LIST OF ILLUSTRATIONS
Reading edition
FIGURE PAGE
1. Principal parts of the eye 14
2. Stereoscopic pictures for combining by converging or diverging the optical axes 41
3. Stereoscopic pictures 41
4. The vertical line appears longer than the equal horizontal line in each case 46
5. The vertical dimension is equal to the horizontal one, but the former appears greater 47
6. The divided or filled space on the left appears longer than the equal space on the right 49
7. The three lines are of equal length 50
8. The distance between the two circles on the left is equal to the distance between the outside edges of the two circles on the right 50
9. Three squares of equal dimensions which appear different in area and dimension 51
10. The vertical distance between the upper circle and the left-hand one of the group is equal to the overall length of the group of three circles 52
11. Two equal semi-circles 53
12. Arcs of the same circle 53
13. Three incomplete but equal squares 53
14. Middle sections of the two lines are equal 54
15. An effect of contrasting areas (Baldwin’s figure) 54
16. An illusion of contrast 55
17. Equal circles which appear unequal due to contrast (Ebbinghaus’ figure) 56
18. Equal circles appearing unequal owing to contrasting concentric circles 56
19. Circles influenced by position within an angle 57
20. Contrasting angles 57
21. Owing to perspective the right angles appear oblique and vice versa 58
22. Two equal diagonals which appear unequal 58
23. Apparent variations in the distance between two parallel lines 59
24. A striking illusion of perspective 60
25. Distortion of a square due to superposed lines 61
26. Distortion of a circle due to superposed lines 62
27. Illustrating fluctuation of attention 65
28. The grouping of the circles fluctuates 66
29. Crossed lines which may be interpreted in two ways 67
30. Reversible cubes 68
31. The reversible “open book” (after Mach) 69
32. A reversible tetrahedron 69
33. Reversible perspective of a group of rings or of a tube 70
34. Schröder’s reversible staircase 70
35. Thiéry’s figure 71
36. Illustrating certain influences upon the apparent direction of vision.
By covering all but the eyes the latter appear to be drawn alike in both sketches 73
37. Zöllner’s illusion of direction 77
38. Parallel lines which do not appear so 79
39. Wundt’s illusion of direction 79
40. Hering’s illusion of direction 80
41. Simple effect of angles 81
42. The effect of two angles in tilting the horizontal lines 83
43. The effect of crossed lines upon their respective apparent directions 83
44. Another step toward the Zöllner illusion 84
45. The two diagonals would meet on the left vertical line 85
46. Poggendorff’s illusion. Which oblique line on the right is the prolongation of the oblique line on the left? 85
47. A straight line appears to sag 86
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
75. A concave hemispherical cup on the left and a convex hemisphere on the right lighted by a light-source of large angle such as a window 150
76. The same as Fig. 75, but lighted by a very small light-source 151
77. Apparent ending of a searchlight beam 161
78. An accurate tracing from a photograph (continual exposure) of the moon rising 171
79. Accurate tracings from a photograph (short exposures at intervals) of the sun setting 172
80. Explanation offered by Smith of the apparent enlargement of heavenly bodies near the horizon 174
81. Explanation of a common mirage 176
82. Illustrating the apparent distortion of a picture frame in which the grain of the wood is visible 190
83. Another example similar to Fig. 82 191
84. From actual photographs of the end-grain of a board 192
85. Exaggerated illusions in architecture 198
86. Illustrating the influence of visual angle upon apparent vertical height 199
87. Irradiation in architecture 200
88. Some simple geometrical-optical illusions in architecture 201
89. By decreasing the exposed length of shingles toward the top a greater apparent expanse is obtained 202
90. An example of a mirror “illusion” 207
91. Another example of “mirror magic” 208
92. A primary stage in the evolution of the use of geometrical-optical illusions on ships 226
93 and 94. Attempts at distortion of outline which preceded the adoption of geometrical-optical illusions 228
95 and 96. Illustrating the use of models by the Navy Department in developing the geometrical-optical illusion for ships 229
97 and 98. Examples of the geometrical-optical illusion as finally applied 231
99. Representative earth backgrounds for an airplane (uncamouflaged) as viewed from above 235
100. Illustrating the study of pattern for airplanes. The photograph was taken from an altitude of 10,000 feet. The insert shows the relative lengths (vertical scale) of an airplane of 50-foot spread at various distances below the observer 239
VISUAL ILLUSIONS