Popular Exposition of Some Scientific Experiments. Part III. - Things that are invisble. - The nature of sight. - Visual Deceptions.

Harper's new monthly magazine 5, 1877

When a beam of light is decomposed by a prism, and a spectrum is formed, the superbly colored image arising - red at the less, violet at the more, refrangible extremity - offers to the curious observes, as Sir David Bewster, who had spent many years in its examination, once said, a world within itself. It is a visible manifestation of the great forces of Nature.

But the visible manifestation which we thus behold is only the portion of what we might witness were our eyes more perfect. Herchel proved that there are invisible radiations below the red capable of affecting us with a sensation of warmth; Scheele and Ritter, that there are other invisible ratiations beyond the violet that can accomplish chemical decompositions.

To this visible world we have, therefore, to add one that is unseen.

But these observers, satisfied with having indicated the existence of forces in those regions, left to their successors the labor of a more thorough investigation - an investigation that has produced many interesting results.

It would require far more space than I have now at my disposal to do justice to these investigations. They have been conducted by some of the ablest philosophers, and many of them are specimens of consumate experimental skill. What I propose to do now is only to describe researchers I have personally made on this subject, and to offer reflections on their bearing.

In the summer of 1842 I made many attempts to photograph the Fraunhofer fixed lines of the spectrum, and at length obtained exceedingly beautiful impressions of them. These were on daguerreotype silver plates - the operation in use at that time. In the following spring (May, 1843) I published in the Philosophical Magazine a description of the method of conducting the experiment and the results it furnished. The following is an abstract of it:

"When a beam of the sun's light, directed horizontally by a heliostat, is admitted into a dark room, and passing through a slit with parallel sides, is received on the surface of a flint-glass prism, which refracts it at the angle of minimum deviation, and, after its passage through the prism, is converged to a focal image on a white screen by the action of an achromatic lens, the resulting spectrum is given in great purity, and Fraunhofer's lines are very distinct. If a photographic surface be set in the place of the white screen, it will exhibit the representation of multitudes of dark lines.

"I can not avoid calling attention to the remarkable circumstance, which has often presented itself to me, of a great change in the relative visibility of Fraunhofer's lines when seen at different times. Sometimes the strong lines in the red ray are so feeble that the eye can barely discover them, and then again they come out as dark as though marked in India ink on the paper. During these changes the other lines may or may not undergo corresponding variations. The same remark applies to the yellow and blue rays. It has seemed to me that the lines in the red are more visible as the sun approaches the horizon, and those at the more refrangible end of the spectrum are plainer in the middle of the day."

I subsequently substantiated this remark, and satisfied myself that many of the lines in the red are due to absorption by the earth's atmosphere, and therefore more sidtinct with a rising or setting sub. Those in the more refrangible regions, the indigo, the blue, and the violet, are due to absorption by the atmosphere of the sun.

The apparatus I have used may be thus more particularly described:

"A sunbeam, passing horizontally from a heliostat mirror into a dark room, was received on a screen with a slit in its centre, the slit being formed by a pair of parallel knife edges, one of which was movable by a micrometer screw, the instrument being, in fact, the common one used for showing diffracted fringes. The screw was adjusted so as to give an aperture 1/52 (?) inch wide, and the light passing through fell upon an equiantigular flint-glass prism placed at a distance of eleven feet. Immediately on the posterior face of the prism the ray was received on an achromatic lens, the object-glass of a telescope, and brought to a focus at the distance of six feet inches, at which an arrangement was adjusted for exposing white paper screens, on which the greater fixed lines might be seen, or sensitive plates substituted for the screens, occupying precisely the same position. The lines on the screens could, therefore, be compared with those on the sensitive surfaces as to position and magnitude with considerable accuracy.

"In order to identify these lines I have made use of the map of the spectrum published by Professor Powell in the Report of the British Association for 1839. With the apparatus as above described they are exceedingly distinct; no difficulty arises in the identification of the more prominent ones. The spectrum with which I have worked occupied upon the screen a space of nearly four inches and a quarter in length from the red to the violet, or, more correctly speaking, from the ray marked in that map A to the one marked k. In stating, however, that no difficulty arises in identifying these lines, I ought to add that I am referring to that particular map. In the figure annexed to Sir John Herschel's "Treatise on Light," in the Encyclopædia Metropolitana, the rays marked G seem to differ from that in the report. But Professor Powel's map being drawn from his personal observations, with reference to the very difficulties, and as it agrees with my own observations and measures, I have employed it, and therefore take the letters he gives.

"It will be understood that the whole spectrum and all its lines can not be obtained at one impression. The difficulty is that the different regions of the spectrum act with different power in producing the proper effect. Thus, if on common yellow iodide of silver the attempt were made to procure all the lines at one trial, it would be found that the blue region would have passed to a state of high solarization, and that all its fine lines were extinguished by being overdone long before any well-marked action could be traced in the less refrangible extremity. It is necessary, therefore, to examine the different reguins in succession, exposing the sensitive surface to each for a suitable length of time."

"The general result of these various photographs may be thus stated: Below the red rays, and in a reguin altogether invisible, are three very large lines, these I designated α, β, γ, they are obtained more perfectly in the morning and evening light, less perfectly in the middle of the day, and hence I inferred that they are probably due to the absorptive action of the earth's atmosphere. Fraunhofer's lines A and B were also depicted, but I did not obtain D, E, F. The line G and its companions were very strongly impressed. But by far the most striking in the whole photograph are those marked H. Then passing beyond the violet and out in the visible limits of the spectrum, four very striking groups made their appearence. To the first line of each of these, in continuation of Fraunhofer's nomenclature, I gave the designations M, N, O, P. In l There are three lines, in M eight, in N three, in O four, and in P five.

"Besides these larger groups, the photographs were crossed by hundreds of minuter lines, so that it was impossible to count them. If nearly six hundred have been counted between A and H, I should think there must be quite as many between H and P. In speaking of these lines as though they were strong individual ones, the statement is to be taken with some limitation. It is quite likely that each of these bolder lines is made up of a great number that are excessively narrow and close together.

"If the absorptive action of the sun's atmosphere be the cause of this phenomenon, that action must take place much more powerfully on the more refrangible and extra-spectral region. THe lines exhibited there are bold and strongly developed."

Scarcely was the paper from which the foregoing extracts are made published in the Philosophical Magazine, when I learned that in France M. E. Becquerel had already photographed the more refrangible lines, and published statements to that effect. But he had not observed those in the less refrangible regions, designated by me α, β, γ.

In fact, the process I was using was one I had recently discovered: it consisted in permitting the daylight to fall along with the sun rays on the photographic surface. The daylight and the sunlight antagonized each other, and these hitherto undiscovered lines made their appearance as positive photographs. The peculiarities of this singular and interesting process I will describe hereafter in one of these papers.

In 1846 MM. Foucault and Fizeau, having repeated the experiment thus originally made by me, presented a communication to the French Academy of Sicences. They had observed the antagonizing action above described, and had seen the ultra-spectrum heat lines,  α, β, γ. They had taken the precaution to deposit with the Academy a sealed envelope containing an account of their discovery, not knowing that it had been made and published long previously in America.

Hereupon, M. E. Becquerel communicated to the same Academy a criticism on their paper. In this he remarks: "M. Draper, in examining the image produced by the action of the spectrum on plates of iodized silver, announced before those gentlemen the existence of protecting rays antagonizing the action of the solar rays, and even acting negtaively on iodide of silver." He strengthened his views by adding some observations that had been made by Sir J. Herschel, who did not assent to the existence of this protecting action, but thought that the daguerreotype impressions could be explained on Newton's theory of the colors on thin plates.

Herschel had made some investigations on the distribution of heat in the spectrum, using paper blackened on one side and moistened with alcohol on the other. He obtained a series of spots or patches, commencing above the yellow and extending far below the red. Some writers on this subject have considered that these observations imply a discovery of the lines  α, β, γ; they forget, however, that Herchel did not use a slit, but the direct image of the sun - an image which was more than a quarer of an inch in diameter, as I know from the specimens he sent me, and which are still in my possession. Under such circumstances it was physically impossible that these or any other of the fixed lines should be seen.

IN 1871 M. Lamanski announced that he had discovered these lines by tha aid of a thermo-multiplier. In his memoir he states that, with the exception of Foucault and Fizeau, no one had made reference to them. Hereupon I draw attention to the memoi I had published in 1843, containing a map or engraving of them.

As has been mentioned, I did not obtain at that time the lines D, E, F. I used that fact as an argument in belhalf of the theory of the physical independence of the luminous and chemical radiations. These were, however, subsequently photographed by my son, Henry Draper.

There are, therefore, many rays emitted by the sun and other shining bodies to which our eyes are entirely blind.

Two different reasons may be alleged for outr inability to perceive such rays: first, they may not be able to reach the retina, the media of the eye not transmitting them; second, the retina may be so constituted as to be unable to receive their impressions.

It has long been known that rays which come from sheet-iron heated by a lamp can not pass either through the cornea or through the crystalline lens. Even of those that are furnished by an Argand flame, used as a luminous source of heat, less than one-fifth pass through the cornea alone, and scarcely one-fiftieth when the crystalline lens is interposed. Cima showed that of the heat rays emitted a flame, less than one-tenth pass through the cornea, lens, and vitreous humor conjointly. Janssen, using a flame, compared the heat transparency of the separate media of  the eye with that of water included between glass plates, showing that there is a perfect accordance between them if taken of equal thickness. From this it is to be concluded that invisible rays to a certain extent reach the retina. Franx, by carefully conducted experiments with a thermo-electric pile, came to the conclusion that a quantity of obscure rays detectable by the thermometer can reach the retina, which therefore must be so constituted as not to be able to perceive them.

This settles the question so fas as the less refrangible or ultra-red rays are concerned. We have then to determine how it is with those at the opposite or more refrangible end of the spectrum. Do these pass through the media of the eye, or are they arrested and never reach the retina?

I made a series of experiments on these rays, and found that they passed through the different media of the eye examined separately, and what is more tothe point, through them all collectively with but little loss. There was no difficulty in obtaining a dark stain on paper made sensitive with chloride of silver, and placed at the back of the eye of an ox, from which the selerotic and pigment had been suitably removed. In a general manner the media of the eye act like water on the transmissibility of these rays.

Admitting from these experiments that invisible as well as visible rays reach the retina, we may next consider the nature of the impression made upon it, and are thus brought directly to an investigation of the act of vision.

There are three hypotheses to be considered:

1. That rays falling on the retina or black pigment impart to those structures a rise of temperature. This may be termed the caloric hypothesis.

2. That raysfalling on the retina occasion a chemical change or metamorphosis in its structure, implyung the occurance of waste in it, and therefore the necessity of repair. This may be termed the chemical hypothesis.

3. That rays falling on the retina throw its parts into a vibratory movement, not necessarily attended by any metamorphosis of tissue, as waves of sound occasion consentaneous pulsations in the auditory apparatus of the ear. This may be termed the mechanical hypothesis.

First: Of the caloric hypothesis of vision.

Comparative anatomy offers certain facts which lend plausibility to this hypothesis. Some of the most remarkable of these relate to the construction of the eye in lower animals. The ocelli, which consist of dark-colored or black spots, or black cup-shaped membranes, containing within them the rudiment of an optic nerve, are te beginning of an organ of vision. There being no optical apparatus for the production of images, the luminous impression must be felt as heat. For this the dark pigment is well desigued. It is an old physical experiment to lay upon the snow on a sunshiny winter day pieces of differently colored cloth. They will melt their way to a greater depth in proportion as their tint is darker: the black, becoming the warmest, sinks deepest; the white, reflecting most of the heat, scarcely melts the snow at all. Now an animal destitute of any visual organ can only be affected by the impressions of light in a very doubtful manner; but if there be upon its exterior a black spot, not only is there a much higher sensitiveness because of the increased absorptive power for heat, but the sphere of consciousness is greatly extended, from the possibility of acquiring a knowledge of directions in space - a knowledge that becomes more and more exavt with the increasing number and symmertical arrangement of these ocelli.

Ifwe apply these principles to a more perfect form of eye, as that of man, we are led to a new interpretation of the function of some of its parts. The black pigment becomes the receiving surface for images of external things, and rays falling upon it, in their diversity of color, brightness, and shade, in the act of becoming extinguished, engender heat. As with the tip of the finger passing over an object we can discover, even in the dark, spaces that are warm and those that are cool, so the rods and cones of Jacob's membrane, acting as tactile organs, convey to the brain a knowledge of the momentary distribution of heat on the dark concave of the eye. The pigment has therefore a far more important office to discharge than that of merely extinguishing stray light and darkening the indise of the globe.

But this calirific hypothesis is not without great difficulties. Heat suffers conduction. If this black pigment officiated as a transformer of light rays into heat by producing extinction, there must unavoidably be a lateral spread from the boundaties of warm to cooler spaces, the edges of images must be nebulous and without sharpness of contour. Moreover, there is reason to believe that the visual apparatus can not take cognizance of heat merely as such. Calorific rays reach the black pigment and raise its temperature without the retina being affected.

Such considerations seem, therefore, to exclude the calorific hypothesis, and prepare us for a n examintaion of the chemical.

Second: Of the chemical hypothesis of vision.

Numerous discoveries made of late years in relation to the chemical actions of light put us in possession of many facts having a bearing on this hypothesesi. A majority of compound substances, both inorganic and organic, suffer chemical modifications when exposed to the access of light, and, what is very significant, these changes are occasioned by definite classes of rays. One substance finds it maximum of action in the violet region, another in the yellow, another in the red. The effect in every instance grades off toward the less and more refrangible spaces respectively.

In these actions of decomposition there is nothing like lateral spreading, nothing answering to conduction. No better proof oof this is necessary than the exquisite sharpness of photographic pictures - a sharpness only limit by the optical imperfections of the lens with which they are made. The molecules on which the light falls are the only ones that experience change; there is no propagation of the effect from part to part - an important particular, because it is what we observe in the case of sight.

The retina, the nervous expansion of the eye, is so constituted that a maximum effect upon it is occasioned by the yellow ray, the action declining on one side to the red, and on the other toward the violet, and ceasing at the extremes of those rays. For this reason, when a solar spectrum is examined by the eye, the yellow is the most brilliant space, there being a decline in intensity fro mit to the two extremes.

In my experiments on the decomposition of carbonic acid by plants in the sunlight, previously related in these papers, the maximum of action was found to be in the yellow, with a gradation of effect toward the red and violet ends of the spectrum respectively. From this it would appear that a relation exists between light and compounds having a carbon nucleus, answering to that observed in the case of the retina of the eye. Such a relation is very well illustrated in the case of other chemical elements, as silver, a metal which is the basis of all ordinary photographic preparations. The ray of maximum action is in the indigo space. Objects viewed by a retina having a silver sensitive nucleus would present an appearance altogether unlike that they would offer to a carbon nucleus. The order of brilliancy in the lights would be no longer the same. The red and yellow parts of objects would be black, that is to say invisible, and other rays beyond the violet would come into view.

Among experiments that I have made on this subject, there is one of much physiological interest. The element phosphorus finds its maximum impression in the more refrangible portion of the spectrum, in that respect resembling silver. Upon a portion of translucent phosphorus, inclosed out of contact of air in a flattened glass tube, into which it had been drawn while melted, and then suffered to solidify, a solar spectrum was cast. The effect of light upon this kind of phosphorus is to turn it eventually to a deep mahogany red, and chemically to throw it from an active into an inactive state. As amorphous phosphorus otherwise prepared, it ceases to shine in the dark. In the experimetns now alluded to, it appeared that this redding takes place in the indigo and violet spaces, so that the fixed lines known by spectroscopists as those about H were beautifully depicted. Now some physiologists have supposed that nerve vesicle tissue owes its property to the presence of unoxidized phosphorus, but if the principles we are contemplating be correct, and this were the case, the most brilliant ray in the spectrum should be the indigo, and not the yellow. Therefore, if vision be performed by chemical change in the substance of the retina, it is carbon and not phosphorus that is concerned.

If we admit that during the act of vision the retina, as a structure with a carbon nucleus, undergoes metamorphosis, the principles of photo-chemistry would lead us to expect that the yellow must be the brightest ray, and a harmony is thus established between this and other functional changes in the body. We also perceive the significance of certain structures of the eye which otherwise would appear to be without meaning. the rapid retrograde metamorphosis which must be taking place in the retina involves the provision of some means for moving away the wasted procuts and of supplying nutrition with the utmost quickness. And this is the office discharged by the choroid.

But such removals and supplies require time. Time, therefore, enters as an element in the visual operation. Sight commences instantaneously, but the image of an obeject may be seen long after the reality has disappeared. This instantaneous commencement of a retnal impression may be very strikingly illustrated. The spark of a Leyden-jar, though it does not last, as is affirmed, the millionth of a second, can without any difficulty be photographed even on so sluggish a compound as silver iodide. On the far more sensitive retina the chemical impression must be practivally contemporaneous with the impinging of the light.

If, after the eyelids have been closed for some time, we suddenly and steadfastly gaze at a bright object, and then quickly close the lids again, a phantom image is perceived existing in the indefinite darkness before us. By degrees the image becomes less and less distinct; in a minute or two it has disappeared.

The chemical hypothesis renders a very clear explanation of this effect - an explanation that commends itself to our attention as casting light in many cases on the curious phenomena of apparitions - henomena that have been not without influence on the history of mankind.

The duration and gradual extiction of the retinal phantoms correspond to the destruction and renovation taking place in the retina itself. The blood supply is very ample, as are likewise the channels for the removal of waste, but the operations require time to be accomplished. As in machines contrived by man, so in natural organs, the practical working does not always come up to the theorerical standard. Theoretically, as the retina suffers change under the incident light, the removal of waste and nutrition should go in an equal manner both as to time and quantity. A marvelous approach to the ideal perfection is attained, for though the action of light must necessarily be cumulative, that is, increasing with the continuance of exposure, objects  do not become brighter and brighter as we look at them, but they attain their predestined distinctness at once. The action of the light, the removal of the waste it is occasioning, and the supply for renovation are all contemporaneously going on with an equal step, or so nearly so that such may be considered to be the practical effect.

Third: Of the mechanical hypothesis of vision.

There is a growing belief among those who are cultivating photo-chemistry that the mode of operation of a ray of light in accomplishing chemical changes is by establishing vibratory movements among the molecules of the subtance affected. As has been affirmed, perhaps fancifully, of certain singers, that they could cause a glass goblet to fly to pieces by a proper intonation of their voice, through the attempt of the glass by resonance to execute incompatible vibrations, so it is thought that an incident ray may break asunder a group of molecules by establishing among them discordant agitations. Chemical decompositions by radiations become thus connected theoretically with vibratory movements.

But these are vibrations not necessarily attended by any destruction of tissue. Waves of sound occasion such pulsations in the apparatus of the ear without prducing any chemical change in the auditory nerve.

If we consider the retina as an elastic shell, of which the parts are put into a purely mechanical movement by the pulsations of light, we abandon without explanation some of the most interesting portions of the structure of the eye. Of what use is that wonderful net-work of vessels constituting the choroid? It is a principle in physiology that the supply of blood to a part is proportional to its functional activity. The elaborate vascular mechanism in juxtaposition with the retina will bear no other interpretation than that that tissue is the seat of incessant chemical changes.

Moreover, physical science in its present state is not sufficiently advanced to furnish the means of clearly comprehending such purely mechanical motions executed by the ultimate particles of things. We may conceive of the comparatively slow swaying of groups of molecules under the influence of normal pulsations in the air, but not of the dance of atoms disturbed by transverse vibrations in the ether. If, therefore, there were no arguments of an anatomical kind to be presented against the admission of this hypothesis, we should be compelled to turn aside from it because of the inadequacy of our knowledge in tracing its conditions to their applications.

This, therefore, is the conclusion at which we finally arrive - that vision depends on chemical changes, especially of oxidation, in the retina, and that they approach in their nature those that we speak of as photographic. There is no difficulty in understanding how such changes may give rise to an influence transmitted along the optic nerve to the brain, when we reflect that the oxidation of a few particles of zinc may accomplish specific mechanical results through many miles of intervening telegraphic wire, producing mechanical motions as in the telegraph of Morse, or chemical changes as in that of Bain.

We have remarked that a critical study of the function of vision can not fail to lead to interesting results respecting the nervous system generally. Guided by that remark, we may perhaps profitably consider further the vestges of visual impressions, and the physical conditions under which they disturb us or spontaneously obtrude themselves on our attention.

The perception of external objects depends on the rays of light entering the eye, and converging so as to produce images, which make an impression on the retina, and through the optic nerve are delivered to the brain. The direction of these influence, so far as the observer is concerned, is from without to within, from the object to the brain.

But the inverse of this is possible. Impressions existing in the brain may take, as it were, and outward direction, and be projected or localized among external forms; or if the eyes be closed, as in sleep, or the obserber be in darkness, they will fill up the empty space before him with scenery of their own.

Inverse vision depends primarily on the condition that former impressions, inclosed in the optic thalami, or registering ganglia at the base of the brain, assume such a degree of relative intensity that they can arrest the attention of the mid. The moment that an equality is established between the intensity of these vestiges and sensations contemporaneously received from the outer world, or that the latter are wholly extinguished, as in sleep, inverse sight occurs, presenting, as the occasions may vary, apparitions, visions, dreams.

From the moral effect that arises, we are very liable to connect these with the supernatural. In truth, however, they are the natural results of the action of the nervous mechanism, which of necessity produces them whenever it is placed, either y normal or morbid or artificial causes, in the proper conditions. It confounds the subjective and the objective together. It can act either directly, as in ordinary vision, or inversely, as in cerebral sight, and in this respect resembles those instruments which equally yield a musical note whether the air is blown through them or drawn in.

The hours of sleep continually present us, in a state of perfect health, illusions that address themselves to the eye rather than to any other organ of sense, and these commonly combine into moving and acting scenes, a dream being truly a drama of the night. In certain states of health apprearances of a like nature intrude themselves before us even in the open day, but these, being corrected by the realities by which they are surrounded, impress us very differently. The want of unison between such images and the things among which they have intruded themselves, the anachronism of their advent, or other obvious incongruites, restrain the mind from delivering itself up to that absolute possesses us in our dreams. Yet, nevertheless, such is the constitution of man, the bravest and the wisest encounter these dictions of their own organization with awe.

The visions of an Arab merchant have ended in tincturing the daily life of halt the people of Asia and Africa for a thousand years. A spectre that came into the camp at Sardis the night before the battle of Philippi unnerved the heart of Brutus, and thereby put an end to the political system that had made the Roman republic the arbiter of the world. A phantom that appeared to Constantine strengthened his hand to that most difficult of all the tasks of a statesman, the destruction of an ancient faith.

Hallucinations are of two kinds - those seen when the eyes are open, and those perceived when they are closed. To the former the designation of apparitions, to the latter that of visions, may be given.

In a physiological sense, simple apparitions may be considered as arising from disturbances or diseases of the retina; visions, from the traces of impressions inclosed at a former time in the corpora quadrigemina and optic thalami.

From flying specks floating before us, the first rudiments of apparitions, it is but a step to the intercalation of simple or even grotesque images among the real objects at which we are looking; and indeed this is the manner in which they always offer themselves, as resting or moving among the actually existing things. Sir W. Scott says of children that lying is natural to them, and that to tell the truth is an ac    quired habit. To them a white object faintly desried in the twilight is easily expanded into a moving and supernatural thing. I do not say how far we are liable to practice this deception upon ourselves in later life.

Insects flying in the air, or rather floating in vacancy before us, present the incipient form of retinal malady. In a more aggravated form is less frequently occurs as producing stars or sparks of light. From the earliest times physicians have observed that it is a "bad sign" when the patient localizes these images. "If the sick man says there be little holes in the curtains or black spots on his bedclothes, then it is plain that his end is at hand."

Sometimes the derangement giving origin to these appearances is not limited to the retina, but involves more or less completely the entire nervous apparatus of the eye. Retinal insanity and cerebral vision occur together. In cases investigated in a philosophical manner by the patients themselves, this complication is often distinctly recognized. Thus Nicolai, the Prussian bookseller, who published in the Memoirs of the Royal Academy of Berlin an interesting account of his sufferings, states that of the apparitions of men and women with which he was troubled, there were some that disappeared on shutting the eyes, but some did not. In such cases there can be no doubt that the disease affected the corpora quadrigemina and the optic thalami as well as the retina.

This condition, in which the receiving centres and registering ganglia at the base of the brain are engaged, is the one that yields the most striking instances of hallucinations in which apparations and visions co-exist. It can in less complicated forms be brought on artificially, as by alcohol in delirium tremens, or by the use of opium or other drugs. In these as in those forms, it is the localization of the phantom among the objects around us that gives power to the illusion. The form of a cloud no bigger than the hand is perhaps first seen floating ever the carpet; but this, as the eye follows it, takes on a distinct contour and a definite shape, and the sufferer sees with dismay a moping raven on some of the more distant articles of furniture. Or, out of an indistinct cloud, faces sometimes of surprising loveliness emerge, a more beautiful one succeeding as a former dies away. "Throw a handkerchief over that bed post," once said a dying friend to me; "there is on it a face too beautiful for me to look at." The mind, ever ready to practice imposture upon itself, will at last accompany the illusion with grotesque or even dreadful inventions. A sarcophagus, painted after the manner of the Egyptians, distresses the visionary with the rolling of its eyes. Martin Luther thus more than once saw the devil under the well-known form popularly assigned to him in the Middle Ages.

As the nervous centres become more profoundly involved, these visions become more impressive. Instead of a solitary phantom intruding itself among recognized realities, as the shade of deceased friend noiselessly steps before us through the unopened door, the complicated scenes of a true drama are displayed. The brain becomes a theatre. According as the travel or the reading of the sick man may have been, the illusion takes a stule: black vistas of Oriental architecture that stretch away into indinite night; temples and fanes and the battlemented walls of cities; colossal Pharaohs sitting in everlasting silence, with their hands upon their knees. "I saw," says De Quincey, in his Confessions of an Opium-Eater,  "as I lay awake in bed, vast processions that passed along in mournful pomp; friezes of never-ending stories, that, to my mind, were as sad as solemn as if they were stories drawn from times before OEdipus or Priam, before Tyre, before Memphis, and, at the same time, a corresponding change took place in my dreams; a theatre seemed suddenly opened and lighted up within my brain, which presented nightly spectacles of more than earthly splendor."

Apparitions are the result of a false interpretation of impressions contemporaneously made on the retina; visions are a presentment of the relics of old ones remaining in the registering ganglia of the brain. We may be convinced of this, not so much from an examination of well related or authenticated cases as from what may be termed the natural history of ghosts. The Greeks and Romans were just as much liable to disorders of the nervous system as we are; but to them supernatural appearances came under mythological forms - Venus and Mars and Minerva. In the dreams of the ascetics of the Middle Ages, the places of these were taken by phantoms of the Virgin and the saints. The forms of such phantoms have changed with changes of the creeds of communities, and we may therefore, with good Reginald Scot, inquire, "If the appraritions which have been seen by true men and brave men in all ages of the world were real existences, what has become of the swarms of them in these latter times?"

One class of apparitions (perhaps it was the first to exist, as it is the last to remain) has survived all these changes - survived them because it is connected with a thing that never ceases, the affection of the human heart. To the people of every age the images of the dead have appeared. They are not infrequent even in our own times. It would be an ungracious task to enter on an examination of the best authenticated of such antecdotes. Inquiries of this kind can scarcely bee freed from the liability to an imputation on personal veracity, perceptive power, or moral courage, and it is not necessary to entangle ourselves with such causes of offense. It is enough for us to perceive that even here incongruities may be pointed out. The Roman saw the shade of his friend clothed in the well-known toga, the European sees his in our own grotesque garb. The spirit of Maupertuis that stood by the bay-window of the library at Berlin had on knee-breeches, silk stockings, and shoes with large silver buckles. To the philosopjer it may perjaps occur that it is very doubtful if, among the awful solemnities of the other world, the fashions ever vary. Shall we carry the vanities of life beyond the grave!

As illustrating the manner in which impressions of the past may emerge from the brain, I shall here furnish an instance bordering closely on the supernatural, and fairly representing the most marvelous of these psychological phenomena. It occured to a physician, who related it, in my hearing, to a circle whose conversation had turned on the subject of personal fear. "What you are saying," he remarked, "may be very true; but I can assure you that the sentiment of fear, in its utmost degree, is much less commmon than you suppose; and though you may be surprised to hear me say it, I know from personal experience that this is certainly so. When I was five or six years old, I dreamed that I was passing by a large pond of water in a very solitary place. On the opposite side of it there stood a great tree that looked as if it had been struck by lightning, and in the pond at another part an old fallen trunk, on one of the prone limbs of which there was a turtle sunning himself. On a sudden a wind arose, which forced me into the pond, and in my dying struggles to extricate myself from its green and slimy waters I awoke, trembling with terror.

"About eight years subsequently, while recovering from a nearly fatal attack of scarlet fever, this dream presented itself to me, identical in all respects, again. Even up to this time I think I had never seen a living tortoise or turtle, but I indistinctly remember that there was a picture of one in the first spelling-book that had been given me. Perhaps on account of my critical condition, this second dream impressed me more dreadfully than the first.

"A dozen years more elapsed. I had become a physician, and was now actively pursuing my professional duties in one of the Southern States. It so fell out that one July afternoon I had to take a long and wearisome ride on horseback. It was Sunday, and extremely hot; the path was solitary, there was not a house for miles. The forest had that intense silence so characteristic of this time of the day; all the wild animals and birds had gone to their retreats to be rid of the heat of the sun. Suddenly at one point of the road I came upon a great stagnant water pool, and casting my eyes across it, there stood a pine-tree blasted by lightning, and on a log that was nearly even with the surface a turtle was basking in the sun. The dream of my infancy was upon me; the bridle fell from my hands, an unutterable fear overshadowed me, and I slunk away from the accursed place.

"Though business occasionally afterward would have drawn me that way, I could not summon resolution to go, and actually have taken roundabout paths. It seemed to me profoundly amazing that the dream that I had had should after twenty years be realized, without respect to difference of scene, or climate, or age. A good clergyman of my acquaintance took the opportunity of improving the circumstance to my spiritual advantage, and in his kind enthusiasm - for he knew that I had been more than once brought to the point of death by such fevers - interpreted my dream that I should die of marsh miasm.

"Most persons have doubtless observed that they suddenly encounter events of a trivial nature, in their course of life, of which they have an indistinct recollection that they have dreamed before. For a long time it seemed to me that this was a case of that kind, and that it might be set down among the mysterious and unaccountable. How wonderful it is that we so often fail to see the simple explanation of things, when that explanations is actually intruding itself before us! And so in this case; it was long before the truth gleamed in upon me, before my reasoning powers shook off the delusive impressions of my senses. But it occurred at last; for I said to myself, Is it more probable that such a mystery is true, or that I have dreamed for the third time that which I had already dreamed of twice before? Have I really seen the blasted tree and the sunning turtle? Are a weary ride of fifty miles, the noontide heat, the silence that could almost be felt, no provocation to a dream? I have ridden under such circumstances many a mile fast asleep, and have awoke and known it; and so I resolved that if ever circumstances carried me to those parts again, I would satify myself as to the matter.

"Acoordinly, after a few years, when an incident led me to travel there, I revisited the well-remembered scene. There was still the stagnant pool, but the blasted pine-tree was gone; and after I had pushed my horse through the marshy thicket as far as I could force him, and then dismounted and pursued a close investigation on foot in every direction around the spot, I was clearly convinced that no pine-tree had ever grown there; not a stamp nor any token of its remains could be seen; and so now I have concluded that at the glimpse of the water, with the readiness of those who are falling asleep, I had adopted an external fact into a dream; that it had aroused the trains of thought which in former years had occupied me, and that, in fine, the mystery was all a delusion, and that I had been frightened with less than a shadow."

The instructive story of this physician teaches us how readily and yet how impressively the remains of old ideas may be recalled; how they may, as it were, be projected into the space beyond us, and take a position among existing realities. For this all that is necessary is that there should be an equalizsation of old impressions with new sensations, and that may be accomplished either by diminishing the force of present sensations, or by increasing the activity of those parts of the barin in which the old impressions are stored up.

Thus, when we are falling asleep, the organs of sense no longer convey their special impressions with the clearness and force that they did in our waking hours, and this gives to the traces that are stored up in the brain the power of drawing upon themselves the attention of the mind.

So likewise in the delirium of fevers, the spectral phantoms which trouble the sick are first seen when the apartment is darkened and kept silent, and especially when the patient closes his eyes. Until the senses are more completely overwhelmed, these shadows will disappear on brightly illuminating the room or on opening the eyes.

So too in the hour of death, when outer things are losing dull ear, and worn-out body, images that have reference to the manner of our past life emerge; the innocent and good being attended in their solemn journey by visions in unison with their former actions and thoughts, the evil, with scenes of terror and despair; and it is right that it should be so.


In this paper I commenced with a narrative of the discovery of the invisible fixed lines of the spectrum, and pursuing the natural suggestions of the subject, have been led to consider the sense of sight. I have shown how we are to explain direct vision, and how the same principles will apply to inverse vision or cerebral sight. Very few topics have a higher interest than this; for cerebral sight, personally or individually considered, presents us with the most potentous and ominous phenomena. It has influenced to an extent that we can scarcely appreciate the history of the human race. The lines invisible to the human eye, but seen by photographic substances, are ready to convey to us a world of information. They can extend all that has been done by the spectrum analysis terrestrial bodies, and reveal new facts respecting the constitution of other worlds. Of stars that we call fixed, they can tell us whether they are in motion or not, whether they are receding from or advancing toward us - information that we have gathered, perhaps less perfectly, from the visible lines.

We should ever bear in mind that the knowledge brought to us by light, the perception of things immediately around us, is but a portion of what we have really acquired. Darkness informs us tof the existence of the unicerse. In the brightness of the day we might learn that there is a sun and a moon, but it is only in the darkness of the night that there is revealed to us the infinity of worlds. From them we gather conceptions of the immensity of space, and learn how absolutely insignificant we individually are.

It is not possible to finish a subject so full of interest as this in a single paper, and so in the following one we will resume its consideration. A shadow can notfall upon a wall without leaving its permanent trace. There exists in nature an ineffaceable record of every act that every man has done.

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