which is dependent upon the intensity of the stimulus._ We have to the present considered only the _simplest_ conditions existing as a result of the effect of a _single_ series of stimuli and also of the interference of its individual members. These elementary conditions are at the basis of an understanding of complicated _interference effects which arise when two series of stimuli interact_. In that these processes can be readily explained by the elementary processes previously described, I will, therefore, dwell but briefly on this subject. From the standpoint already taken it may be readily presumed what will happen when two series of stimuli act upon the same system. When there is interference of _two series of stimuli_, there are two resultant possibilities. In one type the stimuli of the one are active simultaneously with that of the other. In this instance both stimuli would act as a single stimulus of greater intensity, and we have essentially the same condition as exists when a single series is operative. Nevertheless, such cases are practically hardly realized in the physiological happenings of the organism. More often a state exists wherein the single stimuli of one series occur in the intervals of the stimuli of the other. In these cases there is an increase in the frequency of the stimuli applied in a given length of time. We have here, then, in principle the same conditions as when a series of greater frequency is operative. (Figure 55.) The effect of such alteration in the frequency consists in an increase of the velocity of the development of summation or inhibition, as the general scheme (Figure 55) has shown us. Depending upon the special combination of the factors involved in interference, we may have a summation of the exciting effect of each series of stimuli or an inhibition of one series by the exciting effects of the other series. If the frequency of both series is essentially different, we may have here the conditions for periodically increasing and decreasing excitations. Nevertheless these conditions have not been systematically analyzed and experimentally studied. [Illustration: A B Fig. 55. Interference of two series of stimuli. A--Effect of the one series alone. Development of tonus by summation. The dots below the curve indicate the points of time at which the stimuli of the second series will operate. B--Effect resulting from the interference of both series. By the addition of the second series the frequency has been doubled. The result consists in an inhibition. ]