may easily be given. If the time axis is chosen in such a way that it has the direction of the line differential which we are considering, or, in other words, if we reduce the material particle to rest, we shall then have
; this will therefore be measured by the light-seconds clock which is at the same place, and at rest relatively to the material particle. We therefore call
the proper time of the material particle. As opposed to
,
is therefore an invariant, and is practically equivalent to