Two opposite contrasts obtainable by means of a simple apparatus illustrated in [Fig. 62] may be shown simultaneously by means of white, black, and gray papers arranged as in [Fig. 63]. In this figure the gray is represented by the partially black Vs, each of which contains equal amounts of black and of white. When held at some distance this serves as a gray and the same effect is apparent as is described for the case of actually gray Vs. An excellent demonstration may be made by the reader by using two Vs, cut from the same sheet of gray paper, and pasted respectively upon white and black backgrounds, as in [Fig. 63]. It will be apparent that the one amid the black environment appears much brighter than the one (same gray) amid the white environment. This can be demonstrated easily to an audience by means of a figure two feet long. It is interesting to carry the experiment further and place a V of much darker gray on the black background than the V on the white background. The persistency of the illusion is found to be remarkable, for it will exist even when the one V is actually a much darker gray than the other. To become convinced that the two grays are of the same brightness in [Fig. 63], it is only necessary to punch two holes in a white or gray card at such a distance apart that they will lie respectively over portions of the two Vs when the card is laid upon [Fig. 63]. The grays in the holes should now appear alike because their environments are similar.
Fig. 63.—Illustrating brightness-contrast.
The importance of contrasts in brightness and in color cannot be overemphasized, and it appears certain that no one can fully realize their effectiveness without witnessing it in a manner similar to that suggested in [Fig. 62].
Fig. 64.—An effect of brightness-contrast. Note the darkening of the intersections of the white strips.
Many illusions of brightness-contrast are visible on every hand. For example, the point at which the mullions of a window cross will be seen to appear brighter than the remaining portions of them when viewed against a bright sky. Conversely, in [Fig. 64], dark spots appear where the white bars cross. This is purely an illusion and the same type may be witnessed by the observant many times a day. In [Fig. 64] it is of interest to note that the illusion is weak for the crossing upon which the point of sight rests, but by averted vision the illusion is prominent for the other crossings. This is one of the effects which depends upon the location in the visual field.
No brightness-contrasts are seen correctly and often the illusions are very striking. If a series of gray papers is arranged from black to white, with the successive pieces overlapped or otherwise juxtaposed, a series of steps of uniform brightness is not seen. An instrument would determine the brightness of each as uniform, but to the eye the series would appear somewhat “fluted.” That is, where a light gray joined a darker gray the edge of the former would appear lighter than its actual brightness, and the edge of the darker gray would appear darker than it should. This may also be demonstrated by laying a dozen pieces of white tissue paper in a pile in such a manner that a series of 1, 2, 3, 4, etc., thickness would be produced. On viewing this by transmitted light a series of grays is seen, and the effect of contrast is quite apparent. Such a pattern can be made photographically by rotating before a photographic plate a disk with openings arranged properly in steps.