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J. M. CATTELL : TABLE III. 1 SOUND-KEY. LIP-KEY. B C B C R R' R R' R R' R R' 3. IV 1B4 161 175 170 168 167 159 176 168 168 177 165 175 175 157 176 165 176 172 159 199 185 199 189 166 199 187 201 186 165 172 173 172 177 185 171 173 173 177 176 5 7 A. 168 168 170 170 188 188 176 174 AV 19 10 16 10 11 6 13 8 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 For our present purposes it was to a large extent superseded by the use of the lip-key and sound-key.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. TABLE IV. I 5 C i R V R' V R V R' V 7. 1 349 30 346 20 328 32 321 17 30 330 380 37 30 332 372 23 20 327 392 24 27 326 392 14 18 357 32 349 19 393 25 393 16 A. 354 32 350 20 360 27 358 16 Mr. Wolfe's reaction-time on sound was about 150<r. The series made on 30 I. seem to have given rather long times, the 1 To save space in this and some other Tables, I only give the average of the mean- variation for the several series (AV).