move ; the locus Fg, is a locus of unstable 

 position, from which it tends to move. If the body were placed 



f^ 



Fig. 55. 



across F^, it might be torn asunder into two portions, the split 

 coinciding with the locus F^. 



Suppose a number of such bodies to be scattered throughout 

 the medium. Let at first the regions F^ and F^ be entirely outside 

 the space where the bodies are situated: and, in making this 

 supposition we may, if we please, suppose that the loci which we 

 are calhng F^ and F^ are meanwhile situated somewhat farther 

 from the axis than in our figure, that (for instance) F^ is situated 

 where we have drawn Fj,, and that Fj, is still further out. The 

 bodies then tend towards the poles ; but the tendency may be 

 very small if, in Fig. 55, the curve and its intersecting straight fine 

 do not diverge very far from one another beyond Fa', in other 



