more convincing evidence. Again, the stimulation of the retina is ordinarily due to the impinging upon it of light-waves emanating from an external object. Accordingly, when the retina is disturbed by any exceptional cause, such as a blow or electric shock, we have a sensation of light projected outward into space. 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 repetitions; and this has induced an inherent logical necessity to interpret a new experience by the old, an unfamiliar by the familiar. As Mr. Sully well expresses it, these illusions "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.
As our present purpose is to investigate the nature of real deception, of the formation of false beliefs leading to erroneous action, it will be well to note that even such elementary forms of sense-deceptions as those just noted fall under this head. No one allowed the use of his eyes will ever believe that the ball held between the crossed fingers is really double, but children often think that a spoon half immersed in water is really bent. Primitive peoples believed that the moon really grew smaller as it rose above the horizon, and the ancients could count sufficiently upon the ignorance of the people to make use of mirrors and other stage devices for revealing the power of the gods. The ability to correct such errors depends solely upon the possession of certain knowledge, or a confidence in the existence of such knowledge.
Still confining our attention to deceptions produced by exceptional external arrangements, let us pass to more complex instances of them. These, as so many of the types of deception, are found in great perfection in conjuring tricks. When ink is turned into water and water into ink; when a duplicate coin or other article is skillfully introduced in place of the one that has disappeared; when two half-dollars are rolled into one; when a box into which you have just placed an article is opened and found to be empty; when the performer drives a nail through his finger, or when a card which you have just assured yourself is the ace of hearts on second view becomes the king of spades—you are deceived because you are unaware that the addition of a chemical will change the color of liquids; that the piece you now see is different from the one you saw a moment ago; that the one half-