_Practice_ shortens it to a quantity which is for each individual a minimum beyond which no farther reduction can be made. The aforesaid old pauper's time was, after much practice, reduced to 0.1866 sec. _Fatigue_ lengthens it, and _concentration of attention_ shortens it. The _nature of the signal_ makes it vary. I here bring together the averages which have been obtained by some observers: Hirsch. Hankel. Exner. Wundt. Sound 0.149 0.1505 0.1360 0.167 Light 0.200 0.2246 0.1506 0.222 Touch 0.182 0.1546 0.1337 0.213 It will be observed that _sound_ is more promptly reacted on than either _sight_ or _touch_. _Taste_ and _smell_ are slower than either. The _intensity of the signal_ makes a difference. The intenser the stimulus the shorter the time. Herzen compared the reaction from a _corn_ on the toe with that from the skin of the hand of the same subject. The two places were stimulated simultaneously, and the subject tried to react simultaneously with both hand and foot, but the foot always went quickest. When the sound skin of the foot was touched instead of the corn, it was the hand which always reacted first. _Intoxicants_ on the whole lengthen the time, but much depends on the dose. =Complicated Reactions.=--These occur when some kind of intellectual operation accompanies the reaction. The rational place in which to report of them would be under the head of the various intellectual operations concerned. But certain persons prefer to see all these measurements bunched together regardless of context; so, to meet their views, I give the complicated reactions here. When we have to think before reacting it is obvious that there is no definite reaction-time of which we can talk--it all depends on how long we think. The only times we can measure are the _minimum_ times of certain determinate and very simple intellectual operations. The _time required for discrimination_ has thus been made a subject of experimental measurement. Wundt calls it _Unterscheidungszeit_. His subjects (whose simple reaction-time had previously been determined) were required to make a movement, always the same, the instant they discerned _which_ of two or more signals they received. The _excess_ of time occupied by these reactions _over the simple reaction-time_, in which only one signal was used and known in advance, measured, according to Wundt, the time required for the act of discrimination. It was found longer when four different signals were irregularly used than when only two were used. When two were used (the signals being the sudden appearance of a black or of a white object), the average times of three observers were respectively (in seconds) 0.050 0.047 0.079 When four signals were used, a red and a green light being added to the others, it became, for the same observers, 0.157 0.073 0.132