Produced by Susan Skinner, Les Galloway and the Online Distributed Proofreading Team at http://www.pgdp.net (This file was produced from images generously made available by The Internet Archive) LETTERS ON NATURAL MAGIC, ADDRESSED TO SIR WALTER SCOTT, BART. BY SIR DAVID BREWSTER, LL.D., F.R.S. [Illustration: Three figures on hill-top saluting sunrise] SEVENTH EDITION. LONDON: WILLIAM TEGG AND Co., 85, QUEEN STREET. CHEAPSIDE. 1856. CONTENTS. LETTER I. Extent and interest of the subject--Science employed by ancient governments to deceive and enslave their subjects--Influence of the supernatural upon ignorant minds--Means employed by the ancient magicians to establish their authority--Derived from a knowledge of the phenomena of Nature--From the influence of narcotic drugs upon the victims of their delusion--From every branch of science--Acoustics--Hydrostatics--Mechanics--Optics--M. Salverteâs work on the occult sciences--Object of the following letters Page 1 LETTER II. The eye the most important of our organs--Popular description of it--The eye is the most fertile source of mental illusions--Disappearance of objects when their images fall upon the base of the optic nerve--Disappearance of objects when seen obliquely--Deceptions arising from viewing objects in a faint light--Luminous figures created by pressure on the eye, either from external causes or from the fulness of the blood-vessels--Ocular spectra or accidental colours--Remarkable effects produced by intense light--Influence of the imagination in viewing these spectra--Remarkable illusion produced by this affection of the eye--Duration of impressions of light on the eye--Thaumatrope--Improvements upon it suggested--Disappearance of halves of objects or of one of two persons--Insensibility of the eye to particular colours--Remarkable optical illusion described 8 LETTER III. Subject of spectral illusions--Recent and interesting case of Mrs. A.--Her first illusion affecting the ear--Spectral apparition of her husband--Spectral apparition of a cat--Apparition of a near and living relation in grave-clothes, seen in a looking-glass--Other illusions, affecting the ear--Spectre of a deceased friend sitting in an easy-chair--Spectre of a coach-and-four filled with skeletons--Accuracy and value of the preceding cases--State of health under which they arose--Spectral apparitions are pictures on the retina--The ideas of memory and imagination are also pictures on the retina--General views of the subject--Approximate explanation of spectral apparitions 37 LETTER IV. Science used as an instrument of imposture--Deceptions with plane and concave mirrors practised by the ancients--The magicianâs mirror--Effects of concave mirrors--AÃ«rial images--Images on smoke--Combination of mirrors for producing pictures from living objects--The mysterious dagger--Ancient miracles with concave mirrors--Modern necromancy with them, as seen by Cellini--Description and effects of the magic lantern--Improvements upon it--Phantasmagoric exhibitions of Philipstall and others--Dr. Youngâs arrangement of lenses, &c., for the Phantasmagoria--Improvements suggested--Catadioptrical phantasmagoria for producing the pictures from living objects--Method of cutting off parts of the figures--Kircherâs mysterious hand-writing on the wall--His hollow cylindrical mirror for aÃ«rial images--Cylindrical mirror for re-forming distorted pictures--Mirrors of variable curvature for producing caricatures 56 LETTER V. Miscellaneous optical illusions--Conversions of cameos into intaglios, or elevations into depressions, and the reverse--Explanation of this class of deceptions--Singular effects of illumination with light of one simple colour--Lamps for producing homogeneous yellow light--Methods of increasing the effects of this exhibition--Method of reading the inscription of coins in the dark--Art of deciphering the effaced inscription of coins--Explanation of these singular effects--Apparent motion of the eyes in portraits--Remarkable examples of this--Apparent motion of the features of a portrait, when the eyes are made to move--Remarkable experiment of breathing light and darkness 98 LETTER VI. Natural phenomena marked with the marvellous--Spectre of the Brocken described--Analogous phenomena--AÃ«rial spectres seen in Cumberland--Fata Morgana in the Straits of Messina--Objects below the horizon raised and magnified by refraction--Singular example seen at Hastings--Dover Castle seen through the hill on which it stands--Erect and inverted images of distant ships seen in the air--Similar phenomena seen in the Arctic regions--Enchanted coast--Mr. Scoresby recognizes his fatherâs ship by its aÃ«rial image--Images of cows seen in the air--Inverted images of horses seen in South America--Lateral images produced by refraction--AÃ«rial spectres by reflexion--Explanation of the preceding phenomena 127 LETTER VII. Illusions depending on the ear--Practised by the ancients--Speaking and singing heads of the ancients--Exhibition of the Invisible Girl described and explained--Illusions arising from the difficulty of determining the direction of sounds--Singular example of this illusion--Nature of ventriloquism--Exhibitions of some of the most celebrated ventriloquists--M. St. Gille--Louis Brabant--M. Alexandre--Capt. Lyonâs account of Esquimaux ventriloquists 157 LETTER VIII. Musical and harmonic sounds explained--Power of breaking glasses with the voice--Musical sounds from the vibration of a column of air--and of solid bodies--Kaleidophone--Singular acoustic figures produced on sand laid on vibrating plates of glass--and on stretched membranes--Vibration of flat rulers and cylinders of glass--Production of silence from two sounds--Production of darkness from two lights--Explanation of these singular effects--Acoustic automaton--Drozâs bleating sheep--Maillardetâs singing-bird--Vaucansonâs flute-player--His pipe and tabor-player--Baron Kempelenâs talking-engine--Kratzensteinâs speaking-machine--Mr. Willisâs researches 179 LETTER IX. Singular effects in nature depending on sound--Permanent character of speech--Influence of great elevations on the character of sounds, and on the powers of speech--Power of sound in throwing down buildings--Dog killed by sound--Sounds greatly changed under particular circumstances--Great audibility of sounds during the night explained--Sounds deadened in media of different densities--Illustrated in the case of a glass of champagne--and in that of new-fallen snow--Remarkable echoes--Reverberations of thunder--Subterranean noises--Remarkable one at the Solfaterra--Echo at the Menai suspension bridge--Temporary deafness produced in diving-bells--Inaudibility of particular sounds to particular ears--Vocal powers of the statue of Memnon--Sounds in granite rocks--Musical mountain of El-Nakous 212 LETTER X. Mechanical inventions of the ancients few in number--Ancient and modern feats of strength--Feats of Eckeberg particularly described--General explanation of them--Real feats of strength performed by Thomas Topham--Remarkable power of lifting heavy persons when the lungs are inflated--Belzoniâs feat of sustaining pyramids of men--Deception of walking along the ceiling in an inverted position--Pneumatic apparatus in the foot of the house-fly for enabling it to walk in opposition to gravity--Description of the analogous apparatus employed by the gecko lizard for the same purpose--Apparatus used by the Echineis remora, or sucking-fish 244 LETTER XI. Mechanical automata of the ancients--Moving tripods--Automata of DÃ¦dalus--Wooden pigeon of Archytas--Automatic clock of Charlemagne--Automata made by Turrianus for Charles V.--Camusâs automatic carriage made for Louis XIV.--Degenneâs mechanical peacock--Vaucansonâs duck which ate and digested its food--Du Moulinâs automata--Baron Kempelenâs automaton chess-player--Drawing and writing automata--Maillardetâs conjurer--Benefits derived from the passion for automata--Examples of wonderful machinery for useful purposes--Duncanâs tambouring machinery--Wattâs statue-turning machinery--Babbageâs calculating machinery 264 LETTER XII. Wonders of chemistry--Origin, progress, and objects of alchemy--Art of breathing fire--Employed by Barchochebas, Eunus, &c.--Modern method--Art of walking upon burning coals and red-hot iron, and of plunging the hands in melted lead and boiling water--Singular property of boiling tar--Workmen plunge their hands in melted copper--Trial of ordeal by fire--Aldiniâs incombustible dresses--Examples of their wonderful power in resisting flame--Power of breathing and enduring air of high temperatures--Experiments made by Sir Joseph Banks, Sir Charles Blagden, and Mr. Chantrey 227 LETTER XIII. Spontaneous combustion--In the absorption of air by powdered charcoal--and of hydrogen by spongy platinum--Dobereinerâs lamp--Spontaneous combustion in the bowels of the earth--Burning cliffs--Burning soil--Combustion without flame--Spontaneous combustion of human beings--Countess Zangari--Grace Pett--Natural fire-temples of the Guebres--Spontaneous fires in the Caspian Sea--Springs of inflammable gas near Glasgow--Natural light-house of Maracaybo--New elastic fluids in the cavities--of gems--Chemical operations going on in their cavities--Explosions produced in them by heat--Remarkable changes of colour from chemical causes--Effects of the nitrous oxide or Paradise gas when breathed--Remarkable cases described--Conclusion 313 LETTERS ON NATURAL MAGIC; ADDRESSED TO SIR WALTER SCOTT, BART. LETTER I. Extent and interest of the subject--Science employed by ancient governments to deceive and enslave their subjects--Influence of the supernatural upon ignorant minds--Means employed by the ancient magicians to establish their authority--Derived from a knowledge of the phenomena of Nature--From the influence of narcotic drugs upon the victims of their delusion--From every branch of science--Acoustics--Hydrostatics--Mechanics--Optics--M. Salverteâs work on the occult sciences--Object of the following letters. MY DEAR SIR WALTER, As it was at your suggestion that I undertook to draw up a popular account of those prodigies of the material world which have received the appellation of _Natural Magic_, I have availed myself of the privilege of introducing it under the shelter of your name. Although I cannot hope to produce a volume at all approaching in interest to that which you have contributed to the Family Library, yet the popular character of some of the topics which belong to this branch of Demonology may atone for the defects of the following Letters; and I shall deem it no slight honour if they shall be considered as forming an appropriate supplement to your valuable work. The subject of Natural Magic is one of great extent as well as of deep interest. In its widest range, it embraces the history of the governments and the superstitions of ancient times,--of the means by which they maintained their influence over the human mind,--of the assistance which they derived from the arts and the sciences, and from a knowledge of the powers and phenomena of nature. When the tyrants of antiquity were unable or unwilling to found their sovereignty on the affections and interests of their people, they sought to entrench themselves in the strongholds of supernatural influence, and to rule with the delegated authority of Heaven. The prince, the priest, and the sage, were leagued in a dark conspiracy to deceive and enslave their species; and man, who refused his submission to a being like himself, became the obedient slave of a spiritual despotism, and willingly bound himself in chains when they seemed to have been forged by the gods. This system of imposture was greatly favoured by the ignorance of these early ages. The human mind is at all times fond of the marvellous, and the credulity of the individual may be often measured by his own attachment to the truth. When knowledge was the property of only one caste, it was by no means difficult to employ it in the subjugation of the great mass of society. An acquaintance with the motions of the heavenly bodies, and the variations in the state of the atmosphere, enabled its possessor to predict astronomical and meteorological phenomena with a frequency and an accuracy which could not fail to invest him with a divine character. The power of bringing down fire from the heavens, even at times when the electric influence was itself in a state of repose, could be regarded only as a gift from heaven. The power of rendering the human body insensible to fire was an irresistible instrument of imposture; and in the combinations of chemistry, and the influence of drugs and soporific embrocations on the human frame, the ancient magicians found their most available resources. The secret use which was thus made of scientific discoveries and of remarkable inventions, has no doubt prevented many of them from reaching the present times; but though we are very ill informed respecting the progress of the ancients in various departments of the physical sciences, yet we have sufficient evidence that almost every branch of knowledge had contributed its wonders to the magicianâs budget, and we may even obtain some insight into the scientific acquirements of former ages, by a diligent study of their fables and their miracles. The science of _Acoustics_ furnished the ancient sorcerers with some of their best deceptions. The imitation of thunder in their subterranean temples could not fail to indicate the presence of a supernatural agent. The golden virgins whose ravishing voices resounded through the temple of Delphos;--the stone from the river Pactolus, whose trumpet notes scared the robber from the treasure which it guarded;--the speaking head which uttered its oracular responses at Lesbos; and the vocal statue of Memnon, which began at the break of day to accost the rising sun,--were all deceptions derived from science, and from a diligent observation of the phenomena of nature. The principles of _Hydrostatics_ were equally available in the work of deception. The marvellous fountain which Pliny describes in the island of Andros as discharging wine for seven days, and water during the rest of the year;--the spring of oil which broke out in Rome to welcome the return of Augustus from the Sicilian war,--the three empty urns which filled themselves with wine at the annual feast of Bacchus in the city of Elis,--the glass tomb of Belus which was full of oil, and which when once emptied by Xerxes could not again be filled,--the weeping-statues, and the perpetual lamps of the ancients,--were all the obvious effects of the equilibrium and pressure of fluids. Although we have no direct evidence that the philosophers of antiquity were skilled in _Mechanics_, yet there are indications of their knowledge by no means equivocal in the erection of the Egyptian obelisks, and in the transportation of huge masses of stone, and their subsequent elevation to great heights in their temples. The powers which they employed, and the mechanism by which they operated, have been studiously concealed, but their existence may be inferred from results otherwise inexplicable; and the inference derives additional confirmation from the mechanical arrangements which seemed to have formed a part of their religious impostures. When, in some of the infamous mysteries of ancient Rome, the unfortunate victims were carried off by the gods, there is reason to believe that they were hurried away by the power of machinery; and when Apollonius, conducted by the Indian sages to the temple of their god, felt the earth rising and falling beneath his feet, like the agitated sea, he was no doubt placed upon a moving floor capable of imitating the heavings of the waves. The rapid descent of those who consulted the oracle in the cave of Trophonius,--the moving tripods which Apollonius saw in the Indian temples,--the walking statues at Antium, and in the temple of Hierapolis,--and the wooden pigeon of Archytas, are specimens of the mechanical resources of the ancient magic. But of all the sciences _Optics_ is the most fertile in marvellous expedients. The power of bringing the remotest objects within the very grasp of the observer, and of swelling into gigantic magnitude the almost invisible bodies of the material world, never fails to inspire with astonishment even those who understand the means by which these prodigies are accomplished. The ancients, indeed, were not acquainted with those combinations of lenses and mirrors which constitute the telescope and the microscope, but they must have been familiar with the property of lenses and mirrors to form erect and inverted images of objects. There is reason to think that they employed them to effect the apparition of their gods; and in some of the descriptions of the optical displays which hallowed their ancient temples, we recognize all the transformations of the modern phantasmagoria. It would be an interesting pursuit to embody the information which history supplies respecting the fables and incantations of the ancient superstitions, and to show how far they can be explained by the scientific knowledge which then prevailed. This task has, to a certain extent, been performed by M. Eusebe Salverte, in a work on the occult sciences which has recently appeared; but notwithstanding the ingenuity and learning which it displays, the individual facts are too scanty to support the speculations of the author, and the descriptions are too meagre to satisfy the curiosity of the reader.[1] [1] We must caution the young reader against some of the views given in M. Salverteâs work. In his anxiety to account for everything miraculous by natural causes, he has ascribed to the same origin some of these events in sacred history which Christians cannot but regard as the result of divine agency. In the following letters I propose to take a wider range, and to enter into more minute and popular details. The principal phenomena of nature, and the leading combinations of arts, which bear the impress of a supernatural character, will pass under our review, and our attention will be particularly called to those singular illusions of sense, by which the most perfect organs either cease to perform their functions, or perform them faithlessly; and where the efforts and the creations of the mind predominate over the direct perceptions of external nature. In executing this plan, the task of selection is rendered extremely difficult by the superabundance of materials, as well as from the variety of judgments for which these materials must be prepared. Modern science may be regarded as one vast miracle, whether we view it in relation to the Almighty Being by whom its objects and its laws were formed, or to the feeble intellect of man, by which its depths have been sounded, and its mysteries explored; and if the philosopher who is familiarized with its wonders, and who has studied them as necessary results of general laws, never ceases to admire and adore their Author, how great should be their effect upon less gifted minds, who must ever view them in the light of inexplicable prodigies!--Man has in all ages sought for a sign from heaven, and yet he has been habitually blind to the millions of wonders with which he is surrounded. If the following pages should contribute to abate this deplorable indifference to all that is grand and sublime in the universe, and if they should inspire the reader with a portion of that enthusiasm of love and gratitude which can alone prepare the mind for its final triumph, the labours of the author will not have been wholly fruitless. LETTER II. The eye the most important of our organs--Popular description of it--The eye is the most fertile source of mental illusions--Disappearance of objects when their images fall upon the base of the optic nerve--Disappearance of objects when seen obliquely--Deceptions arising from viewing objects in a faint light--Luminous figures created by pressure on the eye, either from external causes or from the fulness of the blood-vessels--Ocular spectra or accidental colours--Remarkable effects produced by intense light--Influence of the imagination in viewing these spectra--Remarkable illusion produced by this affection of the eye--Duration of impressions of light on the eye--Thaumatrope--Improvements upon it suggested--Disappearance of halves of objects or of one of two persons--Insensibility of the eye to particular colours--Remarkable optical illusion described. Of all the organs by which we acquire a knowledge of external nature, the eye is the most remarkable and the most important. By our other senses the information we obtain is comparatively limited. The touch and the taste extend no farther than the surface of our own bodies. The sense of smell is exercised within a very narrow sphere, and that of recognizing sounds is limited to the distance at which we hear the bursting of a meteor and the crash of a thunderbolt. But the eye enjoys a boundless range of observation. It takes cognizance not only of other worlds belonging to the solar system, but of other systems of worlds infinitely removed into the immensity of space; and when aided by the telescope, the invention of human wisdom, it is able to discover the forms, the phenomena, and the movements of bodies whose distance is as inexpressible in language as it is inconceivable in thought. While the human eye has been admired by ordinary observers for the beauty of its form, the power of its movements, and the variety of its expression, it has excited the wonder of philosophers by the exquisite mechanism of its interior, and its singular adaptation to the variety of purposes which it has to serve. The eyeball is nearly globular, and is about an inch in diameter. It is formed externally by a tough opaque membrane called the _sclerotic_ coat, which forms the white of the eye, with the exception of a small circular portion in front called the _cornea_. This portion is perfectly transparent, and so tough in its nature as to afford a powerful resistance to external injury. Immediately within the cornea, and in contact with it, is the _aqueous_ humour, a clear fluid, which occupies only a small part of the front of the eye. Within this humour is the iris, a circular membrane, with a hole in its centre called the _pupil_. The colour of the eye resides in this membrane, which has the curious property of contracting and expanding so as to diminish or enlarge the pupil,--an effect which human ingenuity has not been able even to imitate. Behind the iris is suspended the _crystalline_ lens, in a fine transparent capsule or bag of the same form with itself. It is then succeeded by the _vitreous humour_, which resembles the transparent white of an egg, and fills up the rest of the eye. Behind the vitreous humour, there is spread out on the inside of the eyeball a fine delicate membrane, called the _retina_, which is an expansion of the _optic nerve_, entering the back of the eye and communicating with the brain. A perspective view and horizontal section of the left eye, shown in the annexed figure, will convey a popular idea of its structure. It is, as it were, a small camera obscura, by means of which the pictures of external objects are painted on the retina, and, in a way of which we are ignorant, it conveys the impression of them to the brain. [Illustration: Fig. 1.] This wonderful organ may be considered as the sentinel which guards the pass between the worlds of matter and of spirit, and through which all their communications are interchanged. The optic nerve is the channel by which the mind peruses the hand-writing of Nature on the retina, and through which it transfers to that material tablet its decisions and its creations. The eye is consequently the principal seat of the supernatural. When the indications of the marvellous are addressed to us through the ear, the mind may be startled without being deceived, and reason may succeed in suggesting some probable source of the illusion by which we have been alarmed: but when the eye in solitude sees before it the forms of life, fresh in their colours and vivid in their outline; when distant or departed friends are suddenly presented to its view; when visible bodies disappear and reappear without any intelligible cause; and when it beholds objects, whether real or imaginary, for whose presence no cause can be assigned, the conviction of supernatural agency becomes, under ordinary circumstances, unavoidable. Hence it is not only an amusing but a useful occupation to acquire a knowledge of those causes which are capable of producing so strange a belief, whether it arises from the delusions which the mind practises upon itself, or from the dexterity and science of others. I shall therefore proceed to explain those illusions which have their origin in the eye, whether they are general, or only occasionally exhibited in particular persons, and under particular circumstances. There are few persons aware that when they look with one eye, there is some particular object before them to which they are absolutely blind. If we look with the right eye, this point is always about 15Â° to the right of the object which we are viewing, or to the right of the axis of the eye or the point of most distinct vision. If we look with the left eye, the point is as far to the left. In order to be convinced of this curious fact, which was discovered by M. Mariotte, place two coloured wafers upon a sheet of white paper at the distance of three inches, and look at the left-hand wafer with the right eye at the distance of about 11 or 12 inches, taking care to keep the eye straight above the wafer, and the line which joins the eyes parallel to the line which joins the wafers. When this is done, and the left eye closed, the right-hand wafer will no longer be visible. The same effect will be produced if we close the right eye and look with the left eye at the right-hand wafer. When we examine the retina to discover to what part of it this insensibility to light belongs, we find that the image of the invisible wafer has fallen on the base of the optic nerve, or the place where this nerve enters the eye and expands itself to form the retina. This point is shown in the preceding figure by a convexity at the place where the nerve enters the eye. But though light of ordinary intensity makes no impression upon this part of the eye, a very strong light does, and even when we use candles or highly luminous bodies in place of wafers, the body does not wholly disappear, but leaves behind a faint cloudy light, without, however, giving anything like an image of the object from which the light proceeds. When the objects are _white_ wafers upon a _black_ ground, the white wafer absolutely disappears, and the space which it covers appears to be completely black; and as the light which illuminates a landscape is not much different from that of a white wafer, we should expect, whether we use one or both eyes,[2] to see a black or a dark spot upon every landscape, within 15Â° of the point which most particularly attracts our notice. The Divine Artificer, however, has not left his work thus imperfect. Though the base of the optic nerve is insensible to light that falls directly upon it, yet it has been made susceptible of receiving luminous impressions from the parts which surround it; and the consequence of this is, that when the wafer disappears, the spot which is occupied, in place of being black, has always the same colour as the ground upon which the wafer is laid, being white when the wafer is placed upon a white ground, and red when it is placed upon a red ground. This curious effect may be rudely illustrated by comparing the retina to a sheet of blotting-paper, and the base of the optic nerve to a circular portion of it covered with a piece of sponge. If a shower falls upon the paper, the protected part will not be wetted by the rain which falls upon the sponge that covers it, but in a few seconds it will be as effectually wetted by the moisture which it absorbs from the wet paper with which it is surrounded. In like manner the insensible spot on the retina is stimulated by a borrowed light, and the apparent defect is so completely removed, that its existence can be determined only by the experiment already described. [2] When both eyes are open, the object whose image falls upon the insensible spot of the one eye is seen by the other, so that, though it is not invisible, yet it will only be half as luminous and, therefore two dark spots ought to be seen. Of the same character, but far more general in its effects, and important in its consequences, is another illusion of the eye which presented itself to me several years ago. When the eye is steadily occupied in viewing any particular object, or when it takes a fixed direction while the mind is occupied with any engrossing topic of speculation or of grief, it suddenly loses sight of, or becomes blind to, objects seen indirectly, or upon which it is not fully directed. This takes place whether we use one or both eyes, and the object which disappears will reappear without any change in the position of the eye, while other objects will vanish and revive in succession without any apparent cause. If a sportsman, for example, is watching with intense interest the motions of one of his dogs, his companion, though seen with perfect clearness by indirect vision, will vanish, and the light of the heath or of the sky will close in upon the spot which he occupied. In order to witness this illusion, put a little bit of white paper on a green cloth, and, within three or four inches of it, place a narrow strip of white paper. At the distance of twelve or eighteen inches, fix one eye steadily upon the little bit of white paper, and in a short time a part or even the whole of the strip of paper will vanish as if it had been removed from the green cloth. It will again reappear, and again vanish, the effect depending greatly on the steadiness with which the eye is kept fixed. This illusion takes place when both the eyes are open, though it is easier to observe it when one of them is closed. The same thing happens when the object is luminous. When a candle is thus seen by indirect vision, it never wholly disappears, but it spreads itself out into a cloudy mass, the centre of which is blue, encircled with a bright ring of yellow light. This inability of the eye to preserve a sustained vision of objects seen obliquely, is curiously compensated by the greater sensibility of those parts of the eye that have this defect. The eye has the power of seeing objects with perfect distinctness only when it is directed straight upon them; that is, all objects seen indirectly are seen indistinctly: but it is a curious circumstance, that when we wish to obtain a sight of a very faint star, such as one of the satellites of Saturn, we can see it most distinctly _by looking away from it_, and when the eye is turned full upon it it immediately disappears. Effects still more remarkable are produced in the eye when it views objects that are difficult to be seen from the small degree of light with which they happen to be illuminated. The imperfect view which we obtain of such objects forces us to fix the eye more steadily upon them; but the more exertion we make to ascertain what they are, the greater difficulties do we encounter to accomplish our object. The eye is actually thrown into a state of the most painful agitation, the object will swell and contract, and partly disappear, and it will again become visible when the eye has recovered from the delirium into which it has been thrown. This phenomenon may be most distinctly seen when the objects in a room are illuminated with the feeble gleam of a fire almost extinguished; but it may be observed in daylight by the sportsman when he endeavours to mark upon the monotonous heath the particular spot where moor-game has alighted. Availing himself of the slightest difference of tint in the adjacent heath, he keeps his eye steadily fixed on it as he advances, but whenever the contrast of illumination is feeble, he will invariably lose sight of his mark, and if the retina is capable of taking it up, it is only to lose it a second time. This illusion is likely to be most efficacious in the dark, when there is just sufficient light to render white objects faintly visible, and to persons who are either timid or credulous must prove a frequent source of alarm. Its influence, too, is greatly aided by another condition of the eye, into which it is thrown during partial darkness. The pupil expands nearly to the whole width of the iris, in order to collect the feeble light which prevails; but it is demonstrable that in this state the eye cannot accommodate itself to see near objects distinctly, so that the forms of persons and things actually become more shadowy and confused when they come within the very distance at which we count upon obtaining the best view of them. These affections of the eye are, we are persuaded, very frequent causes of a particular class of apparitions which are seen at night by the young and the ignorant. The spectres which are conjured up are always _white_, because no other colour can be seen, and they are either formed out of inanimate objects which reflect more light than others around them, or of animals or human beings whose colour or change of place renders them more visible in the dark. When the eye dimly descries an inanimate object whose different parts reflect different degrees of light, its brighter parts may enable the spectator to keep up a continued view of it; but the disappearance and reappearance of its fainter parts, and the change of shape which ensues, will necessarily give it the semblance of a living form, and if it occupies a position which is unapproachable, and where animate objects cannot find their way, the mind will soon transfer to it a supernatural existence. In like manner a human figure shadowed forth in a feeble twilight may undergo similar changes, and after being distinctly seen while it is in a situation favourable for receiving and reflecting light, it may suddenly disappear in a position fully before, and within the reach of, the observerâs eye; and if this evanescence takes place in a path or road where there was no side-way by which the figure could escape, it is not easy for an ordinary mind to efface the impression which it cannot fail to receive. Under such circumstances we never think of distrusting an organ which we have never found to deceive us; and the truth of the maxim that âseeing is believingâ is too universally admitted, and too deeply rooted in our nature, to admit on any occasion of a single exception. In these observations we have supposed that the spectator bears along with him no fears or prejudices, and is a faithful interpreter of the phenomena presented to his senses; but if he is himself a believer in apparitions, and unwilling to receive an ocular demonstration of their reality, it is not difficult to conceive the picture which will be drawn when external objects are distorted and caricatured by the imperfect indications of his senses, and coloured with all the vivid hues of the imagination. Another class of ocular deceptions have their origin in a property of the eye which has been very imperfectly examined. The fine nervous fabric which constitutes the retina, and which extends to the brain, has the singular property of being _phosphorescent by pressure_. When we press the eyeball outwards by applying the point of the finger between it and the nose, a circle of light will be seen, which Sir Isaac Newton describes as âa circle of colours like those in the feather of a peacockâs tail.â He adds, that âif the eye and the figure remain quiet, these colours vanish in a second of time; but if the finger be moved with a quavering motion, they appear again.â In the numerous observations which I have made on these luminous circles, I have never been able to observe any colour but white, with the exception of a general red tinge which is seen when the eyelids are closed, and which is produced by the light which passes through them. The luminous circles, too, always continue while the pressure is applied, and they may be produced as readily after the eye has been long in darkness as when it has been recently exposed to light. When the pressure is very gently applied, so as to compress the fine pulpy substance of the retina, light is immediately created when the eye is in total darkness; and when in this state light is allowed to fall upon it, the part compressed is more sensible to light than any other part, and consequently appears more luminous. If we increase the pressure, the eyeball, being filled with incompressible fluids, will protrude all round the point of pressure, and consequently the retina at the protruded part will be _compressed_ by the outward pressure of the contained fluid, while the retina on each side, namely, under the point of pressure and beyond the protruded part, will be drawn towards the protruded part or _dilated_. Hence the part under the finger which was originally compressed is now _dilated_, the adjacent parts _compressed_, and the more remote parts immediately without this _dilated_ also. Now we have observed, that when the eye is, under these circumstances, exposed to light, there is a bright luminous circle shading off externally and internally into total darkness. We are led, therefore, to the important conclusions, that when the retina is compressed in total darkness it gives out light; that when it is compressed when exposed to light, its sensibility to light is increased; and that when it is _dilated under exposure to light, it becomes absolutely blind, or insensible to all luminous impressions_. When the body is in a state of perfect health, this phosphorescence of the eye shows itself on many occasions. When the eye or the head receives a sudden blow, a bright flash of light shoots from the eyeball. In the act of sneezing, gleams of light are emitted from each eye both during the inhalation of the air, and during its subsequent protrusion, and in blowing air violently through the nostrils, two patches of light appear above the axis of the eye and in front of it, while other two luminous spots unite into one, and appear as it were about the point of the nose when the eyes are directed to it. When we turn the eyeball by the action of its own muscles, the retina is affected at the place where the muscles are inserted, and there may be seen opposite each eye, and towards the nose, two semicircles of light, and other two extremely faint towards the temples. At particular times, when the retina is more phosphorescent than at others, these semicircles are expanded into complete circles of light. In a state of indisposition, the phosphorescence of the retina appears in new and more alarming forms. When the stomach is under a temporary derangement accompanied with headache, the pressure of the blood-vessels upon the retina shows itself, in total darkness, by a faint blue light floating before the eye, varying in its shape, and passing away at one side. This blue light increases in intensity, becomes _green_ and then _yellow_, and sometimes rises to _red_, all these colours being frequently seen at once, or the mass of light shades off into darkness. When we consider the variety of distinct forms which in a state of perfect health the imagination can conjure up when looking into a burning fire, or upon an irregularly shaded surface,[3] it is easy to conceive how the masses of coloured light which float before the eye may be moulded by the same power into those fantastic and natural shapes, which so often haunt the couch of the invalid, even when the mind retains its energy, and is conscious of the illusion under which it labours. In other cases, temporary blindness is produced by pressure upon the optic nerve, or upon the retina; and under the excitation of fever or delirium, when the physical cause which produces spectral forms is at its height, there is superadded a powerful influence of the mind, which imparts a new character to the phantasms of the senses. [3] A very curious example of the influence of the imagination in creating distinct forms out of an irregularly shaded surface, is mentioned in the life of Peter Heaman, a Swede, who was executed for piracy and murder at Leith in 1822. We give it in his own words:-- âOne remarkable thing was, one day as we mended a sail, it being a very thin one, after laying it upon deck in folds, I took the tar brush and tarred it over in the places which I thought needed to be strengthened. But when we hoisted it up, I was astonished to see that the tar I had put upon it represented a gallows and a man under it without a head. The head was lying beside him. He was complete, body, thighs, legs, arms, and in every shape like a man. Now, I oftentimes made remarks upon it, and repeated them to the others. I always said to them all, You may depend upon it that something will happen. I afterwards took down the sail on a calm day, and sewed a piece of canvas over the figure to cover it, for I could not bear to have it always before my eyes.â In order to complete the history of the illusions which originate in the eye, it will be necessary to give some account of the phenomena called _ocular spectra_, or _accidental colours_. If we cut a figure out of red paper, and, placing it on a sheet of white paper, view it steadily for some seconds with one or both eyes fixed on a particular part of it, we shall observe the red colour to become less brilliant. If we then turn the eye from the red figure upon the white paper, we shall see a distinct _green_ figure, which is the _spectrum_, or accidental colour of the _red_ figure. With differently coloured figures we shall observe differently coloured spectra, as in the following table:-- COLOUR OF THE COLOUR OF THE ORIGINAL FIGURES. SPECTRAL FIGURES. Red, Bluish-green. Orange, Blue. Yellow, Indigo. Green, Reddish-violet. Blue, Orange-red. Indigo, Orange-yellow. Violet, Yellow. White, Black. Black, White. The two last of these experiments, viz., white and black figures, may be satisfactorily made by using a white medallion on a dark ground, and a black profile figure. The spectrum of the former will be found to be black, and that of the latter white. These ocular spectra often show themselves without any effort on our part, and even without our knowledge. In a highly painted room, illuminated by the sun, those parts of the furniture on which the sun does not directly fall have always the opposite or accidental colour. If the sun shines through a chink in a _red_ window-curtain, its light will appear _green_, varying as in the above table, with the colour of the curtain; and if we look at the image of a candle, reflected from the water in a _blue_ finger-glass, it will appear _yellow_. Whenever, in short, the eye is affected with one prevailing colour, it sees at the same time the spectral or accidental colour, just as when a musical string is vibrating, the ear hears at the same time its fundamental and its harmonic sounds. If the prevailing light is _white_ and _very strong_, the spectra which it produces are no longer black, but of various colours in succession. If we look at the sun, for example, when near the horizon, or when reflected from glass or water so as to moderate its brilliancy, and keep the eye upon it steadily for a few seconds, we shall see, even for hours afterwards, and whether the eyes are open or shut, a spectrum of the sun varying in its colours. At first, with the eye open, it is _brownish-red_ with a _sky-blue_ border, and when the eye is shut, it is _green_ with a _red_ border. The _red_ becomes more brilliant, and the _blue_ more vivid, till the impression is gradually worn off; but even when they become very faint, they may be revived by a gentle pressure on the eyeball. Some eyes are more susceptible than others of these spectral impressions, and Mr. Boyle mentions an individual who continued for years to see the spectre of the sun when he looked upon bright objects. This fact appeared to Locke so interesting and inexplicable, that he consulted Sir Isaac Newton respecting its cause, and drew from him the following interesting account of a similar effect upon himself:--âThe observation you mention in Mr. Boyleâs book of colours, I once made upon myself with the hazard of my eyes. The manner was this: I looked a very little while upon the sun in the looking-glass with my right eye, and then turned my eyes into a dark corner of my chamber, and winked, to observe the impression made, and the circles of colours which encompassed it, and how they decayed by degrees, and at last vanished. This I repeated a second and a third time. At the third time, when the phantasm of light and colours about it were almost vanished, intending my fancy upon them to see their last appearance, I found, to my amazement, that they began to return, and by little and little to become as lively and vivid as when I had newly looked upon the sun. But when I ceased to intend my face upon them, they vanished again. After this I found that as often as I went into the dark, and intended my mind upon them, as when a man looks earnestly to see anything which is difficult to be seen, I could make the phantasm return without looking any more upon the sun; and the oftener I made it return, the more easily I could make it return again. And at length, by repeating this without looking any more upon the sun, I made such an impression on my eye, that, if I looked upon the clouds, or a book, or any bright object, I saw upon it a round bright spot of light like the sun, and, which is still stranger, though I looked upon the sun with my right eye only, and not with my left, yet my fancy began to make an impression upon my left eye as well as upon my right. For if I shut my right eye, and looked upon a book or the clouds with my left eye, I could see the spectrum of the sun almost as plain as with my right eye, if I did but intend my fancy a little while upon it: for at first, if I shut my right eye, and looked with my left, the spectrum of the sun did not appear till I intended my fancy upon it; but by repeating, this appeared every time more easily. And now in a few hoursâ time I had brought my eyes to such a pass, that I could look upon no bright object with either eye but I saw the sun before me, so that I durst neither write nor read; but to recover the use of my eyes, shut myself up in my chamber, made dark, for three days together, and used all means in my power to direct my imagination from the sun. For if I thought upon him, I presently saw his picture, though I was in the dark. But by keeping in the dark; and employing my mind about other things, I began, in three or four days, to have more use of my eyes again; and by forbearing to look upon bright objects, recovered them pretty well; though not so well but that, for some months after, the spectrums of the sun began to return as often as I began to meditate upon the phenomena, even though I lay in bed at midnight with my curtains drawn. But now I have been well for many years, though I am apt to think, if I durst venture my eyes, I could still make the phantasm return by the power of my fancy. This story I tell you, to let you understand, that in the observation related by Mr. Boyle, the manâs fancy probably concurred with the impression made by the sunâs light to produce that phantasm of the sun which he constantly saw in bright objects.â[4] [4] See the _Edinburgh EncyclopÃ¦dia_, Art. ACCIDENTAL COLOURS. I am not aware of any effects that had the character of supernatural having been actually produced by the causes above described; but it is obvious, that if a living figure had been projected against the strong light which imprinted these durable spectra of the sun, which might really happen when the solar rays are reflected from water, and diffused by its ruffled surface, this figure would have necessarily accompanied all the luminous spectres which the fancy created. Even in ordinary lights, strange appearances may be produced by even transient impressions; and if I am not greatly mistaken, the case which I am about to mention is not only one which may occur, but which actually happened. A figure dressed in _black_, and mounted upon a _white_ horse, was riding along, exposed to the bright rays of the sun, which, through a small opening in the clouds, was throwing its light only upon that part of the landscape. The _black_ figure was projected against a white cloud, and the white horse shone with particular brilliancy by its contrast with the dark soil against which it was seen. A person interested in the arrival of such a stranger had been for some time following his movements with intense anxiety, but, upon his disappearance behind a wood, was surprised to observe the spectre of the mounted stranger in the form of a _white_ rider upon a _black_ steed, and this spectre was seen for some time in the sky, or upon any pale ground to which the eye was directed. Such an occurrence, especially if accompanied with a suitable combination of events, might, even in modern times, have formed a chapter in the history of the marvellous. It is a curious circumstance, that when the image of an object is impressed upon the retina only for a few moments, the picture which is left is exactly of the same colour with the object. If we look,
{'title': 'Letters on Natural Magic; Addressed to Sir Walter Scott, Bart. by David Brewster', 'source': 'Project Gutenberg', 'lang': 'en'}