’ 
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 am 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 
eases, the lines of equal tint are the lines of equal density. 
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 
elass 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 Cov- 
romx’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. 1816, p. 82. 
