Page:Popular Science Monthly Volume 11.djvu/586

568 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 decision 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 existence 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, the 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 determined to press the button; second, the transmission of the impulse through the nerves from the brain to the hand; and, third, the excitement 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 "Æneid," 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 nervous 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