Fig. 34
Poggendorf's illustration of the displacement of oblique lines (Figure 33) [{772}] and Zöllner's distortion of parallel lines as illustrated by Figure 34, make it very clear that our judgment of direction must depend on many factors besides our vision, if we are not to make serious mistakes.
These optical illusions might seem to be of little significance, but the Greeks thought them of so much importance and recognized so thoroughly that they could not be corrected, and that the distortions and displacements would inevitably take place, that they deliberately put certain optical corrections into their great architectural monuments in order to avoid these false appearances. These have been traced very accurately in the Parthenon, for instance. In a word, the Greeks, knowing of these optical illusions, in order to make the lines of their buildings appear correct, deliberately made them wrong to a sufficient degree to correct the optical illusion; This frank mode of yielding to a limitation of human nature is a fine lesson for patients to learn if they can only be made to learn it from these illustrations.
It is with regard to colors, however, that we have the best examples of optical illusions depending on the individual and his special anatomy and physiology. Color-blind people are quite sure that they see color, just as other people do, until their defect is demonstrated to them. A man who is color blind for red thinks that he sees that color as other people do, while all that he sees is a particular shade of brightness which, because other people call it red, he has come to call red. When asked to pick out red from a series of other colors he may often succeed. When asked, however, to take a skein of red wool selected for him to a basket containing a number of different colored wools, and to bring back all those that are of the same color, he will select grays and browns and sometimes greens as well as reds, and present them as all matched colors. A man who is color blind for all colors will still think that he sees colors as other people do. The ingenious illustration of the American flag as it appears to people suffering from different forms of color blindness, though they are all persuaded that they see the same kind of flag, is an interesting example of how different may be people's sensations, though their conclusions are the same. It may be seen in many of the text books of analytical or experimental psychology.
Dalton, to whom we owe the atomic theory, was himself color blind for red and made the first investigations in that subject. He was of Quaker origin and found that a great many of his brethren were deficient in color vision. It becomes much easier from this to understand why they resolved to wear nothing but gray. They did not see colors as other people do and therefore could not understand nor sympathize with the joy of other people in color. Dalton tells the story of a Quaker prominent in his sect who once went to town to buy a gray waistcoat and purchased instead one of bright red. When he appeared at meeting in this he was promptly tried for heresy and violation of church regulations.
There is an interesting tendency on the part of people who are themselves defective in certain faculties of sensation, to conclude that when other people are wrapt in admiration of something that they cannot perceive, it is because these other people have some mental defect that leads them to enthuse too easily over their sensations. A story is told of a newspaper man who used to insist that all that was said about the beauty of the song of birds was due to the vivid imagination of the writers, for he could find nothing to admire about the songs of birds. He was placed in a room with a number of fine song birds all round him and it proved that he could not hear any of the higher notes at all. It was easy, then, to understand his condemnation of the enthusiasm of others as hysterical and imaginative. Nearly this same thing is true of many quite intelligent people with regard to music. They hear ordinary sounds, as did the newspaper man, very well. They are tone-deaf however, that is, they are quite unable to hear and appreciate combinations of sounds or even to catch melodious successions of single notes. They cannot recognize one tune from another and often do not know "Yankee Doodle" from the "Doxology," or, at most, know only the most familiar tunes, but they set themselves up very calmly as judges of the intellects of others and conclude that music lovers are really a hysterical set of people who go into ecstasies over certain quite insignificant sensations.
These interesting tendencies are helpful in enabling the physician to understand his patients better. They often serve as texts from which the physician can explain curious things to patients who are prone to draw wrong conclusions from them and often suggestions unfavorable to their health.
These illustrations and their discussion serve to make very clear the distinction between illusions, delusions and hallucinations, which are often confounded. Illusions are deceptions of the senses. If a man walking along a country road where he fears the presence of snakes sees in the gathering twilight a piece of rope coiled, he will almost surely mistake it for a snake. This is an illusion produced by the conditions in which the object is seen. If walking along the same road the next day, more timorous than ever as to snakes, he should see in broad daylight the same coil of rope, he might in his fright not stay long enough to decide whether it was a snake or not, and his illusion would continue, though it would partake somewhat of the nature of a delusion due to fright disturbing his judgment. If, in spite of careful examination, however, of it, such as would satisfy any ordinary mind that it was a coil of rope and not a snake, he should still insist in believing that it was a snake, this would be a delusion. There is always a mental element in delusions. If, having seen nothing, he should insist, owing to fright and [{774}] nervousness or to some other cause, that he sees a snake where there is nothing at all resembling a snake and where evidently whatever is the basis of his idea of the presence of the snake, is within his own mind, then he is suffering from an hallucination.
Illusions may be quite inevitable. Most of the optical illusions continue to appeal to us as truths even when we know that they represent errors of vision. In spite of the fact that we know that the sun and moon are not larger at the horizon than they are at the zenith, by optical illusion we continue to see them of larger size. It is our duty to correct such illusions by information gathered from other sources. To follow an illusion, that is, to give it credit, when we should correct it, is a delusion. To think that because we cannot see red that therefore there is no red, or because we do not hear the sounds of notes of birds that they do not utter any notes, in spite of the fact that we have the testimony of nearly the whole human race to the contrary, is a delusion. When, using the verb in its broadest sense, as "perceive," we seem to see things very differently from the generality of people around us, there is every reason to suspect that there is some specific or individual limitation of our senses which makes us fail to perceive these things as others do. We have to suspect our sources of information then and to correct them by what we can learn from the experience of others. These are important considerations for many of the ideas that patients cherish with regard to themselves and their ills.