368 ON THE DISTRIBUTION OF THE POLARISING FORCE 



The distribution of the polarising force at the angles of 

 square or rectangular plates, characterises a very remarkable 

 state of equilibrium among the cohesive forces ; and this state 

 of equilibrium is not an accidental result of the mode in which 

 the heat either enters or leaves the angles of the glass plate, 

 for the very same distribution takes place when a new angle is 

 formed, by cutting the plate into two parts *. In all these 

 cases, the lines of equal tint are the lines of equal densitv. 



As the two kinds of polarising forces seem to be co-existent 

 in every part of a glass plate, and as each of them, when it ap- 

 pears to exist alone, is merely the resultant of two opposite 

 forces, it is easy to assign a reason for the singular phenome- 

 na which are exhibited, by dividing a plate of glass into two 

 parts. The two forces which reside in every part of the glass 

 cannot be in equilibrio, unless a negative structure is placed 

 between two positive structures ; and therefore each half of the 

 2,'lass plate, or each portion of it of the same shape as the whole 

 plate, must acquire the same property as the plate itself, or 

 have the forces distributed in the same manner. This view of 

 the distribution of the polarising forces is analogous to Cou- 

 lomb's theory of the construction of the magnet. Every ele- 

 mentary portion of the magnet has a north and south pole, and 

 therefore wherever it is broken, the fragment must have a 

 north and south pole, like the magnet of which it formed a 

 part. 



6. On 



* See Phil. Trans. lS16,,p. 82. 



