Friedrich Kottler

“ For the previously defined direction cosines of the tangent we now have
which we will also denote as the components of the velocity vector [1] . Therefore, we are now completely within Euclidean geometry.
The formulas of Wiechert. Let now the world line of a pointlike charge be given, whose coordinates shall always be denoted by ; we look for the potentials and fields in an arbitrary reference point, which is not allowed to be taken infinitely close to the respective location of the light-point, and whose coordinates shall be denoted by . ”

“ Furthermore, a material point of plane world line is connected to a plane or straight trajectory depending on the choice of reference system, the latter being the case when the -axis is directed parallel to the plane of the world line ”

“ Our world line is now
Consequently
thus
in which one recognizes Minkowski's proper time multiplied by , which here appears to be connected in the most simple way with the (imaginary) Euclidean arc of the world line. ”