THE PSYCHOLOGY OF DECEPTION. 147 



more convincing evidence. Again, the stimulation of tlie retina 

 is ordinarily due to the impinging upon it of light-waves emanat- 

 ing from an external object. Accordingly, when the retina is dis- 

 turbed by any excejjtional 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 im- 

 pressed 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 ex- 

 presses 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 ordi- 

 nary, 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 de- 

 ception, 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 be- 

 tween 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 knowl- 

 edge, or a confidence in the existence of such knowledge. 



Still confining our attention to deceptions produced by excep- 

 tional external arrangements, let us pass to more complex in- 

 stances 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 de- 

 ceived 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- 



