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the time of the second observer's reaction on the sound, and by subtracting this from the entire time, we have the reaction-time of the first observer with his speech-organs. When the average of several series is taken the error becomes very small. A further application of this method will be found below. For our present purposes it was to a large extent superseded by the use of the lip-key and sound-key. There are however certain difficulties in the way of using these instruments, especially in the case of inexperienced persons, children or the insane, for example. The method could further be applied to determining the reaction-time, etc., of the lower animals, and also the length of certain reflex processes where the motion can with difficulty be registered. I give in Table IV. the results of four series of reactions made in this way, Mr. H. Wolfe making reactions on the sound.

Mr. Wolfe's reaction-time on sound was about 150α. The series made on 30 I. seem to have given rather long times, the