TEE TIME IT TAKES TO THINK. 489 



has been pressed an interval very nearly constant in length passes 

 before he can press the hand of his neighbor. This interval, which 

 we may call the reaction-time, is made up of a number of factors. A 

 period elapses before the pressure is changed into a nervous message 

 or impulse. This time is very short in the case of touch ; but light 

 working on the retina seems to effect chemical changes in it, and these 

 take up some little time, probably about one fiftieth of a second. After 

 a nervous impulse has been generated it moves along the nerve and 

 spinal cord to the brain, not traveling with immense rapidity like 

 light, but at the rate of an express train. In the brain it must move 

 on to a center having to do with sensation, where changes are brought 

 about, through which a further impulse is sent on to a center having 

 to do with motion, and a motor impulse having been prepared there is 

 sent down to the hand. Another pause, one two hundredth to one one 

 hundredth of a second, now occurs, while the muscle is being excited, 

 after which the fingers are contracted and the reaction is complete. 

 The entire time required is usually from one tenth to one fifth of a 

 second. The reaction-time varies in length with different individuals 

 and for the several senses, but as long as the conditions remain the 

 same the times are very constant, only varying a few thousandths of 

 a second from each other. One may wonder how it is possible to 

 measure such short times and with such gi-eat accuracy. It would not 

 be easy if we had not the aid of electricity ; but when it is called to 

 mind that a movement made in London is almost instantaneously 

 registered in Edinburgh, it will not seem inconceivable that we can 

 record to the thousandth of a second the instant a sense-stimulus is 

 produced and the instant a movement is made. The time passing 

 between these two events can be measured by letting a tuning-fork 

 write on a revolving drum. The tuning-folk can be regulated to 

 vibrate with great exactness, say five hundred times a second ; it writes 

 a wavy line on the drum, each undulation long enough to be divided 

 into twenty equal parts, and thus time can be measured to the ten 

 thousandth of a second. 



The psychologist is chiefly interested in what goes on in the brain 

 and mind. It seems that about one half of the entire reaction-time is 

 spent while brain changes take place, but we know very little as to 

 these changes, or as to how the time is to be allotted among them. 

 It is probable that in the case of the simple reaction the movement 

 can be initiated before the nature of the impression has been perceived. 

 .We can, however, so arrange the conditions of experiment that the 

 observer must know what he has seen, or heard, or felt, before he 

 makes the movement. He can, for example, be shown one of a number 

 of colors, and not knowing beforehand which to expect, be required to 

 lift his finger only when red is presented. By making certain analyses 

 and subtracting the time of the simple reaction from the time in the 

 more complex case, it is possible to determine with considerable 



