Page:Philosophical Review Volume 2.djvu/339

No. 3.] other senses. The noise and smells of cities are not noticed and cannot be fully perceived by those who dwell among them. The dependence of sensation on the time of stimulation may be represented by a curve, and its equation determined. The relation is of special importance as regards feelings of pleasure and pain. These soon reach a maximum and then decline. The Greeks might have called the gods pitiful as well as envious for both pain and pleasure consume themselves. If fortune physique is to be distributed so as to produce the maximum fortune morale, it should not be supplied continuously to one individual.

If the hand by accident touch a piece of hot metal, it will be withdrawn, but not before it has been in contact with the metal long enough to be burned. The time is, indeed, about $1⁄10$ sec. During this short interval a complex series of physiological processes must take place. The physical motion is converted into a nervous impulse, the impulse travels along the nerve and through the brain, a movement is released, the impulse travels back, and the muscle is innervated. The time required for such a reaction under varying conditions may be measured with great exactness. It is usually between one tenth and one fifth of a second, being longest in children, in the aged, and in disease. A reaction of this sort must be regarded as a physiological process, but it has psychological interest, as its length depends on mental conditions. Indeed, a reaction and its duration throw light on two of the most important problems before rational psychology – the nature of volition and the relation between body and mind.

A simple reaction may be modified so as to include the time it takes to perceive a sense-stimulus or to choose a movement.