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 _V_s, 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 _V_s. An excellent demonstration may be made by the reader by using two _V_s, 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 _V_s when the card is laid upon Fig. 63. The grays in the holes should now appear alike because their environments are similar. [Illustration: 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. [Illustration: 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.