568 THE POPULAR SCIENCE MONTHLY. 



approximately true, the various experiments that have been made are 

 not without some curious interest. 



It is well known that the operations of the nervous system require 

 time. The action of the different senses is not instantaneous ; there 

 is always an interval of time after a foreign body touches our skin 

 before we know that it touches us. So also between the mental de- 

 cision to make a movement and its actual execution there is a real 

 though short interval. 



Where upon sight of a star a button is to be pressed, there is, first, 

 the action of the sense of sight which gives us knowledge of the ex- 

 istence of the star ; and, second, action of the will causing pressure of 

 the button. In fact, the physiological and psychical action, in all 

 cases where excitation is followed by the giving of a signal, may be 

 divided into six separate and successive actions. The sensation may 

 be divided into three distinct acts. In the example supposed there is, 

 first, tlie reception of the image of the star upon the retina ; second, the 

 transmission of the stimulus through the nerve from the eye to the 

 brain ; and, third, the mental perception of the existence of the star. 

 The voluntary movement which causes the signal may also be divided 

 into three acts. There is, first, the act of the will by which it is deter- 

 mined to press the button ; second, the transmission of the impulse 

 through the nerves from the brain to the hand ; and, third, the excite- 

 ment of the muscular fibres by which the finger is bent and the button 

 pressed. The entire interval between the excitation and the giving 

 of the signal, during which these six acts occur in succession, has 

 been called the physiological time. It is really the same in amount, 

 in most cases, with the personal equation. That portion of it which 

 is occupied with the purely mental acts of perceiving the signal and 

 determining the signal is called the psychical time. It is the time 

 required to think. 



Attempts have been made to measure each of these six factors, 

 and interesting results have been obtained, though of various degrees 

 of accuracy and trustworthiness. 



The earliest experiments were made with reference to the rapidity 

 of movement through the nerves. The first attempt to measure the 

 velocity of nervous impulses proceeding from the brain under action 

 of the will was made long ago by Haller. He ascertained, by reading 

 aloud with great rapidity extracts from the " ^neid," the average 

 number of letters which he could pronounce in one minute. Then he 

 calculated the length of the nerve from the brain to the muscles of 

 the tongue and mouth. Each letter he regarded as requiring a ner- 

 vous impulse. He was obliged then only to multiply the number of 

 letters spoken in each minute by the length of the nerve. This gave 

 as a result that the rate of nervous transmission from the brain was 

 about 150 feet a second. This experiment was defective in failing to 

 take into account the facts that both the act of willing and that of 



