On viewing a rapidly revolving wheel of an automobile as it proceeds down the street, occasionally it will be seen to cease revolving momentarily. These apparently are accounted for by involuntary eye-movements which take place regardless of the effort made to fixate vision. If the eyelids are almost closed, streamers appear to radiate in various directions from a light-source.
Movements of the eyelids when nearly closed sometimes cause objects to appear to move. These may be accounted for perhaps by the distortion of the moist film which covers the cornea. The effects of these and many others enter into visual illusions, as will be seen here and there throughout the chapters which follow.
A description of the eye by no means suffices to clarify the visual process. Even the descriptions of various phenomena in the preceding chapter accomplish little more than to acquaint the reader with the operation of a mechanism, although they suggest the trend of the explanations of many illusions.
At best only monocular vision has been treated, and it does not exist normally for human beings. A person capable only of monocular vision would be like Cyclops Polyphemus. We might have two eyes, or even, like Argus, possess a hundred eyes and still not experience the wonderful advantages of binocular vision, for each eye might see independently.
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The phenomena of binocular vision are far less physical than those of monocular vision. They are much more obscure, illusory, and perplexing because they are more complexly interwoven with or allied to psychological phenomena. The sense of sight differs considerably from the other senses. The sense of touch requires solid contact usually ; taste involves liquid contact; smell, gaseous contact; and hearing depends upon a relay of vibrations from an object through another medium usually air , resulting finally in contact.
However, we perceive things at a distance through vibration [Pg 30] electromagnetic waves called light conveyed by a subtle, intangible, universal medium which is unrecognizable excepting as a hypothetically necessary bearer of light-waves or, more generally, radiant energy. It also is interesting to compare the subjectiveness and objectiveness of sensations.
The sensation of taste is subjective; it is in us, not in the body tasted. In smell we perceive the sensation in the nose and by experience refer it to an object at a distance. The sensation of hearing is objective; that is, we refer the cause to an object so completely that there is practically no consciousness of sensation in the ear. In sight the impression is so completely projected outward into space and there is so little consciousness of any occurrence in the eye that it is extremely difficult to convince ourselves that it is essentially a subjective sensation.
The foregoing order represents the sense-organs in increasing specialization and refinement.
In the two higher senses—sight and hearing—there is no direct contact with the object and an intricate mechanism is placed in front of the specialized nerve to define and to intensify the impression. In the case of vision this highly developed instrument makes it possible to see not only light but objects. As we go up the scale of vertebrate animals we find that there is a gradual change of the position of the eyes from the sides to the front of the head and a change of the inclination of the optical axes of the two eyes from degrees to parallel.
There is also evident a gradual increase in the fineness of the [Pg 31] bacillary layer of the retina from the margins toward the center, and, therefore, an increasing accuracy in the perception of form. This finally results in a highly organized central spot or fovea which is possessed only by man and the higher monkeys. Proceeding up the scale we also find an increasing ability to converge the optic axes on a near point so that the images of the point may coincide with the central spots of both retinas.
These changes and others are closely associated with each other and especially with the development of the higher faculties of the mind. Binocular vision in man and in the higher animals is the last result of the gradual improvement of the most refined sense-organ, adapting it to meet the requirements of highly complex organisms. It cannot exist in some animals, such as birds and fishes, because they cannot converge their two optical axes upon a near point. When a chicken wishes to look intently at an object it turns its head and looks with one eye.
Such an animal sees with two eyes independently and possibly moves them independently.
The normal position of the axes of human eyes is convergent or parallel but it is possible to diverge the axes. In fact, with practice it is possible to diverge the axes sufficiently to look at a point near the back of the head, although, of course, we do not see the point. The movement of the eyes is rather complex. When they move together to one side or the other or up and down in a vertical plane there is no rotation of the optical axes; that is, no torsion. When the visual plane is elevated and the eyes move to the [Pg 32] right they rotate to the right; when they move to the left they rotate to the left.
When the visual plane is depressed and the eyes move to the right they rotate to the left; when they move to the left they rotate to the right. Through experience we unconsciously evaluate the muscular stresses, efforts, and movements accompanying the motion of the eyes and thereby interpret much through visual perception in regard to such aspects of the external world as size, shape, and distance of objects.
Even this brief glimpse of the principal movements of the eyes indicates a complexity which suggests the intricacy of the explanations of certain visual phenomena. At this point it appears advantageous to set down the principal modes by which we perceive the third dimension of space and of objects and other aspects of the external world.
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They are as follows: 1 extent; 2 clearness of brightness and color as affected by distance; 3 interference of near objects with those more distant; 4 elevation of objects; 5 variation of light and shade on objects; 6 cast shadows; 7 perspective; 8 variation of the visor angle in proportion to distance; 9 muscular effort attending accommodation of the eye; 10 stereoscopic vision; 11 muscular effort attending convergence of the axes of the eyes.
It will be recognized that only the last two are necessarily concerned with binocular vision. These varieties of experiences may be combined in almost an infinite variety of proportions. Wundt in his attempt to explain visual perception considered chiefly three factors: 1 the retinal [Pg 33] image of the eye at rest; 2 the influence of the movements of one eye; and, 3 the additional data furnished by the two eyes functioning together.
There are three fields of vision corresponding to the foregoing. These are the retinal field of vision, the monocular field, and the binocular field. The retinal field of vision is that of an eye at rest as compared with the monocular field, which is all that can be seen with one eye in its entire range of movement and therefore of experience. The retinal field has no clearly defined boundaries because it finally fades at its indefinite periphery into a region where sensation ceases. It might be tiresome to follow detailed analyses of the many modes by which visual perception is attained, so only a few generalizations will be presented.
For every voluntary act of sight there are two adjustments of the eyes, namely, focal and axial. In the former case the ciliary muscle adjusts the lens in order to produce a defined image upon the retina. In axial adjustments the two eyes are turned by certain muscles so that their axes meet on the object looked at and the images of the object fall on the central-spots of the retina. These take place together without distinct volition for each but by the single voluntary act of looking.
Through experience the intellect has acquired a wonderful capacity to interpret such factors as size, form, and distance in terms of the muscular movements in general without the observer being conscious of such interpretations. Binocular vision is easily recognized by holding a finger before the eyes and looking at a point beyond [Pg 34] it.
The result is two apparently transparent fingers. An object is seen single when the two retinal images fall on corresponding points. Direction is a primary datum of sense. The property of corresponding points of the two retinas binocular vision and consequently of identical spatial points in the two visual fields is not so simple.
The one view results in the nativistic , the other in the empiristic theory. Inasmuch as some scientists are arrayed on one side and some on the other, it appears futile to dwell further upon this aspect. After noting after-images, motes floating in the field of view caused by defects in the eye-media and various other things, it is evident that what we call the field of view is the external projection into space of retinal states.
All the variations of the latter, such as images and shadows which are produced in the external field of one eye, are faithfully reproduced in the external field of the other eye. This sense of an external visual field is ineradicable. Even when the eyes are closed the external field is still there; the imagination or intellect projects it outward. Objects at different distances cannot be [Pg 35] seen distinctly at the same time but by interpreting the eye-movements as the point of sight is run backward and forward varying convergence of the axes the intellect practically automatically appraises the size, form, and distance of each object.
Obviously, experience is a prominent factor. The perception of the third dimension, depth or relative distance, whether in a single object or a group of objects, is the result of the successive combination of the different parts of two dissimilar images of the object or group. As already stated, the perception of distance, size, and form is based partly upon monocular and partly upon binocular vision, and the simple elements upon which judgments of these are based are light, shade, color, intensity, and direction.
Although the interpretation of muscular adjustments plays a prominent part in the formation of judgments, the influences of mathematical perspective, light, shade, color, and intensity are more direct. Judgments based upon focal adjustment monocular are fairly accurate at distances from five inches to several yards. Those founded upon axial adjustment convergence of the two axes in binocular vision are less in error than the preceding ones.
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They are reliable to a distance of about feet. Judgments involving mathematical perspective are of relatively great accuracy without limits. Those arrived at by interpreting aerial perspective haziness of atmosphere, reduction in color due to atmospheric absorption, etc. The measuring power of the eye is more liable [Pg 36] to error when the distances or the objects compared lie in different directions.
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A special case is the comparison of a vertical distance with a horizontal one. It is not uncommon to estimate a vertical distance as much as 25 per cent greater than an actually equal horizontal distance. In general, estimates of direction and distance are comparatively inaccurate when only one eye is used although a one-eyed person acquires unusual ability through a keener experience whetted by necessity. A vertical line drawn perpendicular to a horizontal one is likely to appear bent when viewed with one eye. Its apparent inclination is variable but has been found to vary from one to three degrees.
The error in the estimate of size is in reality an error in the estimation of distance except in those cases where the estimate is based directly upon a comparison with an object of supposedly known size.
Chronology of Albert A. Michelson and Acknowledgements
An amusing incident is told of an old negro who was hunting for squirrels. He shot several times at what he supposed to be a squirrel upon a tree-trunk and his failure to make a kill was beginning to weaken his rather ample opinion of his skill as a marksman. Similarly, a gnat in the air might appear to be an airplane under certain favorable circumstances.
It is interesting to note that the estimated size of the disk of the sun or moon varies from the size of a saucer to that of the end of a barrel, [Pg 37] although a pine tree at the horizon-line may be estimated as 25 feet across despite the fact that it may be entirely included in the disk of the sun setting behind it.
Double images play an important part in the comparison of distances of objects. Suppose two horizontal wires or clotheslines about fifty feet away and one a few feet beyond the other.