where [nu] is the direction of the normal (drawn outwards) to the
remaining face of the tetrahedron, and (x, [nu]) ... denote the angles
which this normal makes with the axes. Hence X_[nu], ... for any
direction [nu] are expressed in terms of X_x,.... When the above
result is interpreted for a very small portion in the shape of a cube,
having its edges parallel to the co-ordinate axes, it leads to the
equations