Page:Philosophical Review Volume 2.djvu/340

It takes about 1/10 sec. to perceive a color and a little longer to recognize a picture or word. A word can be recognized as quickly as a single letter. A word is named more quickly than a letter, and much more quickly than a color or an object. The processes of perception and movement may be regarded as psycho-physical, but it is also possible to measure the time needed to remember, to form a judgment, and in the association of ideas. These times are purely mental; we know of them only on the side of consciousness, being in complete ignorance of the cerebral processes which may accompany them. It takes about 1/4 sec. to translate words into a foreign language with which the observer is familiar, 2/5 sec. to call to mind the language of a given author, 1/2 sec. to multiply the numbers of the multiplication table, etc.; 1/2 sec. is required to estimate the length of a given line or to decide which of two eminent men is thought the greater. The time required for one idea to suggest another under conditions similar to those of ordinary thought may be measured, and the times are of use in studying the laws of association. Psychology is a science, not an art, but time-measurements such as we have been considering have practical applications in æsthetics (as in the rhythm of music and poetry); in medicine (as in the diagnosis of diseases of the nervous system); in political economy (as in distributing wealth so as to secure the maximum amount of happiness), and in education (as in adjusting methods so as to increase the rate of thought).

14. There is a least noticeable difference in intervals of time.

Intervals of time may seem the same to common observation, while a difference may be readily measured with a clock. Savages living near the equator would not notice that days are longer in summer than in winter, nor do we notice the variation in the solar day. It is possible to measure the error made in comparing intervals of time, and to study its variation under varying circumstances. 3/4 sec. has been found to be the interval, which can be most accurately perceived. An interval