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“Twisted-cord” illusion. These are straight cords 89 51. “Twisted-cord” illusion. These are concentric circles 89 52. A spiral when rotated appears to expand or contract, depending upon direction of rotation 90 53. Angles affect the apparent length of lines 91 54. The horizontal line appears to tilt downward toward the ends 92 55. The horizontal line appears to sag in the middle 92 56. The Müller-Lyer illusion 93 57. Combined influence of angles and contrasting lengths 95 58. Two equal oblique lines appear unequal because of their different positions 95 59. An illusion of area 96 60. Five equal areas showing the influence of contour upon judgment of area 97 61. Showing the effect of directing the attention 98 62. Simple apparatus for demonstrating the remarkable effects of contrasts in brightness and color 115 63. Illustrating brightness-contrast 117 64. An effect of brightness-contrast. Note the darkening of the intersections of the white strips 118 65. The phenomenon of irradiation 121 66. An excellent pattern for demonstrating color-contrast 126 67. By rotating this Mason (black and white) disk color-sensations are produced 133