Page:Philosophical Review Volume 2.djvu/338

324 on which variation depends. Such knowledge has not only theoretic interest, but has, also practical bearings in adjusting education, occupation, and mode of life. The measurement of mental time is proved possible by the measurements of mechanics, while its rate would seem to have as much theoretic and practical interest as the durations of any physical motions. Let us see what has actually been accomplished by experiment.

An impression must last a certain time in order that it may be perceived. Inertia and diffusion in the sense-organs and paths of conduction may partly account for this fact, but it can be shown by experiment that stimuli lasting too short a time to be distinctly perceived still affect the course of mental life. Such sub-conscious mental modifications should not be relegated to physiology; they may prove as fruitful for psychology as has been the study of sub-tangible phenomena for physical science. The time a stimulus must work in order that it may just be perceived, may be measured. Thus, it has been found that colors such as we see in daily life must work on the retina from about .001 to .003 sec., varying with the color and observer. If the time be shorter than this, no color is seen: as the time is taken longer, the intensity and saturation of the color increase, until, the time being about $1⁄10$ sec., a maximum is reached. As the time of stimulation is further increased, the intensity and saturation of the color decline and after a certain time disappear altogether. A similar relation obtains with the