These illusions are of special interest because they represent not only failures of the senses to convey truth, but because they illustrate how easy it is for the mind to be led astray by the senses. People often declare that they have seen things with their own eyes or in some other way have definite sensory knowledge of them, yet these illusions make it clear that it is perfectly possible for such sensory phenomena to convey quite mistaken information. People who are suffering from many symptoms are persuaded that they must pay attention to their sensations. The main purpose of the psychotherapeutist often is to have them neglect their sensations and correct them by means of information gathered from other sources. We do this with regard to our sensory illusions, why not also with regard to many sensations which are probably quite as mistaken, in certain individuals at least, as these universal illusions of mankind. The argument from analogy holds very well and can be used to decided advantage in many cases.
A startling illusion which makes it clear that care is needed in interpreting our sensations, is the so-called tube illusion or experiment. If a sheet of note paper be rolled into a tube of something less than an inch in diameter and then held close to one eye, both eyes being kept open, while the hand opposite to the eye before which the tube is held is placed palm faceward against the side of the tube about its middle, a hole will be seen, as it were, through the palm of the hand. This false vision is as clear as can be and persists after any number of repetitions of the experiment. It merely illustrates two-eyed vision. We have a picture in each eye and we combine them. When the pictures cannot be combined for any reason, optical illusions result. There are many more optical illusions than we think and there are many reasons besides two-eyed vision for them.
Other illusions of two-eyed vision may be illustrated rather easily. If [{767}] two dots are made on a sheet of paper about two inches apart and the eyes look at them in a dreamy way, looking far beyond the paper, with vision more or less fixed between them, after a few moments a number of things happen. Usually the two dots exhibit a tendency to float together.
Fig. 26
After an interval four dots will be seen—each of the dots having a picture in each eye. Then only one dot may be seen because the pictures combine. Sometimes three dots will be seen. When the dots swim toward one another, a curious feeling of insecurity comes over the experimenter, showing how much our sense of stability is dependent on vision and illustrating why vision from a height is so disturbing because objects cannot be properly fixed on the distant background.
Fig. 27
Just as the two dots may be made to come together, so, after a little practice, a bird may be made to go into a cage (Fig, 27) or an apple made to go onto a plate (Fig. 28),
