, less slanting than
, at least if we consider the case of an observer moving along the embankment with a speed inferior to that of light. At a definite instant, say at one second after zero hour, the point-event defining the observer’s position will be
, that is to say, a point on his world-line at a height of one inch above
. The point-event of the light wave at the same instant will be
, also at one inch above