Illusions by James Sully (story reading txt) 📕
And here at once there forces itself on our attention the question, What exactly is to be understood by the term "illusion"? In scientific works treating of the pathology of the subject, the word is confined to what are specially known as illusions of the senses, that is to say, to false or illusory perceptions. And there is very good reason for this limitation, since such illusions of the senses are the most palpable and striking symptoms of mental disease. In addition to this,
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[37] The perception of magnitude is closely connected with that of distance, and is similarly apt to take an illusory form. I need only refer to the well-known simple optical contrivances for increasing the apparent magnitude of objects. I ought, perhaps, to add that I do not profess to give a complete account of optical illusions here, but only to select a few prominent varieties, with a view to illustrate general principles of illusion. For a fuller account of the various mechanical arrangements for producing optical illusion, I must refer the reader to the writings of Sir D. Brewster and Helmholtz.
[38] Painters are well aware that the colours at the red end of the spectrum are apt to appear as advancing, while those of the violet end are known as retiring. The appearance of relief given by a gilded pattern on a dark blue as ground, is in part referable to the principle just referred to. In addition, it appears to involve a difference in the action of the muscles of accommodation in the successive adaptations of the eye to the most refrangible and the least refrangible rays. (See Brücke, Die Physiologie der Farben, sec. 17.)
[39] Helmholtz tells us (Populäre wissenschaftliche Vorträge, 3tes Heft, p. 64) that even in a stereoscopic arrangement the presence of a wrong cast shadow sufficed to disturb the illusion.
[40] Among the means of giving a vivid sense of depth to a picture, emphasized by Helmholtz, is diminishing magnitude. It is obvious that the perceptions of real magnitude and distance are mutually involved. When, for example, a picture represents a receding series of objects, as animals, trees, or buildings, the sense of the third dimension, is rendered much more clear.
[41] A striking example of this was given in a painting, by Andsell, of a sportsman in the act of shooting, exhibited in the Royal Academy in 1879.
[42] This is at least true of all near objects.
[43] Helmholtz remarks (op. cit., p. 628) that the difficulty of seeing the convex cast as concave is probably due to the presence of the cast shadow. This has, no doubt, some effect: yet the consideration urged in the text appears to me to be the most important one.
[44] Populäre wissenschaftliche Vorträge, 3tes Heft, pp. 71, 72.
[45] See, on this point, some excellent remarks by G.H. Lewes, Problems of Life and Mind, third series, vol. ii. p. 275.
[46] To some extent this applies to the changes of apparent magnitude due to altered position. Thus, we do not attend to the reduction of the height of a small object which we are wont to handle, when it is placed far below the level of the eye. And hence the error people make in judging of the point in the wall or skirting which a hat will reach when placed on the ground.
[47] I refer to the experiments made by Exner, Wundt, and others, in determining the time elapsing between the giving of a signal to a person and the execution of a movement in response. "It is found," says Wundt, "by these experiments that the exact moment at which a sense-impression is perceived depends on the amount of preparatory self-accommodation of attention." (See Wundt, Physiologische Psychologie, ch. xix., especially p. 735. et seq.)
[48] Quoted by Helmholtz, op. cit., p. 626.
[49] When the drawing, by its adherence to the laws of perspective, does not powerfully determine the eye to see it in one way rather than in the other (as in Figs. 5 to 7), the disposition to see the one form rather than the other points to differences in the frequency of the original forms in our daily experience. At the same time, it is to be observed that, after looking at the drawing for a time under each aspect, the suggestion now of the one and now of the other forces itself on the mind in a curious and unaccountable way.
[50] Ueber die phantastischen Gesichsterscheinungen, p. 45.
[51] Another side of histrionic illusion, the reading of the imitated feelings into the actors' minds, will be dealt with in a later chapter.
[52] In a finished painting of any size this preparation is hardly necessary. In these cases, in spite of the great deviations from truth in pictorial representation already touched on, the amount of essential agreement is so large and so powerful in its effect that even an intelligent animal will experience an illusion. Mr. Romanes sends me an interesting account of a dog, that had never been accustomed to pictures, having been put into a state of great excitement by the introduction of a portrait into a room, on a level with his eye. It is not at all improbable that the lower animals, even when sane, are frequently the subjects of slight illusion. That animals dream is a fact which is observed as long ago as the age of Lucretius.
[53] This kind of illusion is probably facilitated by the fact that the eye is often performing slight movements without any clear consciousness of them. See what was said about the limits of sensibility, p. 50.
[54] Mental Physiology, fourth edit., p. 158.
[55] In persons of very lively imagination the mere representation of an object or event may suffice to bring about such a semblance of sensation. Thus, M. Taine (op. cit., vol. i. p. 94) vouches for the assertion that "one of the most exact and lucid of modern novelists," when working out in his imagination the poisoning of one of his fictitious characters, had so vivid a gustatory sensation of arsenic that he was attacked by a violent fit of indigestion.
[56] Mentioned by Dr. Carpenter (Mental Physiology, p. 207), where other curious examples are to be found.
[57] See Annales Médico-Psychologiques, tom. vi. p. 168, etc.; tom. vii. p. 1, etc.
[58] I have already touched on the resonance of a sense-impression when the stimulus has ceased to act (see p. 55). The remarks in the text hold good of all such after-impressions, in so far as they take the form of fully developed percepts. A good example is the recurrence of the images of microscopic preparations, to which the anatomist is liable. (See Lewes, Problems of Life and Mind, third series, vol. ii. p. 299.) Since a complete hallucination is supposed to involve the peripheral regions of the nerve, the mere fact of shutting the eye would not, it is clear, serve as a test of the origin of the illusion.
[59] That subjective sensation may become the starting-point in complete hallucination is shown in a curious instance given by Lazarus, and quoted by Taine, op. cit., vol. i. p. 122, et seq. The German psychologist relates that, on one occasion in Switzerland, after gazing for some time on a chain of snow-peaks, he saw an apparition of an absent friend, looking like a corpse. He goes on to explain that this phantom was the product of an image of recollection which somehow managed to combine itself with the (positive) after-image left by the impression of the snow-surface.
[60] For an account of Mr. Galton's researches, see Mind, No. xix. Compare, however, Professor Bain's judicious observations on these results in the next number of Mind. The liability of children to take images for percepts, is illustrated by the experiences related in a curious little work, Visions, by E.H. Clarke, M.D. (Boston, U.S., 1878), pp. 17, 46, and 212.
[61] A common way of describing the relation of the hallucinatory to real objects, is to say that the former appear partly to cover and hide the latter.
[62] Griesinger remarks that the forms of the hallucinations of the insane rarely depend on sense-disturbances alone. Though these are often the starting-point, it is the whole mental complexion of the time which gives the direction to the imagination. The common experience of seeing rats and mice running about during a fit of delirium tremens very well illustrates the co-operation of peripheral impressions not usually attended to, and possibly magnified by the morbid state of sensibility of the time (in this case flying spots, muscæ volitantes), with emotional conditions. (See Griesinger, loc. cit., p. 96.)
[63] Wundt (Physiologische Psychologie, p. 652) tells us of an insane woodman who saw logs of wood on all hands in front of the real objects.
[64] It is stated by Baillarger (Mémoires de l'Académie Royale de Médicine, tom. xii. p. 273, etc.) that while visual hallucinations are more frequent than auditory in healthy life, the reverse relation holds in disease. At the same time, Griesinger remarks (loc. cit., p. 98) that visual hallucinations are rather more common than auditory in disease also. This is what we should expect from the number of subjective sensations connected with the peripheral organ of vision. The greater relative frequency of auditory hallucinations in disease, if made out, would seem to depend on the close connection between articulate sounds and the higher centres of intelligence, which centres are naturally the first to be thrown out of working order. It is possible, moreover, that auditory hallucinations are quite as common as visual in states of comparative health, though more easily overlooked. Professor Huxley relates that he is liable to auditory though not to visual hallucinations. (See Elementary Lessons in Physiology, p. 267.)
[65] See Baillarger, Mémoires de l'Académie Royale de Médicine, tom. xii. p. 273, et seq.
[66] See Baillarger, Annales Médico-Psychologiques, tom. vi. p. 168 et seq.; also tom. xii. p. 273, et seq. Compare Griesinger, op. cit. In a curious work entitled Du Démon de Socrate (Paris, 1856), M. Lélut seeks to prove that the philosopher's admonitory voice was an incipient auditory hallucination symptomic of a nascent stage of mental alienation.
[67] This is well brought out by Dr. J. Hughlings Jackson, in the papers in Brain, already referred to.
[68] Friend, vol. i. p. 248. The story is referred to by Sir W. Scott in his Demonology and Witchcraft.
[69] See E.B. Tylor, Primitive Culture, ch. xi.; cf. Herbert Spencer, Principles of Sociology, ch. x.
[70] For a fuller account of the different modes of dream-interpretation, see my article "Dream," in the ninth edition of the Encyclopædia Britannica.
[71] For a fuller account of the reaction of dreams on waking consciousness, see Paul Radestock, Schlaf und Traum. The subject is touched on later, under the Illusions of Memory.
[72] For an account of the latest physiological hypotheses as to the proximate cause of sleep, see Radestock, op. cit., appendix.
[73] Plutarch, Locke, and others give instances of people who never dreamt. Lessing asserted of himself that he never knew what it was to dream.
[74] The error touched on
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