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system at time +t, the forces exerted upon the ions have the same direction and magnitude, as the corresponding forces in the first system at time -t. Now, if also the remaining forces that act on the ions in both cases — and in the same instances — are the same, then we can conclude, that the second state of motion is realizable in any way.

By means of similar considerations the possibility of motion can be demonstrated, which is the "mirror image" of a given motion with respect to a fixed plane.

We call $P_{2}$ the mirror image of a point $P_{1}$ and denote the magnitudes that are valid for two system — namely for the first in $P_{1}$ and for the second in $P_{2}$ — by $\rho _{1}$, ${\mathfrak {v}}_{1}$, ${\mathfrak {d}}_{1}$, ${\mathfrak {H}}_{1}$ and $\rho _{2}$, ${\mathfrak {v}}_{2}$, ${\mathfrak {d}}_{2}$, ${\mathfrak {H}}_{2}$. There it should constantly be $\rho _{2}=\rho _{1}$, and the vectors ${\mathfrak {v}}_{2}$, ${\mathfrak {d}}_{2}$, ${\mathfrak {H}}_{2}$ should be the mirror images of the vectors ${\mathfrak {v}}_{1}$, ${\mathfrak {d}}_{1}$ and $-{\mathfrak {H}}_{1}$.

That the second state of motion can now conveniently be called "mirror image", requires no explanation. If the forces of non-electric origin are of such a manner, so that the vectors by which they can be represented in both cases behave like objects and their mirror images, then the second motion will be possible as soon as the first one is possible.