Page:A history of the theories of aether and electricity. Whittacker E.T. (1910).pdf/351

 Comparing this equation with that which determines the value of, we have

or substituting for $$\bar w$$,

The isotropy with respect to x and z gives the equation But by integration by parts we obtain the equation

and by the condition of incompressibility the second member may be written

So we have

On account of the isotropy, we may write $1⁄3$ for

so and, therefore,

The deviation from isotropy shown by this equation is very small, because of the smallness of. The equation is therefore not restricted to the initial values of the two members,