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Treatise on Physiological Optics, Volume III
By Hermann von Helmholtz, James P. C. Southall
Dover Publications, Inc.Copyright © 2005 Dover Publications, Inc.
All rights reserved.
The Theory of the Perceptions of Vision
§26. Concerning the Perceptions in General
The sensations aroused by light in the nervous mechanism of vision enable us to form conceptions as to the existence, form and position of external objects. These ideas are called visual perceptions. In this third subdivision of Physiological Optics we must try to analyze the scientific results which we have obtained concerning the conditions which give rise to visual perceptions.
Perceptions of external objects being therefore of the nature of ideas, and ideas themselves being invariably activities of our psychic energy, perceptions also can only be the result of psychic energy. Accordingly, strictly speaking, the theory of perceptions belongs properly in the domain of psychology. This is particularly true with respect to the mode of the mental activities in the case of the perceptions and with respect to the determination of their laws. Yet even here there is a wide field of investigation in both physics and physiology, inasmuch as we have to determine, scientifically as far as possible, what special properties of the physical stimulus and of the physiological stimulation are responsible for the formation of this or that particular idea as to the nature of the external objects perceived. In this part of the subject, therefore, we shall have to investigate the special properties of the retinal images, muscular sensations, etc., that are concerned in the perception of a definite position of the observed object, not only as to its direction but as to its distance; how the perception of the form of a body of three dimensions depends on certain peculiarities of the images; and under what circumstances it will appear single or double as seen by both eyes, etc. Thus, our main purpose will be simply to investigate the material of sensation whereby we are enabled to form ideas, in those relations that are important for the perceptions obtained from them. This problem can be solved entirely by scientific methods. At the same time, we cannot avoid referring to psychic activities and the laws that govern them, as far as they are concerned with the perception of the senses. But the discovery and description of these psychic activities will not be regarded as an essential part of our present task, because then we might run the risk of losing our hold of established facts and of not adhering steadily to a method founded on clear, well-recognized principals. Thus, for the present at least, I think the psychological domain of the physiology of the senses should be kept separate from pure psychology, whose province really is to establish as far as possible the laws and nature of the processes of the mind.
Still we cannot altogether avoid speaking of the mental processes that are active in the sense-perceptions, if we wish to see clearly the connection between the phenomena and to arrange the facts in their proper relation to one another. And hence, to prevent any misconception of the plan I have in mind, I intend to devote the latter part of this chapter to a discussion of the conclusions which I think can be inferred with respect to these mental processes. And yet we know by experience that people very seldom come to any agreement as to abstract questions of this nature. The keenest thinkers, philosophers like KANT for instance, have long ago analyzed these relations correctly and demonstrated them, and yet there is no permanent and general agreement about them among educated people. And, therefore, in the subsequent chapters devoted specially to the theory of the visual perceptions, I shall endeavour to avoid all reference to opinions as to mental activity, as involving questions that always have been, and perhaps always will be, subjects of debate between the various metaphysical schools; so as not to distract the reader's attention from those facts about which an agreement may possibly be reached, by wrangling over abstract propositions that are not necessarily involved in the problem before us.
Here I shall merely indicate at the outset certain general characteristics of the mental processes that are active in the sense-perceptions, because they will be constantly encountered in connection with the various subjects to be considered. Without some previous explanation of their general significance and wide range of activity, the reader might be apt in some special case to regard them as paradoxical and incredible.
The general rule determining the ideas of vision that are formed whenever an impression is made on the eye, with or without the aid of optical instruments, is that such objects are always imagined as being present in the field of vision as would have to be there in order to produce the same impression on the nervous mechanism, the eyes being used under ordinary normal conditions. To employ an illustration which has been mentioned before, suppose that the eyeball is mechanically stimulated at the outer corner of the eye. Then we imagine that we see an appearance of light in front of us somewhere in the direction of the bridge of the nose. Under ordinary conditions of vision, when our eyes are stimulated by light coming from outside, if the region of the retina in the outer corner of the eye is to be stimulated, the light actually has to enter the eye from the direction of the bridge of the nose. Thus, in accordance with the above rule, in a case of this kind we substitute a luminous object at the place mentioned in the field of view, although as a matter of fact the mechanical stimulus does not act on the eye from in front of the field of view nor from the nasal side of the eye, but, on the contrary, is exerted on the outer surface of the eyeball and more from behind. The general validity of the above rule will be shown by many other instances that will appear in the following pages.
In the statement of this rule mention is made of the ordinary conditions of vision, when the visual organ is stimulated by light from outside; this outside light, coming from the opaque objects in its path that were the last to be encountered, and having reached the eye along rectilinear paths through an uninterrupted layer of air. This is what is meant here by the normal use of the organ of vision, and the justification for using this term is that this mode of stimulation occurs in such an enormous majority of cases that all other instances where the paths of the rays of light are altered by reflections or refractions, or in which the stimulations are not produced by external light, may be regarded as rare exceptions. This is because the retina in the fundus of the firm eyeball is almost completely protected from the actions of all other stimuli and is not easily accessible to anything but external light. When a person is in the habit of using an optical instrument and has become accustomed to it, for example, if he is used to wearing spectacles, to a certain extent he learns to interpret the visual images under these changed conditions.
Incidentally, the rule given above corresponds to a general characteristic of all sense-perceptions, and not simply to the sense of sight alone. For example, the stimulation of the tactile nerves in the enormous majority of cases is the result of influences that affect the terminal extensions of these nerves in the surface of the skin. It is only under exceptional circumstances that the nerve-stems can be stimulated by more powerful agencies. In accordance with the above rule, therefore, all stimulations of cutaneous nerves, even when they affect the stem or the nerve-centre itself, are perceived as occurring in the corresponding peripheral surface of the skin. The most remarkable and astonishing cases of illusions of this sort are those in which the peripheral area of this particular portion of the skin is actually no longer in existence, as, for example, in case of a person whose leg has been amputated. For a long time after the operation the patient frequently imagines he has vivid sensations in the foot that has been severed. He feels exactly the places that ache on one toe or the other. Of course, in a case of this sort the stimulation can affect only what is left of the stem of the nerve whose fibres formerly terminated in the amputated toes. Usually, it is the end of the nerve in the scar that is stimulated by external pressure or by contraction of the scar tissue. Sometimes at night the sensations in the missing extremity get to be so vivid that the patient has to feel the place to be sure that his limb is actually gone.
Thus it happens, that when the modes of stimulation of the organs of sense are unusual, incorrect ideas of objects are apt to be formed; which used to be described, therefore, as illusions of the senses. Obviously, in these cases there is nothing wrong with the activity of the organ of sense and its corresponding nervous mechanism which produces the illusion. Both of them have to act according to the laws that govern their activity once for all. It is rather simply an illusion in the judgment of the material presented to the senses, resulting in a false idea of it.
The psychic activities that lead us to infer that there in front of us at a certain place there is a certain object of a certain character, are generally not conscious activities, but unconscious ones. In their result they are equivalent to a conclusion, to the extent that the observed action on our senses enables us to form an idea as to the possible cause of this action; although, as a matter of fact, it is invariably simply the nervous stimulations that are perceived directly, that is, the actions, but never the external objects themselves. But what seems to differentiate them from a conclusion, in the ordinary sense of that word, is that a conclusion is an act of conscious thought. An astronomer, for example, comes to real conscious conclusions of this sort, when he computes the positions of the stars in space, their distances, etc., from the perspective images he has had of them at various times and as they are seen from different parts of the orbit of the earth. His conclusions are based on a conscious knowledge of the laws of optics. In the ordinary acts of vision this knowledge of optics is lacking. Still it may be permissible to speak of the psychic acts of ordinary perception as unconscious conclusions, thereby making a distinction of some sort between them and the common so-called conscious conclusions. And while it is true that there has been, and probably always will be, a measure of doubt as to the similarity of the psychic activity in the two cases, there can be no doubt as to the similarity between the results of such unconscious conclusions and those of conscious conclusions.
These unconscious conclusions derived from sensation are equivalent in their consequences to the so-called conclusions from analogy. Inasmuch as in an overwhelming majority of cases, whenever the parts of the retina in the outer corner of the eye are stimulated, it has been found to be due to external light coming into the eye from the direction of the bridge of the nose, the inference we make is that it is so in every new case whenever this part of the retina is stimulated; just as we assert that every single individual now living will die, because all previous experience has shown that all men who were formerly alive have died.
But, moreover, just because they are not free acts of conscious thought, these unconscious conclusions from analogy are irresistible, and the effect of them cannot be overcome by a better understanding of the real relations. It may be ever so clear how we get an idea of a luminous phenomenon in the field of vision when pressure is exerted on the eye; and yet we cannot get rid of the conviction that this appearance of light is actually there at the given place in the visual field; and we cannot seem to comprehend that there is a luminous phenomenon at the place where the retina is stimulated. It is the same way in case of all the images that we see in optical instruments.
On the other hand, there are numerous illustrations of fixed and inevitable associations of ideas due to frequent repetition, even when they have no natural connection, but are dependent merely on some conventional arrangement, as, for example, the connection between the written letters of a word and its sound and meaning. Still to many physiologists and psychologists the connection between the sensation and the conception of the object usually appears to be so rigid and obligatory that they are not much disposed to admit that, to a considerable extent at least, it depends on acquired experience, that is, on psychic activity. On the contrary, they have endeavoured to find some mechanical mode of origin for this connection through the agency of imaginary organic structures. With regard to this question, all those experiences are of much significance which show how the judgment of the senses may be modified by experience and by training derived under various circumstances, and may be adapted to the new conditions. Thus, persons may learn in some measure to utilize details of the sensation which otherwise would escape notice and not contribute to obtaining any idea of the object. On the other hand, too, this new habit may acquire such a hold that when the individual in question is back again in the old original normal state, he may be liable to illusions of the senses.
Facts like these show the widespread influence that experience, training and habit have on our perceptions. But how far their influence really does extend, it would perhaps be impossible to say precisely at present. Little enough is definitely known about infants and very young animals, and the interpretation of such observations as have been made on them is extremely doubtful. Besides, no one can say that infants are entirely without experience and practice in tactile sensations and bodily movements. Accordingly, the rule given above has been stated in a form which does not anticipate the decision of this question. It merely expresses what the result is. And so it can be accepted even by those who have entirely different opinions as to the way ideas originate concerning objects in the external world.
Another general characteristic property of our sense-perceptions is, that we are not in the habit of observing our sensations accurately, except as they are useful in enabling us to recognize external objects. On the contrary, we are wont to disregard all those parts of the sensations that are of no importance so far as external objects are concerned. Thus in most cases some special assistance and training are needed in order to observe these latter subjective sensations. It might seem that nothing could be easier than to be conscious of one's own sensations; and yet experience shows that for the discovery of subjective sensations some special talent is needed, such as PURKINJE manifested in the highest degree; or else it is the result of accident or of theoretical speculation. For instance, the phenomena of the blind spot were discovered by MARIOTTE from theoretical considerations. Similarly, in the domain of hearing, I discovered the existence of those combination tones which I have called summation tones. In the great majority of cases, doubtless it was accident that revealed this or that subjective phenomenon to observers who happened to be particularly interested in such matters. It is only when subjective phenomena are so prominent as to interfere with the perception of things, that they attract everybody's attention. Once the phenomena have been discovered, it is generally easier for others to perceive them also, provided the proper precautions are taken for observing them, and the attention is concentrated on them. In many cases, however—for example, in the phenomena of the blind spot, or in the separation of the overtones and combination tones from the fundamental tones of musical sounds, etc.—such an intense concentration of attention is required that, even with the help of convenient external appliances, many persons are unable to perform the experiments. Even the after-images of bright objects are not perceived by most persons at first except under particularly favourable external conditions. It takes much more practice to see the fainter kinds of after-images. A common experience, illustrative of this sort of thing, is for a person who has some ocular trouble that impairs his vision to become suddenly aware of the so-called mouches volantes in his visual field, although the causes of this phenomenon have been there in the vitreous humor all his life. Yet now he will be firmly persuaded that these corpuscles have developed as the result of his ocular ailment, although the truth simply is that, owing to his ailment, the patient has been paying more attention to visual phenomena. No doubt, also, there are cases where one eye has gradually become blind, and yet the patient has continued to go about for an indefinite time without noticing it, until he happened one day to close the good eye without closing the other, and so noticed the blindness of that eye.
When a person's attention is directed for the first time to the double images in binocular vision, he is usually greatly astonished to think that he had never noticed them before, especially when he reflects that the only objects he has ever seen single were those few that happened at the moment to be about as far from his eyes as the point of fixation. The great majority of objects, comprising all those that were farther or nearer than this point, were all seen double.
Excerpted from Treatise on Physiological Optics, Volume III by Hermann von Helmholtz, James P. C. Southall. Copyright © 2005 Dover Publications, Inc.. Excerpted by permission of Dover Publications, Inc..
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