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VISUAL ILLUSIONS

_THEIR CAUSES, CHARACTERISTICS AND APPLICATIONS_

BY M. LUCKIESH

DIRECTOR OF APPLIED SCIENCE, NELA RESEARCH LABORATORIES, NATIONAL LAMP WORKS OF GENERAL ELECTRIC CO.

AUTHOR OF "COLOR AND ITS APPLICATIONS," "LIGHT AND SHADE AND THEIR APPLICATIONS," "THE LIGHTING ART," "THE LANGUAGE OF COLOR," "ARTIFICIAL LIGHT--ITS INFLUENCE UPON CIVILIZATION," "LIGHTING THE HOME," ETC.

100 ILLUSTRATIONS

NEW YORK D. VAN NOSTRAND COMPANY EIGHT WARREN STREET 1922

COPYRIGHT, 1922, BY D. VAN NOSTRAND COMPANY

PREFACE

Eventually one of the results of application to the analysis and measurement of the phenomena of light, color, lighting, and vision is a firmly entrenched conviction of the inadequacy of physical measurements as a means for representing what is perceived. Physical measurements have supplied much of the foundation of knowledge and it is not a reflection upon their great usefulness to state that often they differ from the results of intellectual appraisal through the visual sense. In other words, there are numberless so-called visual illusions which must be taken into account. All are of interest; many can be utilized; and some must be suppressed.

Scientific literature yields a great many valuable discussions from theoretical and experimental viewpoints but much of the material is controversial. The practical aspects of visual illusions have been quite generally passed by and, inasmuch as there does not appear to be a volume available which treats the subject in a condensed manner but with a broad scope, this small volume is contributed toward filling the gap.

The extreme complexity of the subject is recognized, but an attempt toward simplicity of treatment has been made by confining the discussion chiefly to static visual illusions, by suppressing minor details, and by subordinating theory. In other words, the intent has been to emphasize experimental facts. Even these are so numerous that only the merest glimpses of various aspects can be given in order to limit the text to a small volume. Some theoretical aspects of the subject are still extremely controversial, so they are introduced only occasionally and then chiefly for the purpose of illustrating the complexities and the trends of attempted explanations. Space does not even admit many qualifications which may be necessary in order to escape criticism entirely.

The visual illusions discussed are chiefly of the static type, although a few others have been introduced. Some of the latter border upon motion, others upon hallucinations, and still others produced by external optical media are illusions only by extension of the term. These exceptions are included for the purpose of providing glimpses into the borderlands.

It is hoped that this condensed discussion, which is ambitious only in scope, will be of interest to the general reader, to painters, decorators, and architects, to lighting experts, and to all interested in light, color, and vision. It is an essential supplement to certain previous works.

M. LUCKIESH

November, 1920.

CONTENTS

CHAPTER PAGE

I. Introduction 1

II. The eye 13

III. Vision 29

IV. Some types of geometrical illusions 44

V. Equivocal figures 64

VI. The influence of angles 76

VII. Illusions of depth and of distance 102

VIII. Irradiation and brightness-contrast 114

IX. Color 124

X. Lighting 144

XI. Nature 164

XII. Painting and decoration 179

XIII. Architecture 195

XIV. Mirror Magic 205

XV. Camouflage 210

LIST OF ILLUSTRATIONS

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. Schroeder's reversible staircase 70

35. Thiery'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. Zoellner'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 Zoellner 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 Mueller-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

I

INTRODUCTION

Seeing is deceiving. Thus a familiar epigram may be challenged in order to indicate the trend of this book which aims to treat certain phases of visual illusions. In general, we do not see things as they are or as they are related to each other; that is, the intellect does not correctly interpret the deliverances of the visual sense, although sometimes the optical mechanism of the eyes is directly responsible for the illusion. In other words, none of our conceptions and perceptions are quite adequate, but fortunately most of them are satisfactory for practical purposes. Only a part of what is perceived comes through the senses from the object; the remainder always comes from within. In fact, it is the visual sense or the intellect which is responsible for illusions of the various types to be discussed in the following chapters. Our past experiences, associations, desires, demands, imaginings, and other more or less obscure influences create illusions.

An illusion does not generally exist physically but it is difficult in some cases to explain the cause. Certainly there are many cases of errors of judgment. A mistaken estimate of the distance of a mountain is due to an error of judgment but the perception of a piece of white paper as pink on a green background is an error of sense. It is realized that the foregoing comparison leads directly to one of the most controversial questions in psychology, but there is no intention on the author's part to cling dogmatically to the opinions expressed. In fact, discussions of the psychological judgment involved in the presentations of the visual sense are not introduced with the hope of stating the final word but to give the reader an idea of the inner process of perception. The final word will be left to the psychologists but it appears possible that it may never be formulated.

In general, a tree appears of greater length when standing than when lying upon the ground. Lines, areas, and masses are not perceived in their actual physical relations. The appearance of a colored object varies considerably with its environment. The sky is not perceived as infinite space nor as a hemispherical dome, but as a flattened vault. The moon apparently diminishes in size as it rises toward the zenith. A bright object appears larger than a dark object of the same physical dimensions. Flat areas may appear to have a third dimension of depth. And so on.

Illusions are so numerous and varied that they have long challenged the interest of the scientist. They may be so useful or even so disastrous that they have been utilized or counteracted by the skilled artist or artisan. The architect and painter have used or avoided them. The stage-artist employs them to carry the audience in its imagination to other environments or to far countries. The magician has employed them in his entertainments and the camoufleur used them to advantage in the practice of deception during the recent war. They are vastly entertaining, useful, deceiving, or disastrous, depending upon the viewpoint.

Incidentally, a few so-called illusions will be discussed which are not due strictly to errors of the visual sense or of the intellect. Examples of these are the mirage and certain optical effects employed by the magician. In such cases neither the visual sense nor the intellect errs. In the case of the mirage rays of light coming from the object to the eye are bent from their usual straight-line course and the object appears to be where it really is not. However, with these few exceptions, which are introduced for their specific interest and for the emphasis they give to the "true" illusion, it will be understood that illusions in general as hereinafter discussed will mean those due to the visual mechanism or to errors of judgment or intellect. For the sake of brevity we might say that they are those due to errors of visual perception. Furthermore, only those of a "static" type will be considered; that is, the vast complexities due to motion are not of interest from the viewpoint of the aims of this book.

There are two well-known types of misleading perceptions, namely illusions and hallucinations. If, for example, two lines appear of equal length and are not, the error in judgment is responsible for what is termed an "illusion." If the perceptual consciousness of an object appears although the object is not present, the result is termed an "hallucination." For example, if something is seen which does not exist, the essential factors are supplied by the imagination. Shadows are often wrought by the imagination into animals and even human beings bent upon evil purpose. Ghosts are created in this manner. Hallucinations depend largely upon the recency, frequency, and vividness of past experience. A consideration of this type of misleading perception does not advance the aims of this book and therefore will be omitted.

The connection between the material and mental in vision is incomprehensible and apparently must ever remain so. Objects emit or reflect light and the optical mechanism known as the eye focuses images of the objects upon the retina. Messages are then carried to the brain where certain molecular vibrations take place. The physiologist records certain physical and chemical effects in the muscles, nerves, and brain and behold! there appears consciousness, sensations, thoughts, desires, and volitions. How? and, Why? are questions which may never be answered.

It is dangerous to use the word _never_, but the ultimate answers to those questions appear to be so remote that it discourages one from proceeding far over the hazy course which leads toward them. In fact, it does not appreciably further the aims of this book to devote much space to efforts toward explanation. In covering this vast and complex field there are multitudes of facts, many hypotheses, and numerous theories from which to choose. Judgment dictates that of the limited space most of it be given to the presentation of representative facts. This is the reasoning which led to the formulation of the outline of chapters.

Owing to the vast complex beyond the physical phenomena, physical measurements upon objects and space which have done so much toward building a solid foundation for scientific knowledge fail ultimately to provide an exact mathematical picture of that which is perceived. Much of the author's previous work has been devoted to the physical realities but the ever-present differences between physical and perceptive realities have emphasized the need for considering the latter as well.

Illusions are legion. They greet the careful observer on every hand. They play a prominent part in our appreciation of the physical world. Sometimes they must be avoided, but often they may be put to work in various arts. Their widespread existence and their forcefulness make visual perception the final judge in decoration, in painting, in architecture, in landscaping, in lighting, and in other activities. The ultimate limitation of measurements with physical instruments leaves this responsibility to the intellect. The mental being is impressed with things as perceived, not with things as they are. It is believed that this intellectual or judiciary phase which plays such a part in visual perception will be best brought out by examples of various types of static illusions coupled with certain facts pertaining to the eye and to the visual process as a whole.

In special simple cases it is not difficult to determine when or how nearly a perception is true but in general, agreement among normal persons is necessary owing to the absence of any definite measuring device which will span the gap between the perception and the objective reality. Illusions are sometimes called "errors of sense" and some of them are such, but often they are errors of the intellect. The senses may deliver correctly but error may arise from imagination, inexperience, false assumptions, and incorrect associations, and the recency, frequency, and vividness of past experience. The gifts of sight are augmented by the mind with judgments based upon experience with these gifts.

The direct data delivered by the visual sense are light, intensity, color, direction. These may be considered as simple or elemental sensations because they cannot be further simplified or analyzed. At this point it is hoped that no controversy with the psychologist will be provoked. In the space available it appears unfruitful to introduce the many qualifications necessary to satisfy the, as yet uncertain or at least conflicting, definitions and theories underlying the science of psychology. If it is necessary to add darkness to the foregoing group of elemental visual sensations, this will gladly be agreed to.

The perceptions of outline-form and surface-contents perhaps rank next in simplicity; however, they may be analyzed into directions. The perception of these is so direct and so certain that it may be considered to be immediate. A ring of points is apparently very simple and it might be considered a direct sense-perception, but it consists of a number of elemental directions.

The perception of solid-form is far more complex than outline-form and therefore more liable to error. It is judged partially by binocular vision or perspective and partly by the distribution of light and shade. Colors may help to mold form and even to give depth to flat surfaces. For example, it is well known that some colors are "advancing" and others are "retiring."

Perhaps of still greater complexity are the judgments of size and of distance. Many comparisons enter such judgments. The unconscious acts of the muscles of the eye and various external conditions such as the clearness of the atmosphere play prominent parts in influencing judgment. Upon these are superposed the numerous psycho-physiological phenomena of color, irradiation, etc.