We can endure for a short time, without giving any visible response, the psychological stimulation which is supplied by a comical spectacle; but if the latter continues sufficiently long in a sufficiently ludicrous manner, our appropriate emotion rapidly runs up to a point at which it becomes uncontrollable, and we burst into an explosion of ill-timed laughter. But in this case of psychological tickling, as in the previous case of physiological tickling, some persons are much more susceptible than others. Nevertheless, there can be no doubt that from the excitable tissues of a plant, through those of a jelly-fish and a frog, up even to the most complex of our psychological processes, we have in this recently discovered principle of the summation of stimuli a very remarkable uniformity of occurrence. _Effects of Temperature on Excitability._ I shall now conclude this chapter with a brief statement of the effects of temperature on the excitability of the Medusæ; and before stating my results, I may observe that in all my experiments in this connection I changed the temperature of the Medusæ by drawing off the water in which they floated with a siphon, while at the same time I substituted water of a different temperature from that which I thus abstracted. In this way, without modifying any of the other conditions to which the animals were exposed, I was able to observe the effects of changing the temperature alone. With regard to the effect of temperature on the latent period of stimulation, the following table, setting forth the results of one among several experiments, explains itself. Period of latent stimulation of the deganglionated tissues of Aurelia aurita as affected by temperature:-- -------------------------------+------------------------------- Temperature of water (Fahr.). | Period of latent stimulation. -------------------------------+------------------------------- 70° | 1/5 second 50° | 1/3 second 35° | 2/5 second 20° | 1/2 second -------------------------------+------------------------------- In the case of each observation, several shocks were administered before the latent period was taken, in order to decrease this period to its _minimum_ by the staircase action. When this is not done, the latent period at 20° may be as long as 1-1/5 seconds; but soon after this irritability disappears. The extraordinary sluggishness of the latent period at very low temperatures is fully equalled by the no less extraordinary sluggishness of the contraction. In order to render apparent the degree in which both these effects are produced, I here append a pair of tracings which were procured from the same piece of tissue when exposed to the different temperatures named. (N.B.--The seconds are wrongly marked in Fig. 7; they ought to be the same as in Fig. 6.)