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From these merely fictitious "tensions" we can, as we saw, directly derive the interaction between charged bodies and electrodynamic actions. It is also to be recommended, to operate with them, when the phenomena are periodic and when we only wish to know the averages of the ponderomotive forces during a full period; the last member of (15) namely doesn't contribute anything to these values.

In this way we come to Maxwell's theorem on the pressure generated by motion of light.

The reversibility of motions and the mirror image of motion.

§ 18. For subsequent applications we include the following considerations at this place.

Let a system of moving ions be given, and $\rho _{1}$, ${\mathfrak {v}}_{1}$, ${\mathfrak {d}}_{1}$ and ${\mathfrak {H}}_{1}$ are the various relevant magnitudes within. We may denote the corresponding magnitudes for a second system by $\rho _{2}$, ${\mathfrak {v}}_{2}$, ${\mathfrak {d}}_{2}$ and ${\mathfrak {H}}_{2}$, and we want to imagine that in an arbitrary point, these magnitudes are at time +t in agreement with the magnitudes $\rho _{1}$, $-{\mathfrak {v}}_{1}$, ${\mathfrak {d}}_{1}$ and $-{\mathfrak {H}}_{1}$ at time -t.

We can easily see that, as regards $\rho _{2}$ and ${\mathfrak {v}}_{2}$, those conditions can be satisfied by a real motion of the ions, and namely the system of these ions must completely be in agreement with the first system; the same configurations with the same interval must occur one after the other, as in that first system, but in opposite order; in other words, we obtain the motions of the ions in the second system, when we reverse the motions given at first.

Furthermore, since ${\mathfrak {d}}_{2}$ and ${\mathfrak {H}}_{2}$ satisfy the conditions (I), (II), (III) and (IV), thus the condition of the aether as determined by these vectors, is in agreement with the motion of the ions.

Eventually it follows from equation (V), that in the second