Visual Illusions: Their Causes, Characteristics and Applications

by Matthew Luckiesh

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.