It must be mentioned, however, that Weber’s law does not hold good over an unlimited range of intensities of stimulation. If the sun were twice as bright, it would not appear brighter to the eye. For such extreme intensities the law is no longer valid. Neither is it valid for exceedingly low intensities; it makes no difference to the eye whether the wall of a dark room is illuminated from a distance of three or four yards by the glow of one cigarette or a dozen. The logarithmic equation applies only to a certain--quite large--range of medium intensities. For this range our sensitiveness to change is not only constant, but also greatest. Changes in illumination within this range can be perceived as soon as the stimulus increases or decreases by about one hundred and fiftieth. Weber’s law has still another practical significance. A thing which we recognize by the aid of the differences in illumination of its parts (as, for example, a stone relief) or by its differences in loudness (as a rhythm beaten on a drum) always retains, not the same absolute differences, but the same quotients or proportions of the different light or tone values, however our distance from the thing varies. Weber’s law, then, enables us to perceive the identity of the thing although the absolute light or tone values have undergone change. If our nervous activities were not regulated in accordance with Weber’s law, the relief and the rhythm might become unrecognizable at a greater distance, and the relief also at dusk. A further important relation between our mental life and the external world consists in our much greater sensitiveness to the moving and changing than to the stable and permanent. A pencil point moved over the skin under slight pressure gives us a perception of the length and direction of the line traversed more accurate than the impression received from the edge of a screwdriver pressed on the skin. On the peripheral parts of the retina the sizes and distances of things are not easily perceived; but no difficulty is experienced in noticing a waving handkerchief or a starting animal. Only the small central part of the retina is adapted to the perception of the motionless.