Part 4

48. Distortions of contour due to contact with other contours 87

49. An illusion of direction 88

50. "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

68. For demonstrating retiring and advancing colors 137

69. By combining these stereoscopically the effect of metallic lustre (similar to graphite in this case) is obtained 141

70. A bas-relief lighted from above 146

71. An intaglio lighted from above 147

72. A bas-relief lighted from the left 148

73. An intaglio lighted from the left 149

74a. A disk (above) and a sphere (below) lighted from overhead 145

b. A disk and a sphere lighted by perfectly diffused light 145