(87) ω (S - Ṡ) = (εμ - 1) Ω
When the matter is at rest at a space-time point, ω = 0, then the equation 86) denotes the existence of the following equations
Zy = Yz, Xz = Zx, Yx = Xy,
and from 83),
Tx = Ω₁, Ty = Ω₂, Tz = Ω₃
Xt = εμΩ₁, Yt = εμΩ₂, Zt = εμΩ₃
Now by means of a rotation of the space co-ordinate system round the null-point, we can make,
Zy = Yz = 0, Xz = Zx = 0, Xx = Xy = 0,
According to 71), we have
(88) Xx + Yy + Zz + Tt = 0,