75 8 THE POPULAR SCIENCE MONTHLY. 



seventh of a second ; that a sound on the ear is indicated by the hand 

 in one sixth of a second ; and that, when light irritates the eye, one 

 fifth of a second elapses before the hand moves. The mechanism of 

 such a process is the following : Suppose the sound " A " is heard by 

 the ear. After a latent period it is translated to some nerve-cells and 

 hence to the brain. From the brain it goes to other cells, ganglion- 

 cells, and to other nerves, and then to the different muscles of the 

 chest and larynx, and then follows the audible response " A." Now, 

 since this whole process requires only one sixth of a second, the ques- 

 tion arises, How much of it is psychical ? To answer it, the experi- 

 ment is repeated, but with this difference, that the particular sound to 

 be used is unknown to the experimenter. Before the sound can be 

 repeated by him, therefore, a distinct act of discrimination is required, 

 and the time taken is longer. Calling the time in the first experiment 

 a, and in the second b, the difference b a is the time required for two 

 distinct actions ; one, that of distinguishing the sound, and the other, 

 that of willing the corresponding movement. If, now, it be agreed 

 that only the sound " A " shall be responded to when called, these may 

 be separated, since, no other sound being responded to, the latter action 

 is eliminated. If the time now required be called c, the difference 

 c a represents the time required for forming a judgment, and c b 

 the time required for a volition. In making these measurements, Don- 

 ders used an instrument devised by him, called a noemotachograph, 

 and also a modification of it, called a noernotachometer. By these in- 

 struments different points of the body can be irritated, different sounds 

 can be produced, and different colors or letters can be shown, all by 

 the electric spark. By subtracting the simple physiological time from 

 the time given in any experiment, the time necessary for recognition 

 may be obtained. By an addition to the apparatus, a second stimulus 

 may be made to follow the first, either on the same or on a different 

 sense, thus enabling the time necessary for a simple thought to be de- 

 termined. As a result of his experiments, Donders found that the 

 value b a in the case of a simple dilemma was seventy-five thou- 

 sandths of a second, this being the time required for recognition and 

 subsequent volition. In the same way c a has been shown to be 

 forty thousandths of a second, being the time required for simple rec- 

 ognition ; there are left thirty-five thousandths of a second as the time 

 required for volition. Moreover, by independent measurement with 

 the noemotachometer, exactly the same time, one twenty-fifth of a 

 second, is found necessary to enable a judgment to be formed about 

 the priority of two impulses acting on the same sense. If they act on 

 different senses, more time is necessary. So, also, more time is re- 

 quired to recognize a letter by seeing its form than by hearing its 

 sound. A man of middle age, then, thinking not so very quickly, re- 

 quires one twenty-fifth of a second for a simple thought. 



Another important fact concerning nervous action is that its amount 



