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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. Salverte's work on the occult sciences--Object of the following letters Page 1

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

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

Science used as an instrument of imposture--Deceptions with plane and concave mirrors practised by the ancients--The magician's mirror--Effects of concave mirrors--A?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. Young's 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--Kircher's mysterious hand-writing on the wall--His hollow cylindrical mirror for a?rial images--Cylindrical mirror for re-forming distorted pictures--Mirrors of variable curvature for producing caricatures 56

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

Natural phenomena marked with the marvellous--Spectre of the Brocken described--Analogous phenomena--A?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 father's ship by its a?rial image--Images of cows seen in the air--Inverted images of horses seen in South America--Lateral images produced by refraction--A?rial spectres by reflexion--Explanation of the preceding phenomena 127

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--Droz's bleating sheep--Maillardet's singing-bird--Vaucanson's flute-player--His pipe and tabor-player--Baron Kempelen's talking-engine--Kratzenstein's speaking-machine--Mr. Willis's researches 179

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

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--Belzoni's 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

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--Aldini's 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

Spontaneous combustion--In the absorption of air by powdered charcoal--and of hydrogen by spongy platinum--Dobereiner's 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

NATURAL MAGIC;

ADDRESSED TO

SIR WALTER SCOTT, BART.

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. Salverte's work on the occult sciences--Object of the following letters.

MY DEAR SIR WALTER,

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 magician's 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.

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.

We must caution the young reader against some of the views given in M. Salverte's 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.

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.

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.

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 15? 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 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.

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.

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.

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.

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:--

"One 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."

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.

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:--"The observation you mention in Mr. Boyle's 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 hours' 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 man's fancy probably concurred with the impression made by the sun's light to produce that phantasm of the sun which he constantly saw in bright objects."

As the revolving card is virtually transparent, so that bodies beyond it can be seen through it, the power of the illusion might be greatly extended by introducing into the picture other figures, either animate or inanimate. The setting sun, for example, might be introduced into a landscape; part of the flame of a fire might be seen to issue from the crater of a volcano, and cattle grazing in a field might make part of the revolutionary landscape. For such purposes, however, the form of the instrument would require to be completely changed, and the rotation should be effected round a standing axis by wheels and pinions, and a screen placed in front of the revolving plane with open compartments or apertures, through which the principal figures would appear. Had the principle of this instrument been known to the ancients, it would doubtless have formed a powerful engine of delusion in their temples, and might have been more effective than the optical means which they seem to have employed for producing the apparitions of their gods.

In certain diseased conditions of the eye, effects of a very remarkable kind are produced. The faculty of seeing objects double is too common to be noticed as remarkable; and though it may take place with only one eye, yet, as it generally arises from a transient inability to direct the axis of both eyes to the same point, it excites little notice. That state of the eye, however, in which we lose sight of half of every object at which we look, is more alarming and more likely to be ascribed to the disappearance of part of the object than to a defect of sight. Dr. Wollaston, who experienced this defect twice, informs us that, after taking violent exercise, he "suddenly found that he could see but half of a man whom he met, and that on attempting to read the name of JOHNSON over a door, he saw only SON, the commencement of the name being wholly obliterated from his view." In this instance, the part of the object which disappeared was towards his left; but on a second occurrence of the same affection, the part which disappeared was towards his right. There are many occasions on which this defect of the eye might alarm the person who witnessed it for the first time. At certain distances from the eye one of two persons would necessarily disappear; and by a slight change of position either in the observer or the person observed, the person that vanished would reappear, while the other would disappear in his turn. The circumstances under which these evanescences would take place could not be supposed to occur to an ordinary observer, even if he should be aware that the cause had its origin in himself. When a phenomenon so strange is seen by a person in perfect health, as it generally is, and who has never had occasion to distrust the testimony of his senses, he can scarcely refer it to any other cause than a supernatural one.

If one of the two travellers who, in the fable of the chameleon, are made to quarrel about the colour of that singular animal, had happened to possess this defect of sight, they would have encountered at every step of their journey, new grounds of dissension, without the chance of finding an umpire who could pronounce a satisfactory decision. Under certain circumstances, indeed, the arbiter might set aside the opinions of both the disputants, and render it necessary to appeal to some higher authority,

Independent of its interest as an example of the marvellous in vision, the preceding fact may be considered as a proof that the retina retains its power to its very termination near the ciliary processes, and that the law of visible direction holds true even without the range of ordinary vision. It is therefore possible that a reflecting surface favourably placed on the outside of the eye, or that a reflecting surface in the inside of the eye, may cause a luminous image to fall nearly on the extreme margin of the retina, the consequence of which would be, that it would be seen in the back of the head, half way between a vertical and a horizontal line.

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.

The preceding account of the different sources of illusion to which the eye is subject is not only useful as indicating the probable cause of any individual deception, but it has a special importance in preparing the mind for understanding those more vivid and permanent spectral illusions to which some individuals have been either occasionally or habitually subject.

In these lesser phenomena, we find the retina so powerfully influenced by external impressions, as to retain the view of visible objects long after they are withdrawn: we observe it to be so excited by local pressures of which we sometimes know neither the nature nor the origin, as to see in total darkness moving and shapeless masses of coloured light; and we find, as in the case of Sir Isaac Newton, and others, that the imagination has the power of reviving the impressions of highly luminous objects, months and even years after they were first made. From such phenomena, the mind feels it to be no violent transition to pass to those spectral illusions which, in particular states of health, have haunted the most intelligent individuals, not only in the broad light of day, but in the very heart of the social circle.

This curious subject has been so ably and fully treated in your Letters on Demonology, that it would be presumptuous in me to resume any part of it on which you have even touched; but as it forms a necessary branch of a Treatise on Natural Magic, and as one of the most remarkable cases on record has come within my own knowledge, I shall make no apology for giving a full account of the different spectral appearances which it embraces, and of adding the results of a series of observations and experiments on which I have been long occupied, with the view of throwing some light on this remarkable class of phenomena.

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