EYES with normal sight have different illusions, as well as eyes with abnormal sight. An illusion is not caused by the same strain that causes an error of refraction. To imagine that one is looking at something which has no existence is different essentially from a distortion of the rays of light received on the lens. To be unable, at any time, to distinguish between two colors, differs from a reaction which makes a black letter seem some other color, and perhaps a color that is quite variable. There are illusions as to size and form; and these may relate only to the object when it is at various distances from the eye. These illusions may be influenced by environment, or by circumstances. There are illusions as to numbers, two or several objects being seen when only one is present; and the relative arrangements may be different. The illusions may vary when only one eye is used; and differ with one eye from the illusion seen with the other. Letters in words may change places, and the changes may vary. These actual illusions are not errors in refraction, but are caused by some abnormal function in the visual center in the brain—in other words, they are conceived by the mind.

An unique illustration of these illusions is the experience of some persons when practicing the eyes on the special cards made for use with the stereoscope. There is one card with the letters O N on the left half of the card, and the letters N E on the right half. It has happened with several patients that with both eyes open, they have imagined that they saw the letters N E with the left eye, and the letters O N with the right eye. They could not possibly have seen the letters that way. The transposition therefore must have been made in the mind.

It must have been imagined. It was distinctly an illusion. That is not hard to realize; although, of course, it cannot be explained. In every case the patient has succeeded in fusing the letters, so that one letter N forms a perfect union with the other N, so there appears to be only one N, and when thus fused they see the word ONE. This is accomplished by relaxing the abnormal tension in the mind, so that the fusion function works normally.

There is an interesting variety of illusion which is quite common. Many explanations are given for it. The technical Latin name for the condition means "flying flies." There seem to be floating specks which are generally dark or black, but may look like air bubbles, or have different colors, and often are connected in chains. They generally appear to be in motion, but do not change their positions. They seem to be going, but do not go. They are present more or less constantly in the different kinds of abnormal vision; but may be present at times when the eyes have normal sight.

These strange things are only illusions, and are caused by some tension in the mind. I can speak personally on this subject, because I was troubled with them for years. They were quite variable, in their presence and absence, and in their conduct. Sometimes I have studied them, with my eyes closed, and it has happened often that in the midst of my observations they would simply disappear. I soon observed that they were not accompanied by any symptoms except the interference with vision, and I realized early that they must be some kind of a nervous disturbance.

Patients have described the same experience. I asked one patient, a woman of forty, who came with a chronic inflammation of the iris—a condition which was very painful and made her almost blind—if she had such specks in her eyes. She said: "Yes, I have flocks of them in both eyes, and there are some big grandfathers among them that seem to shut out the light." The iritis was cured by the Bates Treatments, and the specks disappeared also. My own specks still appear sometimes, for short periods only, but are never annoying, as they used to be.

There are many differing illusions of color. Looking at a black or white or colored surface, or at a strong light, as a 300 watt or a 1000 watt electric light, and then closing the eyes, one will see various colored after-images, and generally these images will change colors. I have looked into the noonday sun and seen most interesting after-images—streaks of brilliant colors, generally perpendicular, followed by vague blocks of varying colors. I have found that my sight is always improved in a few minutes, after the effects of the stimulation have subsided, so that I can see small letters at a greater distance. Looking at the sun, until one becomes accustomed to it, may cause an unpleasant reaction, and even lower the vision for a time. These after-effects are all illusions. They are not seen by the eyes, because the eyes are closed. They are caused by some tension in the mind—the same as abnormal reactions in other parts of the nervous system, which are quite common, are known to be caused by tension. Since most of these illusions are apparent when the eyes are closed, it is obvious that they are produced in the mind, in the same way that an illusion appears when the eye is open, and seems to see an object which is not present. Various illusions can be produced by deliberate efforts of straining the eyes. It is worth while to make such an experiment, simply staring hard, for instance, at a short line of heavy large letters. The distinct benefit in such a procedure is that it impresses on the mind a new knowledge of the mechanism of one's function of vision, and is helpful in working out the cure.