_f__{_y z_}^* = _f__{_x l_} etc.
and _f²_ = ρ² + ρ^*², (_f__f_^*) = 2ρρ^*.
| _f_ |² and (_f__f_^*) may be said to be invariants of the six vectors,
for their values are independent of the choice of the system of
co-ordinates.
[M. N. S.]
Note 12.
Light-velocity as a maximum.
Page 23, and Electro-dynamics of Moving Bodies, p. 17.