motion is imparted to the swing, but the inclosing walls, which are painted to represent a forest scene, are themselves revolved forward and then backward about the axis from which the swing is suspended. As, however, we have no experience with oscillating trees, we unconsciously infer the oscillations to be and feel them in our own persons. In another application of the same illusion we seem to be let down into the bowels of the earth; but after a slight actual descent the car remains stationary while the illuminated sides of the shaft, which are suitably painted, are moved panorama-like in an upward direction. In brief, we are creatures of the average; we are adjusted for the most probable event; our organism has acquired the habits impressed upon it by the most frequent experiences; and this has induced an inherent logical necessity to interpret a new experience by the old, an unfamiliar by the familiar. In describing illusions of the above type, Mr. Sully aptly says that they "depend on the general mental law that when we have to do with the unfrequent, the unimportant, and therefore unattended to, and the exceptional, we employ the ordinary, the familiar, and the well known as our standard." Illusion arises when the rule thus applied fails to hold; and whether or not we become cognizant of the illusion depends upon the ease with which the exceptional character of the particular instance can be recognized, or the inference to which it leads be opposed by presumably more reliable evidence. II