and
, provided with a contrivance such as two mirrors inclined at 90° and adjusted so that light from
and
would be reflected into his vertical line of sight ([Fig. 3]).
Two events such as lightning-strokes are then to be defined as simultaneous for the observer at
if rays of light from them reach the observer at the same moment (psychologically): i.e. if he sees the strokes in his mirror-contrivance simultaneously.