In empty space this becomes \[ \Wave F_{\mu\nu} = 2B_{\mu\nu\alpha\epsilon} F^{\epsilon\alpha} \Tag{(74.42)} \] for an infinite world. For a curved world undisturbed by attracting matter, in which $G_{\mu}^{\epsilon} = \lambda g_{\mu}^{\epsilon}$, $B_{\mu\nu\alpha\epsilon} = \frac{1}{3}\lambda(g_{\mu\nu} g_{\alpha\epsilon} - g_{\mu\alpha} g_{\nu\epsilon})$, the result is \[ (\Wave + \tfrac{4}{3}\lambda)F_{\mu\nu} = 0. \Tag{(74.43)} \] It need not surprise us that the velocity of propagation of electromagnetic potential and of electromagnetic force is not the same (cf.~\Eq{(74.33)} and~\Eq{(74.43)}). The former is not physically important since it involves the arbitrary convention $\kappa_{\alpha}^{\alpha} = 0$. But the result~\Eq{(74.42)} is, I think, unexpected. It shows that the equations of propagation of electromagnetic force involve the Riemann-Christoffel tensor; and therefore this is not one of the phenomena for which the ordinary Galilean equations can be immediately generalised by the principle of equivalence. \PageSep{177} This naturally makes us uneasy as to whether we have done right in adopting the invariant equations of propagation of light ($ds = 0$, $\delta\int ds = 0$) as true in all circumstances; but the investigation which follows is reassuring. \Subsection[(c)]{Propagation of a wave-front.} The conception of a ``ray'' of light in physical optics is by no means elementary. Unless the wave-front is of infinite extent, the ray is an abstraction, and to appreciate its meaning a full discussion of the phenomena of interference fringes is necessary. We do not wish to enter on such a general discussion here; and accordingly we shall not attempt to obtain the formulae for the tracks of rays of light for the case of general coordinates \Foreign{ab initio}. Our course will be to reduce the general formulae to such a form, that the subsequent work will follow the ordinary treatment given in works on physical optics.