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64 OB; and this time the screen CD, carried along with the frame in this movement, is no longer interposed in the path of the rays. The half-plate AO therefore receives the action of the spark while subjected to the rays.

This being understood, the experiment is as follows: first the plate is kept in the first of the above-indicated positions during five seconds, then in the second position also for five seconds; it is then brought back to the first position, and the double operation just described is repeated several times.

After an interval of time equal to an even multiple of five seconds—for instance, one hundred seconds—each of the half-plates has been exposed to the spark for an equal period, only, while AO was exposed, "N" rays were in action, and while OB was exposed there were none.

Thanks to an arrangement of guides and buffer-stops, the to-and-fro motion of the frame can be executed with perfect certainty and regularity, in spite of the darkness. A metronome is used to regulate the action.