366 ON THE DISTRIBUTION OF THE POLARISING FORCE 



attentive consideration. If the polarising forces depend solely 

 on the mechanical condition of the particles of the glass, then 

 it necessarily follows, that the central parts of the glass, which 

 were in a state of variable expansion when it had a circular 

 shape, are in a state of variable compression when the glass has 

 received an elliptical form, and we are presented with a new 

 law relative to the equilibrium of the cohesive forces in solids 

 of variable density. But if the variation of density is merely 

 the means of developing a new agent in the same manner as 

 heat excites electricity in the tourmaline, or as pressure excites 

 it in calcareous spar, then we cannot avoid regarding this agent 

 in the same light as the electrical and magnetical fluid which 

 are decomposed by certain mechanical operations, and distri- 

 bute themselves according to regular laws. 



But whatever be the origin of the polarising forces, it be- 

 comes a matter of great importance to discover the law of their 

 distribution, when they are not controuled by opposite actions r 

 and to apply this law to the explanation of the phenomena 

 which they develope when they are either modified or extin- 

 guished by the external form of the body in which they re- 

 side. 



Those who have studied the papers to which I have already 

 referred, cannot have failed to remark, that when the polari- 

 sing forces are unconstrained in their developement, a negative 

 structure is generated in the middle of the plate, when a posi- 

 tive structure is generated at one or both of its edges ; and that 

 the intensity of the negative, is to the intensity of either of the 

 positive structures, as 10 to 16.02. 



In order to apply this principle to a circular plate, let EHFG, 

 Fig. 12. be a plate of this description, whose centre is O. Then 

 if this plate were a part of the rectangular plate ABDC, ??in y op 

 would be the lines of no-polarisation which separate the internal 



negative 



