All visual objects suddenly recede to the apparent distance of the horizon and remain in that position several minutes, returning at the end of this period to their original positions. This return movement is very slow at the beginning, but the latter phase of the movement is quite rapid. If the subject closes her eyes while the objects appear at their distant position she cannot even imagine those objects located anywhere except at their apparent distant position.
In all cases (encountered by Carr) the motion in both directions is an actual experience reality and the subject was helpless as to initiating, stopping, or modifying the course of the illusion in any way. Objects and even visual images (which are subject to the same illusions) decrease in size in proportion to the amount of backward movement and grow larger again on their return movement. The objects are always clearly defined as if in good focus. In this particular case the illusion occurred about twice a year, under a variety of conditions of illumination, at various times of the day, but apparently under conditions of a rather pronounced fatigue.
In regard to the variation in the size of objects, many who have experienced these illusions of depth testify that the size seems to change in proportion to the apparent distance, according to the law of perspective. Some persons appear in doubt as to this change and a few have experienced the peculiar anomaly of decreasing size as the objects apparently approached.
Many persons who have experienced these peculiar illusions report no change in the distinctness of objects; almost as many are uncertain regarding this point; and as many report a change in distinctness. Apparently there are phases of hallucinatory origin so that there is a wide variety of experiences among those subject to this type of illusion.
According to Carr’s investigation internal conditions alone are responsible for the illusion with more persons than those due to external conditions alone. With some persons a combination of internal and external conditions seem to be a necessity. Fixation of vision appears to be an essential objective condition for many observers. That is, the illusion appeared while fixating a speaker or singer in a church or a theater. With others the illusion occurs while reading. Some reported that fixation upon checkered or other regularly patterned objects was an essential condition. Among the subjective conditions reported as essential are steady fixation, concentration of attention, complete mental absorption, dreamy mental abstraction, and fatigue.
Ocular defects do not appear to be essential, for the illusions have been experienced by many whose eyes were known to be free from any abnormalities.
Period of life does not appear to have any primary influence, for those who are subject to these peculiar illusions often have experienced them throughout many years. In some cases it is evident that the illusions occur during a constrained eye position, while lying down, immediately upon arising from bed in the morning, and upon opening the eyes after having had them closed for some time. However, the necessity for these conditions are exceptional.
The control of these illusions of depth, that is, the ability to create or to destroy them, appears to be totally lacking for most of those who have experienced them. Some can influence them, a few can destroy them, a few can indirectly initiate them, but those who can both create and destroy them appear to be rare.
It may seem to the reader that the latter part of this chapter departs from the main trend of this book, for most of these illusions of depth are to a degree of hallucinatory origin. Furthermore it has been the intention to discuss only those types of illusions which are experienced quite uniformly and universally. The digression of this chapter is excused on the basis of affording a glimpse along the borderland of those groups of illusions which are nearly universally experienced. Many other phases of depth illusions have been recorded in scientific literature. The excellent records presented by Carr could be drawn upon for further glimpses, but it appears that no more space should be given to this exceptional type. The reader should be sufficiently forewarned of this type and should be able to take it into account if peculiarities in other types appear to be explainable in this manner. However, in closing it is well to emphasize the fact that the hallucinatory aspect of depth illusions is practically absent in types of illusions to which attention is confined in other chapters.
VIII
IRRADIATION AND BRIGHTNESS-CONTRAST
Many interesting and striking illusions owe their existence to contrasts in brightness. The visual phenomenon of irradiation does not strictly belong to this group, but it is so closely related to it and so dependent upon brightness-contrast that it is included. A dark line or spot will appear darker in general as the brightness of its environment is increased; or conversely, a white spot surrounded by a dark environment will appear brighter as the latter is darkened. In other words, black and white, when juxtaposed, mutually reinforce each other. Black print on a white page appears much darker than it really is. This may be proved by punching a hole in a black velvet cloth and laying this hole over a “black” portion of a large letter. The ink which appeared so black in the print, when the latter was surrounded by the white paper, now appears only a dark gray. Incidentally a hole in a box lined with black velvet is much darker than a piece of the black velvet surrounding the hole.
The effects of brightness-contrast are particularly striking when demonstrated by means of lighting, a simple apparatus being illustrated diagrammatically in Fig. 62. For example, if a hole H is cut in an opaque white blotting paper and a large piece of the white blotting paper is placed at C, the eye when placed before the opening at the right will see the opening at H filled with the background C. The hole H may be cut in thin metal, painted a dull white, and may be of the shape of a star. This shape provides an intimacy between the hole and its environment which tends to augment the effects of contrasts. R and F are respectively the rear and front lamps. That is, the lamps R illuminate C, which “fills” the hole and apparently is the hole; and the lamps F illuminate the diffusing white environment E. The two sets of lamps may be controlled by separate rheostats, but if the latter are unavailable the lamps (several in each set) may be arranged so that by turning each one off or on, a range of contrasts in brightness between E and H (in reality C) may be obtained. (By using colored lamps and colored papers as discussed in Chapter IX the marvelous effects of color-contrast may be superposed upon those of brightness-contrast.)
Fig. 62.—Simple apparatus for demonstrating the remarkable effects of contrasts in brightness and color.
If, for example, C is very feebly illuminated and E is very bright, C will be pronounced black; but when the lamps F are extinguished and no light is permitted to reach E, the contrast is reversed, and C may actually appear “white.” Of course, it is obvious that white and black are relative terms as encountered in such a case. In fact in brightness-contrasts relative and not absolute values of brightness are usually the more important. In order to minimize the stray light which emerges from H, it is well to paint the inside of both compartments black with the exception of sufficiently large areas of C and E. The use of black velvet instead of black paint is sometimes advisable. It is also well to screen the lamps as suggested in the diagram. This simple apparatus will demonstrate some very striking effects of contrasts in brightness and will serve, also, to demonstrate even more interesting effects of contrasts in color.
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 Vs, 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 Vs. An excellent demonstration may be made by the reader by using two Vs, 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 Vs when the card is laid upon Fig. 63. The grays in the holes should now appear alike because their environments are similar.
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.
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.
No brightness-contrasts are seen correctly and often the illusions are very striking. If a series of gray papers is arranged from black to white, with the successive pieces overlapped or otherwise juxtaposed, a series of steps of uniform brightness is not seen. An instrument would determine the brightness of each as uniform, but to the eye the series would appear somewhat “fluted.” That is, where a light gray joined a darker gray the edge of the former would appear lighter than its actual brightness, and the edge of the darker gray would appear darker than it should. This may also be demonstrated by laying a dozen pieces of white tissue paper in a pile in such a manner that a series of 1, 2, 3, 4, etc., thickness would be produced. On viewing this by transmitted light a series of grays is seen, and the effect of contrast is quite apparent. Such a pattern can be made photographically by rotating before a photographic plate a disk with openings arranged properly in steps.
Many demonstrations of the chief illusion of brightness-contrast are visible at night under glaring lighting conditions. It is difficult or impossible to see objects beyond automobile headlights, and adjacent to them, in the visual field. Objects similarly located in respect to any surface sufficiently bright are more or less obscured. Characters written upon a blackboard, placed between two windows, may be invisible if the surfaces seen through the window are quite bright, unless a sufficient quantity of light reaches the blackboard from other sources. Stage-settings have been changed in perfect obscurity before an audience by turning on a row of bright lights at the edge of the stage-opening. The term “blinding light” owes its origin to this effect of brightness-contrast.
The line of juncture between a bright and a dark surface may not be seen as a sharp line, but as a narrow band of gray. When this is true it is possible that an undue amount of area is credited to the white. In preceding paragraphs we have seen the peculiar effect at the border-lines of a series of grays. This may have something to do with the estimate; however, irradiation may be due to excitation of retinal rods and cones adjacent to, but not actually within the bright image.
A remarkable effect which may be partially attributable to irradiation can be produced by crossing a grating of parallel black lines with an oblique black line. At the actual crossings the black appears to run up the narrow angle somewhat like ink would under the influence of surface tension. This is particularly striking when two gratings or even two ordinary fly-screens are superposed. The effect is visible when passing two picket-fences, one beyond the other. If a dark object is held so that a straight edge appears to cross a candle-flame or other light-source, at this portion the straight edge will appear to have a notch in it.
Irradiation in general has been defined as the lateral diffusion of nervous stimuli beyond the actual stimulus. It is not confined to the visual sense but irradiation for this sense is a term applied to the apparent enlargement of bright surfaces at the expense of adjacent darker surfaces. The crescent of the new moon appears larger in radius than the faint outline of the darker portion which is feebly illuminated chiefly by light reflected from the earth’s surface. A filament of a lamp appears to grow in size as the current through it is slowly increased from a zero value; that is, as it increases in brightness. In Fig. 65 the small inner squares are of the same size but the white square appears larger than the black one. It seems that this apparent increase is made at the expense of the adjacent dark area. This phenomenon or illusion is strongest when the brightness is most intense, and is said to be greatest when the accommodation is imperfect. A very intense light-source may appear many times larger than its actual physical size.
Fig. 65.—The phenomenon of irradiation.
Doubtless a number of factors may play a part in this phenomenon. It appears possible that there is a rapid spreading of the excitation over the retina extending quite beyond the border of the more intensely stimulated region, but this must be practically instantaneous in order to satisfy results of experiments. Eye-movements may play some part for, despite the most serious efforts to fixate the point of sight, a fringe will appear on the borders of images which is certainly due to involuntary eye-movements.
Irradiation has also been ascribed to spherical aberration in the eye-lens and to diffraction of light at the pupil. Printed type appears considerably reduced in size when the pupil is dilated with atropin and is restored to normal appearance when a small artificial pupil is placed before the dilated pupil. It has been suggested that chromatic aberration in the eye-lens is a contributory cause, but this cannot be very important, for the illusion is visible with monochromatic light which eliminates chromatic aberration. The experimental evidence appears to indicate that the phenomenon is of a physical nature.
There are variations in the effects attributable to radiation, and it is difficult to reduce them to simple terms. Perhaps it may aid the reader to have before him the classification presented by Boswell.[4] He describes the varieties of irradiation as follows:
1. Very rapid spreading of the excitation over the retina extending far beyond the border of the stimulated region and occurring immediately upon impact of the stimulating light.
2. Irradiation within the stimulated portion of the retina after the form of a figure becomes distinctly perceptible.
3. Emanations of decreasing intensity extend themselves outward and backward from a moving image until lost in the darkness of the background.
4. A well known form of irradiation which occurs when a surface of greater intensity enlarges itself at the expense of one of less intensity.
5. A form having many of the characteristics of the first type, but occurring only after long periods of stimulation, of the magnitude of 30 to 60 seconds or more.
IX
COLOR
In order to simplify the presentation of the general subject, discussions of color have been omitted in so far as possible from the preceding chapters. There are almost numberless phenomena involving color, many of which are illusions, or seemingly so. It will be obvious that many are errors of sense; some are errors of judgment; others are errors due to defects of the optical system of the eye; and many may be ascribed to certain characteristics of the visual process. It is not the intention to cover the entire field in detail; indeed, this could not be done within the confines of a large volume. However, substantial glimpses of the more important phases of color as related to illusions are presented in this chapter. In the early chapters pertaining to the eye and to vision some of the following points were necessarily touched upon, but the repetition in the paragraphs which follow is avoided as much as possible.
Simultaneous Contrast.—That the life of color is due to contrast is demonstrable in many ways. If a room is illuminated by deep red light, at first this color is very vivid in consciousness; however, gradually it becomes less saturated. After a half hour the color is apparently a much faded red but upon emerging from the room into one normally lighted, the latter appears very markedly greenish in tint. The reason that the pure red light does not appear as strongly colored as it really is, is due to the lack of contrast. In a similar manner at night we see white objects as white even under the yellowish artificial light. The latter appears very yellow in color when it is first turned on as daylight wanes but as darkness falls and time elapses it gradually assumes a colorless appearance.
An apparatus constructed after the plan of Fig. 62 is very effective for demonstrating the remarkable effects of color-contrast but some additions will add considerably to its convenience. If the lamps F are divided into three circuits, each emitting, respectively, red, green, and blue primary colors, it is possible by means of controlling rheostats to illuminate E, the environment, with light of any hue (including purple), of any saturation, and of a wide range of intensities or resulting brightnesses. Thus we have a very simple apparatus for quickly providing almost numberless environments for H. The same scheme can be applied to lamps R, with the result that a vast array of colors may be seen through the hole H. If the hole is the shape of the star in Fig. 66 it will be found very effective. The observer will actually see a star of any desired color amid an environment of any desired color. Care should be taken to have the star cut in very thin material in order to eliminate conspicuous boundary lines. It is quite satisfactory to use a series of colored papers on a slide at C and ordinary clear lamps at R. By means of this apparatus both contrasts—hue and brightness—may be demonstrated. Of course, for black and white only brightness-contrast is present; but in general where there is color-contrast there is also brightness-contrast. The latter may be reduced or even eliminated if the brightness of the star and of its surroundings are made equal, but it is difficult to make a satisfactory balance in this respect. Assuming, however, that brightness-contrast is eliminated, we have left only hue and saturation contrast, or what will be termed (rather loosely, it is admitted) color-contrast.
Fig. 66.—An excellent pattern for demonstrating color-contrast.
If the surroundings are dark and, for example, an orange star is seen alone, it does not appear very colorful. However, if the surroundings are now made bright with white light, the star appears quite saturated. With blue or green light the orange star appears even more intensely orange, but when the color-contrast is reduced, as in the case of yellow or red surroundings, the vividness of the orange star again decreases. This may be summarized by stating that two widely different colors viewed in this manner will mutually affect each other so that they appear still more different in hue. If their hues are close together spectrally this effect is not as apparent. For example, if orange and green are contrasted, the orange will appear reddish in hue and the green will appear bluish.
Let us now assume the star to be white, and that the surroundings are of any color of approximately the same brightness. The star which is really white will now appear decidedly tinted and of a hue approximately complementary to that of the surroundings. When the latter are of a green color the white star will assume a purplish tinge; when red the white star will appear of a blue-green tint; when yellow the white star will appear bluish. This is an illusion in any sense of the term.
The strength of this illusion caused by simultaneous contrast is very remarkable. For example, if a grayish purple star is viewed amid intense green surroundings it will appear richly purple, but when the surroundings are changed to a rich purple the grayish purple star will even appear greenish. The apparent change of a color to its complementary by merely altering its environment is really a remarkable illusion.
The importance of simultaneous contrast is easily demonstrated upon a painting by isolating any colored object from its surroundings by means of a hole in a gray card. For example, an orange flower-pot amid the green foliage of its surroundings will appear decidedly different in color and brightness than when viewed through a hole in a white, black, or gray cardboard. By means of colored papers the same color may be placed in many different environments and the various contrasts may be viewed simultaneously. The extent of the illusion is very evident when revealed in this simple manner. However, too much emphasis cannot be given to Figs. 62 and 66 as a powerful means for realizing the greatest effects.
After-images.—After looking at bright objects we see after-images of the same size and form which vary more or less in color. These after-images are due to persistence or fatigue of the visual process, depending upon conditions. After looking at the sun for a moment a very bright after-image is seen. Undoubtedly this at first is due to a persistence of the visual process, but as it decays it continuously changes color and finally its presence is due to fatigue.
After-images may be seen after looking intently at any object and then directing the eyes toward a blank surface such as a wall. A picture-frame will be seen as a rectangular after-image; a checkered pattern will be seen as a checkered after-image. When these after-images are projected upon other objects it is obvious that the appearance of the latter is apparently altered especially when the observer is not conscious of the after-image. The effects are seen in paintings and many peculiar phenomena in the various arts are directly traceable to after-images.
It appears unnecessary to detail the many effects for the explanations or at least the general principles of after-images are so simple that the reader should easily render an analysis of any given case.
Let us assume that vision is fixed upon a green square upon a gray or white background. Despite the utmost effort on the part of the observer to gaze fixedly upon this green square, the latter will begin to appear fringed with a pinkish border. This is due to the after-image of the green square and it is displaced slightly due to involuntary eye-movements. After gazing as steadily as possible for a half minute, or even less, if the point of sight is turned to the white paper a pink square is seen upon it. Furthermore, this pink square moves over the field with the point of sight. This is the type most generally noticed.
After-images have been classified as positive and negative. The former are those in which the distribution of light and shade is the same as in the original object. Those in which this distribution is reversed, as in the photographic negative, are termed “negative.” After-images undergo a variety of changes in color but in general there are two important states. In one the color is the same as in the original object and in the other it is approximately complementary to the original color. In general the negative after-image is approximately complementary in color to the color of the original object.
After-images are best observed when the eyes are well rested, as in the morning upon awakening. With a little practice in giving attention to them, they can be seen floating in the air, in the indefinite field of the closed eyes, upon a wall, or elsewhere, and the changes in the brightness and color can be readily followed. Negative after-images are sometimes very persistent and therefore are more commonly noticed than positive ones. The positive after-image is due to retinal inertia, that is, to the persistency of the visual process after the actual stimulus has been removed. It is of relatively brief duration. If an after-image of a window is projected on a white area it is likely to appear as a “negative” when projected upon a white background, and as a “positive” upon a dark background, such as is readily provided by closing the eyes. It may be of interest for the reader to obtain an after-image of a bright surface of a light-source and study its color changes with the eye closed. Upon repeating the experiment the progression of colors will be found to be always the same for the same conditions. The duration of the after-image will be found to vary with the brightness and period of fixation of the object.
It is interesting to note that an after-image is seen with difficulty when the eyes are in motion, but it becomes quite conspicuous when the eyes are brought to rest.
An after-image due to the stimulation of only one eye sometimes seems to be seen by the other eye. Naturally this has given rise to the suggestion that the seat of after-images is central rather than peripheral; that is, in the brain rather than at the retina. However, this is not generally the case and the experimental evidence weighs heavily against this conclusion.
If Fig. 52 is revolved about its center and fixated for some time striking effects are obtained upon looking away suddenly upon any object. The latter will appear to shrink if the spiral has seemed to run outward, or to expand if the spiral has seemed to run inward. These are clearly after-images of motion.
As stated elsewhere, we may have illusions of after-images as well as of the original images. For example, if a clearly defined plane geometrical figure such as a cross or square is bright enough to produce a strong after-image, the latter when projected upon a perspective drawing will appear distorted; that is, it is likely to appear in perspective.
A simple way of demonstrating after-images and their duration is to move the object producing them. For example, extinguish a match and move the glowing end. If observed carefully without moving the eye a bluish after-image will be seen to follow the glowing end of the match. In this case the eyes should be directed straight ahead while the stimulus is moving and the observation must be made by averted or indirect vision.
Growth and Decay of Sensation.—Although many after-images may not be considered to be illusions in the sense in which the term is used here, there are many illusions in which they at least play a part. Furthermore, it is the intention throughout these chapters to adhere to a discussion of “static” illusions, it is difficult to avoid touching occasionally upon motion. The eyes are in motion most of the time, hence, certain effects of an illusory nature may be superposed upon stationary objects.
The persistence of vision has been demonstrated by every small boy as he waved a glowing stick seized from a bonfire. Fireworks owe much of their beauty to this phenomenon. A rapidly revolving spoked wheel may appear to be a more or less transparent disk, but occasionally when a rapid eye-movement moves the point of sight with sufficient speed in the direction of motion, the spokes reappear momentarily. Motion-pictures owe their success to this visual property—the persistence of vision. If a lantern-slide picture be focused upon black velvet or upon a dark doorway, the projected image will not be seen. However, if a white rod be moved rapidly enough in the plane of the image, the latter may be seen in its entirety. The mixture of colors, by rotating them on disks, owes its possibility to the persistence of the color-sensations beyond the period of actual stimulation. The fact that it takes time for sensations of light to grow and decay is not as important here as the fact that the rates of growth, and also of decay, vary for different colors. In general, the growth and the decay are not of similar or uniform rates. Furthermore, the sensation often initially “overshoots” its final steady value, the amount of “overshooting” depending upon the intensity and color of the stimulus. These effects may be witnessed in their extensive variety by rotating disks so constructed that black and various colors stimulate the retina in definite orders.
An interesting case of this kind may be demonstrated by rotating the disk shown in Fig. 67. Notwithstanding the fact that these are only black and white stimuli, a series of colored rings is seen varying from a reddish chocolate to a blue-green. Experiment will determine the best speed, which is rather slow under a moderate intensity of illumination. The reddish rings will be outermost and the blue-green rings innermost when the disk is rotated in one direction. Upon reversing the direction of rotation the positions of these colored rings will be reversed. By using various colors, such as red and green for the white and black respectively, other colors will be produced, some of which are very striking. The complete explanation of the phenomenon is not clear, owing to the doubt which exists concerning many of the phenomena of color-vision, but it appears certain that the difference in the rates of growth and decay of the various color-sensations (the white stimulus includes all the spectral hues of the illuminant) is at least partially, if not wholly, responsible.
Fig. 67.—By rotating this Mason (black and white) disk color-sensations are produced.
An interesting effect, perhaps due wholly or in part to the differences in the rates of growth and decay of color-sensations, may be observed when a colored pattern is moved under a low intensity of illumination, the eyes remaining focused upon a point in space at about the same distance as the object. A square of red paper pasted in the center of a larger piece of blue-green paper is a satisfactory object. On moving this object gently, keeping the point of sight fixed in its plane of movement, the central red square will appear to shake like jelly and a decided trail of color will appear to cling to the lagging edge of the central square. Perhaps chromatic aberration plays some part in making this effect so conspicuous.
A similar case will be noted in a photographic dark-room illuminated by red light upon observing the self-luminous dial of a watch or clock. When the latter is moved in the plane of the dial, the greenish luminous figures appear separated from the red dial and seem to lag behind during the movement. For such demonstrations it is well to experiment somewhat by varying the intensity of the illumination and the speed of movement. Relatively low values of each appear to be best.
Although the various color-sensations grow and decay at different rates, the latter depend upon conditions. It appears that blue-sensation rises very rapidly and greatly overshoots its final steady value for a given stimulus. Red ranks next and green third in this respect. The overshooting appears to be greater for the greater intensity of the stimulus. The time required for the sensation to reach a steady value depends both upon the spectral character and the brightness of the color but is usually less than a second.
Chromatic Aberration.—It is well known that the eye focuses different spectral colors at different points. This is true of any simple lens and the defect is overcome in the manufacture of optical instruments by combining two lenses consisting respectively of glasses differing considerably in refractive index. If a white object is viewed by the eye, it should appear with a purplish fringe; however, the effect is observed more readily by viewing a light-source through a purple filter which transmits only violet and red light. The light-source will have a red or a violet fringe, depending upon the accommodation or focus of the eye.
This effect is perhaps best witnessed on viewing a line spectrum such as that of the mercury arc, focused upon a ground glass. The violet and blue lines are not seen in good focus when the eyes are focused upon the green and yellow lines. Furthermore, the former can be seen in excellent focus at a distance too short for accommodating the eyes to the green and the yellow lines. This experiment shows that the focal length of the optical system of the eye is considerably shorter for the spectral hues of shorter wave-length (violet, blue) than for those of longer wave-length (such as yellow). Narrow slits covered with diffusing glass and illuminated respectively by fairly pure blue, green, yellow, and red lights may be substituted.
The effect may be demonstrated by trying to focus fine detail such as print when two adjacent areas are illuminated by blue and red lights respectively. It is also observed when fine detail such as black lines are held close to the eye for colored fringes are seen. This optical defect is responsible for certain visual illusions.
An excellent demonstration of chromatic aberration in the eye is found by viewing fine detail through a purple filter. Now if a red filter be superposed on the purple one only the red light is transmitted. Notwithstanding the decrease in illumination or rather of light reaching the eye, measurement shows that finer detail can be discriminated than in the first case. A similar result is found on superposing a blue filter upon the purple one.
Retiring and Advancing Colors.—For years the artist and the decorator have felt that certain colors seem to advance nearer than others or that the latter seem to retire more than the former. The author[5] obtained actual measurements of this phenomenon, but the evidence also indicated that the effects were not the same for all persons. The phenomenon is very noticeable in the case of the image of a colored lantern-slide projected upon a screen and is readily observed when the image consists of letters of various colors. In the case of red and green letters, for example, the former appear (to most persons) to be considerably nearer the observer than the green letters. It has appeared to the writer that the illusion is apparent even for white letters upon a dark background. In general, the colors whose dominant hues are of the shorter wave-lengths (violet, blue, blue-green, green) are retiring and those whose dominant hues are of the longer wave-lengths (yellow, orange, red) are advancing.
Fig. 68.—For demonstrating retiring and advancing colors.
In order to obtain experimental measurements two light-tight boxes, each containing a light-source, were arranged to run independently upon tracks. Over the front end of each a diaphragm was placed so that the observer saw two characters as in Fig. 68. A saturated red filter was placed over one and a saturated blue filter over the other. In a dark room the observer saw a blue E and a red H standing out in the darkness. One of these boxes was fastened so as to be immovable and the observer moved the other to and fro by means of a cord over pulleys until the two characters appeared equi-distant from him. This was done for a series of distances of the stationary box from the observer’s eye. Nearly all the observers (without being acquainted with the positions) were obliged to set the red H further behind the blue E in order that both appeared at the same distance. This added distance for the red H was as much as 2.4 feet when the blue E was at a distance of 24 feet. In other words the difference in the positions of the two was as much as 10 per cent of the total distance in this case.
Many other interesting data were obtained but most of these are not particularly of interest here. Some of the experiments tended to show the effect of certain optical defects in the eye and the variations and even reversal of the effect for some persons were accounted for by differences in the curvatures, etc., of certain eye-media for the observers. These details are not of interest here but it may be of interest to know that the phenomenon may be accounted for by the chromatic aberration in the eye. This may not be the true explanation, or it may be only partially correct. Perhaps some of the illusion is purely psychological in origin. Certainly the illusion is very apparent to most careful observers.
Color-sensibility of the Retina.—This aspect was touched upon in Chapter III, but the differences in the sensibility of various areas of the retina to various colors are of sufficient importance to be discussed further. The ability to distinguish light and color gradually fades or decreases at the periphery of the visual field, but the actual areas of the fields of perception vary considerably, depending upon the hue or spectral character of the light reaching the retina. The extreme peripheral region of the visual field is “color-blind”; that is, color ceases to be perceived before brightness-perception vanishes in the outskirts of the visual field. These fields for various colors depend in size and contour not only upon the hue or spectral character of the light-stimuli but also upon the intensity and perhaps upon the size of the stimuli. There is some disagreement as to the relative sizes of these fields but it appears that they increase in size in the following order: green, red, blue, white (colorless). The performances of after-images, and the rates of growth and decay of sensation vary for different colors and for different areas of the retina, but it would be tedious to peruse the many details of these aspects of vision. They are mentioned in order that the reader may take them into account in any specific case.
As already stated, the central part of the visual field—the fovea upon which we depend for acute vision—contains a yellowish pigmentation, which is responsible for the term “yellow spot.” This operates as a yellow filter for this central area and modifies the appearance of visual fields quite the same as if a similar yellow filter was placed in the central position of the field of vision. The effect of the selectivity of the “yellow spot” is noticeable in viewing certain colors.
Purkinje Effect.—The relative sensibility of the retina varies for different colors with a change in brightness; or it may be better to state that the relative sensations for various colors alters as the brightness values are reduced to a low intensity. For example, if a reddish purple (consisting of red and blue or violet rays) be illuminated in such a manner that the intensity of illumination, and consequently its brightness, may be reduced from normal to a low value (approximating moonlight conditions), it will be seen to vary from reddish purple to violet. In doing this its appearance changes through the range of purples from reddish to violet. This can be accomplished by orientation of the purple surface throughout various angles with respect to the direction of light or by reducing the illumination by means of screens.
In general the Purkinje effect may be described as an increasing sensibility of the retina for light of shorter wave-lengths (violet, blue, green) as the brightness decreases, or a corresponding decreasing sensibility for light of longer wave-lengths (yellow, orange, red). The effect may be seen on any colored surfaces at twilight illumination. A blue and a red flower, which appear of the same brightness before sunset will begin to appear unequal in this respect as twilight deepens. The red will become darker more rapidly than the blue if there are no appreciable changes in the color of the daylight. Finally all color disappears. It is better to perform this experiment under artificial light, in order that the spectral character of the illuminant may be certain to remain constant. In this case rheostats must not be used for dimming the light because of the attendant changes in color or quality of the light.
The Purkinje effect may be noticed by the careful observer and it is responsible for certain illusions. Apparently it cannot operate over one portion of the retina, while the remainder is stimulated by normal intensities of light.
Retinal Rivalry.—Many curious effects may be obtained by stimulating the two retinas with lights, respectively different in color. For example, it is interesting to place a blue glass before one eye and a yellow or red one before the other. The two independent monocular fields strive for supremacy and this rivalry is quite impressive. For a moment the whole field may appear of one color and then suddenly it will appear of the other color. Apparently the fluctuation of attention is a factor. Usually it does not seem to be possible to reach a quiescent state or a perfect mixture of the two colors in this manner. The dependence of one monocular field upon the other, and also their independence, are emphasized by this experiment. It is of interest to consider the illusions of reversible perspective and others in Chapter V in this connection.