That which has often occurred is likely to recur. But it does not regularly recur in the same manner. There are exceptions. It happens that certain things occur in surroundings different from their usual surroundings. These things are then perceived, that is, grouped together and supplemented by images, in harmony with their usual surroundings. But the perception is then in discord with the actual surroundings. To the inhabitant of the plains the colors of things appear rather saturated, and the outlines sharp, when these things are at a small distance from the observer. Walking toward them, he is soon able to lay hands on them. But when the air happens to be unusually moist, and because of its diminished weight, free from the particles of dust which have settled because of their weight, things look unusually near, and on walking toward them he discovers that it takes more time to reach them than he expected. The same happens when he goes to the mountains for his vacation, because there the air is always comparatively free from dust. We have here a foreseeing of what ordinarily becomes the subsequent experience, but fails to become it in this instance. There is another kind of illusion based on the fact that sensations which have been imagined just before the stimuli became effective, are thereby favored and become unusually vivid. This law of attention holds good also when the stimuli are not in exact correspondence with the preceding images. In such a case the perception is more or less assimilated to those images, so that the same stimuli result in somewhat different percepts according to circumstances. “How heavy it is!” said a friend of Davy’s, when the discoverer of potassium placed a little piece of this metal on his finger. Potassium is so light that it floats on water, but the metallic appearance produced the image of pressure and changed the sensation into a percept of something heavy. When two pieces of gray paper, equally bright but of slightly different coloring, are put before me side by side, and I ask myself: is not the yellowish paper lighter than the bluish paper, immediately it seems to be lighter. But I begin to doubt and ask myself: is not the yellowish paper darker than the other; and immediately it looks darker.