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 great 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 ease 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