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After thinking the matter over for some time you then offer the

following suggestion with which to test simultaneity. By

measuring along the rails, the connecting line AB should be

measured up and an observer placed at the mid-point M of the

distance AB. This observer should be supplied with an arrangement

(e.g. two mirrors inclined at 90°) which allows him visually to

observe both places A and B at the same time. If the observer

perceives the two flashes of lightning at the same time, then

they are simultaneous.