_Contents_: The quality of the stimulus. Positive and negative alterations of the factors which act as vital conditions. Extent of the alteration in vital conditions or intensity of the stimulus. Threshold stimuli, sub-threshold, submaximal, maximal and supermaximal intensities of stimulus. Relations between the intensity of stimulus and the amount of response. The _Weber_ and _Fechner_ law. All or none law. Time relations of the course of the stimulus. Form of individual stimulus. Absolute and relative rapidity in the course of the stimulus. Duration of the stimulus after reaching its highest point. Adaptation to persistent stimuli. Series of individual stimuli. Rhythmical stimuli. The _Nernst_ law. We have found that stimuli are alterations in the external vital conditions and that the irritability of living substance consists in the capability to respond to stimuli by changes of the vital processes. It now behooves us in the interest of experimental research to investigate the relations between the nature of the alterations in the external vital conditions on the one hand, and that of the alterations of the vital process on the other; that is to say, to systematically study the effects of stimulation on the living organism. For this purpose it is above all necessary to become acquainted with the almost countless numbers of alterations which take place in the external vital conditions of an organism, and to create a systematic scheme of stimulation which differentiates and presents in comprehensive order those various elementary factors which, among the innumerable varieties of stimuli, would prove effectual. For this purpose it is necessary to select the various factors which are involved in an alteration of the external vital conditions. The first of these factors is the _quality of the stimulus_. The external vital conditions are, in short, a series of chemical factors, such as foodstuffs, water and oxygen; the presence of a certain temperature; the existence of a certain light intensity; the existence of a definite static pressure; and finally the presence of an equal osmotic pressure. The stimulus according to its quality can be differentiated into chemical, thermal, photic, mechanical and osmotic varieties. To these must be added other forms of stimuli not ordinarily operative, for instance, many uncommon chemicals, and certain kinds of rays. The form of stimulation, par excellence, which has acquired the greatest importance for the _experimental_ investigation of life, is electricity. In its manifold forms it permits, as no other, of such fine gradations of intensity and duration that it has become in the hand of the physiologist an invaluable means of research.