In M. Niepce's account of the process, after describing the preparation of the asphalte varnish, he says:—"A tablet ofplated silver, or well-cleaned and warmglass, is to be highly polished, on which a thin coating of varnish is to be applied cold, with a light roll of very soft skin. This will impart to it a fine vermilion colour, and cover it with a very thin and equal coating. The plate is then placed on heated iron, which is wrapped round with several folds of paper, from which, by this method, all moisture has been previously expelled. When the varnish has ceased to simmer, the plate is withdrawn from the heatand left to cool and dry in a gentle temperature, and protected from a damp atmosphere. The plate, thus prepared, may be immediately subjected to the action of the luminous fluid in the focus of the camera; but even after having been thus exposed a length of time sufficient for receiving the impressions of external objects, nothing is apparent to show that these impressions exist. The forms of the future picture remain still invisible. The next operation then is to disengage the shrouded image, and this is accomplished by a solvent."
The solvent employed was a mixture of one part of oil of lavender, and ten parts of oil of petroleum. The solvent was poured over the plate, and allowed to remain. M. Niepce continues: "The operator, observing it by reflected light, begins to perceive the images of the objects to which it has been exposed gradually unfolding their forms, though still veiled by the supernatant fluid, continually becoming darker from saturation with the varnish."
The time required for the exposure of the plates in the camera was six or eight hours. For the purpose of darkening the pictures, M. Niepce used iodine, and it has been supposed that the use of iodine for that purpose suggested the employment of it to his partner.
The process adopted by M. Daguerre was, to deposit a film of iodine on a highly polished silver plate, by exposing the plate to the vapour of iodine in a dark box. The prepared plate was then placed in the camera, and after an exposure of ten minutes or more, according to the brightness of the day, an impressionwas made on the iodised silver, but too faint to be visible. To bring out the image thus invisibly impressed, the plate was exposed to the vapour of mercury, in a closed box. The mercury adhered to the parts on which the light had acted, and left the other parts of the plate untouched; and by this means a beautiful representation was produced, in which the deposited mercury represented the lights of the picture, and the polished silver the shadows. The iodised silver remaining on the plate not acted on by light, was washed away by a solution of hyposulphite of soda, and the picture could then be exposed without injury.
Nothing can exceed the delicacy of delineation by such a Daguerreotype; for the fine surface of the highly polished silver seems to exhibit the impressions of the smallest objects that emit rays of light. The length of time required to produce an impression was, however, a serious obstacle to the use of the process, as originally invented, for taking portraits. Numerous attempts were consequently made to obtain a more sensitive material. Bromine was tried, in addition to iodine, and with such complete success, that a few seconds were sufficient to effect an impression on the plate, which could be forcibly brought out by the vapour of mercury.
It was in 1840 that portraits were first taken by the Daguerreotype process in this country. In the first instance, a concave mirror was employed to concentrate the rays of light on the plate, instead of a lens; and the author has now in his possession aportrait taken in this manner, by "Wolcott's reflecting apparatus." The object of using a concave mirror was to be able to concentrate a greater number of the rays of light than could be done by a lens, and thus to form a brighter image. At the time that portrait was taken, the means had not been discovered of making the mercury adhere to the plate, and a feather would brush it away. Soon afterwards, however, M. Fizeau ingeniously contrived to fix the images on the plate by gilding it. This was done by pouring on to the plate a few drops of a diluted solution of muriate of gold, and holding it horizontally over the flame of a spirit lamp; by which means the gold was deposited and formed a thin, beautiful film of the metal over the surface, and thus not only made the picture more durable, but gave it increased effect.
The French government, fully appreciating the importance of the invention, determined to purchase it from the patentee, and to throw it open to the public. An account of the invention was published in June, 1839; and in the following month an arrangement was entered into, to the effect that, in consideration of M. Daguerre making the process fully known, a pension of 6,000 francs should be granted to him for life, and a pension of 4,000 francs to M. Isidore Niepce, the nephew of the original inventor of Photography, his uncle having died before the final success was attained.
It was generally supposed at the time, that by the grant of those pensions the invention was thrown open to the whole world, as represented by the FrenchMinister; but, nevertheless, M. Daguerre patented the process in other countries, and France alone reaped the benefit of a free use of the invention.
Whilst M. Daguerre was thus successfully working out to perfection the plan of producing beautiful naturally-impressed pictures on iodised silver surfaces, Mr. Fox Talbot was at the same time nearly attaining the same results. The following is the account given by himself of hisresearches:3—"Having in the year 1834 discovered the principles of Photography on paper, I some time afterwards made some experiments on metal plates; and in 1838 I discovered a method of rendering a silver plate sensitive to light, by exposing it to iodine vapours. I was at that time, therefore, treading in the footsteps of M. Daguerre, without knowing that he, or indeed any other person, was pursuing, or had commenced or thought of, the art which we now call Photography. But as I was not aware of the power of mercurial vapour to bring out the latent impressions, I found my plates of iodised silver deficient in sensibility, and therefore continued to use in preference my photogenic drawing paper. This was in 1838. Some time after—in August, 1839—M. Daguerre published an account of his perfected process, which reached us during the meeting of the British Association; and I took the opportunity to lay before the Section the facts which I had myself ascertained in Metallic Photography."
Whilst to M. Daguerre must be awarded the honourof having first brought to perfection the method of rendering permanent the images of the camera on metal plates, Mr. Fox Talbot may claim to be the first who perfected similar images on paper, which the comparative roughness of the surface alone prevented from being as delicately beautiful as the pictures of the Daguerreotype. He commenced his experiments in Photography in 1834; and on the 31st of January, 1839, he read a paper before the Royal Society, entitled, "Some Account of the Art of Photogenic Drawing; or, a process by which natural objects may be made to delineate themselves without the aid of the artist's pencil."
Mr. Talbot had not then succeeded in obtaining the impressions of images focused in the camera; what he had succeeded in doing was to fix upon paper the shadows of objects placed upon it, and exposed to the light of the sun. The paper was first dipped into a solution of common salt, and then wiped dry, to diffuse the salt uniformly through the substance of the paper. A solution of nitrate of silver was then spread over one surface with a soft brush, and dried carefully before a fire in a darkened room. The strength of the solution was regulated by first obtaining a saturated solution of the nitrate of silver, and afterwards diluting it with six or eight times its volume of water. The objects to be copied, such as leaves, lace, or other flat surfaces, were pressed against the prepared paper by a glass fixed in a frame, and exposure to light quickly darkened all the parts of the paper, excepting those shaded by the objects. The image thus impressedwas what is termed a "negative," the dark parts which excluded the light being left white on the paper, and the parts through which the light passed being darkened. To produce a picture corresponding with the natural lights and shades, the process was repeated, substituting the picture first obtained, on thin transparent paper, for the original object, by which means the lights and shadows were reversed.
The chloride of silver, formed on the surface of the sensitive paper by the combination of the common salt and nitrate of silver, being insoluble in water, great difficulty was experienced in washing it away, so as to prevent the whole surface from afterwards darkening on exposure to light. The application of hyposulphite of soda, for the purpose of making the pictures durable, was suggested by Sir John Herschel, and it answers remarkably well, as it dissolves the chloride of silver. A solution of ammonia is nearly equally efficacious in removing the chloride.
The Calotype process, by which the images of the camera can be fixed upon paper, was invented by Mr. Talbot, in 1840. It is thus described:—Dissolve 100 grains of crystallized nitrate of silver in 6 ounces of distilled water. Procure some fine writing paper, and wash one side of it with the solution, laid on with a soft brush; then dry the paper cautiously, by holding it at a distance from the fire. When dry, dip the paper into a solution of iodide of potassium, containing 500 grains dissolved in 1 pint of water, and let it remain in the solution two or three minutes. Then dip it into a vessel of water; remove the water on thesurface by blotting paper, and dry it by a fire, in the dark or by candle-light. The paper thus prepared is called "iodised paper;" it is not very sensitive to light, and may be kept for some time without spoiling. Next dissolve 100 grains of crystallized nitrate of silver in 2 ounces of distilled water; add to the solution one-sixth of its volume of strong acetic acid, and call that mixture A. Then make a strong solution of crystallized gallic acid in cold water, and let that solution be called B. Mix equal volumes of A and B together in small quantities at a time. That mixture Mr. Talbot calls gallo-nitrate of silver, and with it wash over the surface of the iodised paper. Allow the paper to remain half a minute, and then dip it into water, and again dry it lightly with blotting paper. The paper thus prepared is very sensitive, and will receive an impression in the camera in the shortest possible time. The impression is at first invisible, but it may be brought out by laying the paper aside in the dark, or by washing it once more in the gallo-nitrate of silver, and holding it at a short distance from the fire. To fix the picture, the paper is first washed in water and lightly dried, and then soaked in a solution of hyposulphite of soda for a few minutes, by which means the iodised silver is removed, and after being again washed in water and dried, the process is completed. The picture thus produced is a negative one, and requires to be transferred in the manner before stated. The original Calotype may, by that means, serve to produce a great number of pictures.
Mr. Talbot's patent was sealed on the 8th ofFebruary, 1841. In his specification, he claimed the use of gallic acid, and he succeeded in enforcing his claim in a Court of Law, though it appeared that on the 10th of April, 1839, photographs of objects taken in the solar microscope in five minutes, by the Rev. J. B. Reade, were shown at the London institution, which were described to have been produced by an infusion of galls, and fixed with hyposulphite of soda. It must be mentioned, however, to Mr. Talbot's honour, that on a representation to him by the President of the Royal Society that the art of Photography was impeded in its progress in this country by patent monopolies, he generously made a present to the public of all his inventions and discoveries, reserving to himself only the privilege of taking portraits.
The transfer from one paper to another of the picture obtained in the camera, and the comparative roughness of the surface of the paper itself, prevent Calotypes from exhibiting that sharpness and delicacy of definition which are so admirable in a Daguerreotype. Several attempts were therefore made to obtain a more smooth surface for the reception of the image; but without much success, until glass was adopted for the purpose. To make that material available, it is necessary to coat it with some substance that will absorb the sensitive solution. In the first instance, the white of eggs was employed with considerable success. Albumen has, however, been supplanted by collodion—a solution of gun-cotton in ether—which is found to be peculiarlysuitable for the reception of the sensitive preparation of silver.
In conducting the collodion process, the collodion is first iodised by adding to it iodide of potassium and iodide of silver, dissolved in alcohol. The iodised collodion is then poured over a plate of glass that has been carefully cleaned, and is moved about horizontally until a perfectly uniform film is spread over the surface, to which it adheres firmly. The plate is afterwards dipped into a solution of nitrate of silver, which renders it so highly sensitive to impressions of light, that it will receive an image in less than a second. The image is latent, until it is developed by pouring over the plate a mixture of pyro-gallic acid in distilled water, acetic acid, and nitrate of silver. The impression is fixed with hyposulphite of soda.
The pictures produced by the collodion process are negatives, which serve admirably for transferring positive pictures on to sensitive paper. But, if required, the negative picture can be readily changed into a positive one, by converting the darkened silver into white metallic silver, by a mixture of protosulphate of iron and pyro-gallic acid. In a short time a white metallic image is obtained, which, when relieved by a background of black velvet or black varnish, equals in delicacy of finish the most beautiful Daguerreotypes.
Many attempts have been made, but hitherto without success, to obtain photographs coloured, as well as shaded, by nature. The opinions of thosewho have most studied the subject differ as to the possibility of ever attaining that desired object. Sir John Herschel has so far shown that it is not impossible, as to have impressed the colours of the solar spectrum on paper, by the mere action of light; and parts of the images of objects fixed on the screen of the camera are also sometimes coloured. These facts induce us to hope that in the progress of discovery some means may be found of obtaining naturally-coloured photographs, notwithstanding it has been pronounced, by good authorities, to be an absolute impossibility.
Specimens of coloured photographs were exhibited by Mr. Mercer at the recent meeting of the British Association, which showed that by the use of various chemical preparations that are sensitive to light, photographs may be shaded in colours. The principal re-agents employed were salts of iron, and by immersing the paper in suitable menstrua, after the image had been impressed in the camera, the picture was developed in any colour required; the same tint being spread over the whole. One purpose to which it was suggested this coloured photographic process is applicable, is printing on woven fabrics, the action of light serving as a mordant to fix the colours.
Photography has been already applied to various uses, and it is capable of being rendered much more valuable. To the meteorologist it affords the means of registering the rise and fall of the mercury in the barometer and thermometer, and, by a self-registeringapparatus, the changes of temperature and of atmospheric pressure are marked upon paper that records the time at which the changes occur. It may also be applied, in the same manner, to register the directions of the wind, and the times of its changes. The sun impresses his own image upon paper; and the spots on his surface, thus correctly delineated, can be compared with those seen in pictures of the sun at other times; and the foundation is laid for more correct knowledge of the nature of those appearances, and of the motion of the sun himself. Photographs of the moon and planets present exact representations of those heavenly bodies, as seen through the most powerful telescope; and, with the assistance of the stereoscope, the figure of the moon is shown in its true globular form, as it can be seen by no other means. It has been proposed, indeed, by the aid of Photography, to extend our knowledge of the stars far beyond the reach of telescopic vision; for as the image focused on the screen of the camera is composed of rays from every object on the body of a star, it might be possible to see those objects by greatly magnifying the image. It remains, however, for the further progress of discovery and invention, to arrive at so delicate a delineation by photographic processes, as to obtain landscapes of the moon, and portraits of the inhabitants of Jupiter!
One of the latest advances in the art of Photography has been the engraving on steel-plates by the action of light, by which means more forcible effects have been obtained than by the impressions of lightupon paper. Mr. Fox Talbot has distinguished himself in thus fixing the images on steel, as he was the first to impress them upon paper. In his method of doing so, he covers the steel plate with a solution of isinglass and bichromate of potass, and placing a collodion negative picture upon it, he exposes it to the action of light. When the picture is sufficiently impressed, he etches it into the plate by means of bichloride of platinum. M. Niepce, the nephew of the original inventor of Photography, has produced the same effect by reviving the first processes adopted by his uncle; using, as he did, bitumen, dissolved in essential oil of lavender, to cover the plates. Two other foreign photographers, M. Poitevin and M. Pretschi, have also successfully directed their attention to engraving the images of the camera, which has now obtained a high degree of perfection.
It is well worth notice that these most recent improvements in Photography are but further developments of the original designs of M. Niepce, who not only succeeded in etching the pictures impressed by the light of the sun on his metal tablets, but made use of a glass surface, on which the now generally adopted collodion process depends.
There are no optical illusions more extraordinary than those shown in the exhibition of Dissolving Views. The effects of the changes in the diorama are only such as are seen in nature, the same scene being represented under different circumstances, and the marvel in that case is that such beautiful and natural effects can be produced on the same canvas. But Dissolving Views set nature at defiance, and exhibit metamorphoses as great as can be conceived by the wildest fancy.
Whilst, for instance, the spectator is looking at the interior of a church, he sees the objects gradually assuming different appearances. The columns that support the vaulted roof begin to fade away, and their places are occupied by other forms, which gradually become better defined and stronger, and a tree, a house, or, it may be, a rock, thrusts the columns out of view, and the roof dims into blue sky, chequered with clouds. The original view thus entirely disappears, and the scene is changed from the interior of a church to open country, or to a rocky valley. Thisis done, not by changing at once one scene into another, but by substituting different individual objects, which at first appear like faint shadows, and then, becoming more and more vivid, at length altogether supplant their predecessors on the field of view, and will, in their turn, be extinguished by others.
It sometimes happens that some strongly marked object resists apparently the efforts made to dispossess it, and in the midst of a mountainous scene will be observed the form of a chandelier or of a statue, that occupied a distinguished place in the church that has just vanished. In a short time, however, these relics disappear, and the mountain, the valley, and the lake are freed from the incongruous images of the former scene.
These effects are produced in a manner as simple as they are extraordinary. All that is requisite is to have two magic lanterns fitted on to a stand, with their tubes inclined towards each other, so that both discs of light may exactly coincide, and form on the screen a single disc. If paintings on glass, representing different views, be then placed in each lantern, with the lenses adjusted to bring the rays to a focus on the screen, the two images will be so mingled together as to present only a confused mixture of colours. Suppose one of the views to be the interior of a church, and the other to be a mountain scene;—the pillars of the church will be mingled with trees and rocks, and in the midst of the confusion there may perhaps be discerned a strongly painted chandelier or an altarpiece. When an opaque shade is placed before the lens of either of the lanterns, to prevent the light from reaching the screen, the previous confusion becomes instantly clear and distinct, and the church or the landscape is seen without any interfering images. If the opaque screen be gradually withdrawn from one lens, and at the same time drawn in an equal degree over the other, the different objects will again be mingled, and those in the one scene will predominate over those in the other in proportion to the relative quantities of light permitted to issue from each lantern to the screen. The two first of the accompanying drawings are thus blended together in the third, when the screen is half withdrawn from each.
It is usual to fix the opaque shade, which alternately covers and exposes the two magic lanterns, on to a central pin, so that it may be moved vertically up or down. The shade is so arranged, that in raising the end to cover the lens of one lantern, the farther end descends, and exposes, in an equal degree, the other lens. During the time that either of the views is altogether concealed, the painting is changed; and in this manner an unlimited number of metamorphoses may be effected.
It requires no expensive apparatus to show the effect of Dissolving Views on a small scale. Two common magic lanterns are quite sufficient for the purpose of private exhibition, and the angle at which they should be fixed on their stand may be readily ascertained after a few trials. To make the transformation more extraordinary, a man's face may be painted on one glass and a landscape on the other; and, when the change is made from the face to the landscape, a strongly painted eye or nose may be seen occupying the centre of the view, long after the other features have disappeared, until all the rays of light from that painting have been excluded. The change from youth to age, from beauty to ugliness, may also be shown with striking effect.
It will be observed that the principle, on which the metamorphoses of Dissolving Views depend, is similar to that which produces the variations in the diorama. In both cases there are two paintings on the same space, either of which may be shown at pleasure by different dispositions of the light; thechief difference between them being that the Dissolving Views are seen altogether by reflected light, whilst in the diorama the paintings at the back and front are shown alternately by reflected and by transmitted light.
No invention, on being first brought out, created so general a sensation as the Kaleidoscope. Every person, who could buy or make one, had a Kaleidoscope. Men, women, and children—rich and poor; in houses or walking in the streets; in carriages, or on coaches—were to be seen looking into the wonder-working tube, admiring the beautiful patterns it produced, and the magical changes which the least movement of the glass occasioned.
It was in the year 1814 that Sir David Brewster discovered the principle on which the effects of the Kaleidoscope depend, whilst he was engaged in experiments on the polarization of light by successive reflections between plates of glass. The reflectors were in some cases inclined to each other, and he remarked the circular arrangement of the images of a candle round a centre. In afterwards repeating the experiments of M. Biot on the action of fluids on light, he placed the fluids in a trough formed by two plates of glass cemented together at an angle. The eye being placed at one end, some of the cement which pressed through between the plates appeared to be arranged in a circular figure. The symmetry ofthis figure being very remarkable, Sir David Brewster undertook to investigate the cause of the phenomenon, and the result of his investigations was the invention of the instrument to which he gave the name of Kaleidoscope, from the Greek words καλος {kalos}, beautiful, ειδος {eidos}, a form, and σκοπεω {skopeô}, tosee.4
The Kaleidoscope in its simplest form consists of two equal strips of plate glass, about 8 inches long and 2 inches wide, silvered on one side, to act as reflectors. These glasses are placed one over the other exactly, and then the edges on one side being separated, whilst the two other edges are kept close together, they are fixed by means of separating pieces of wood and string at the angle required. The glasses are then fitted into a metal tube, which has an eye-hole at one end, and at the other end of the tube there is fixed a small cell of ground glass, to contain pieces of differently stained glass or other objects, that are to be multiplied by reflections into beautiful symmetrical figures. In the better kind of Kaleidoscopes, the cell containing the objects may be turned round, by which means the pieces of glass shift their positions, and the figures instantly change. The same effect is produced, though in a less agreeable manner, in the common kind of instruments, by turning the tube.
To form by the combined reflections from the two glasses a perfectly symmetrical figure, the sector comprised between the inclined sides of the glasses may consist of any even aliquot part of a circle. Inthe accompanying diagram, the ends of the flat silvered glassesa c,b c, are inclined at an angle of 60 degrees; therefore the circle is completed by the junction of six sectors. In such a Kaleidoscope, the circular figure will be formed by three reflections from each glass.
To make the formation of the circular figure by repeated reflections more intelligible, we will consider it as composed of the smallest possible number of equal divisions, as in the second diagram, in which the circle is divided into quadrants. In such an arrangement of the reflectors, the figure seen on looking through the central aperture will consist of four parts. In the first place, the objects included in the spacea b c, between the inclined glasses, will be seen directly by rays of light from the objects themselves; viz., the small crossd, and the trianglee. The same field ofview will be reflected from both mirrors, by which reflection the cross on one side will seem to be doubled, and the triangle on the other will have another similar one added to it, to make a complete rhomb. The cross will also be reflected by the mirror on the right side, and the triangle by the one on the left. The images of the objects contained within the spacea b c, being thus presented by reflection on both sides, they become the objects for further reflections from parts of the mirrors still nearer the spectator. Thus the imagesd1on both sides are reflected to form the single imaged2, and the imagese1are in the same manner reflected to form the second imagee2.
When the angle formed by the inclination of the mirrors divides the circle into a greater number of sectors, the reflections of the images are repeated, from points nearer and nearer to the eye, and the circle is thus completed, however numerous the sectors may be; but at each repetition of the reflection, the images will become more dim, since, owing to the imperfection of reflecting surfaces, a portion of the light is absorbed at each reflection.
In the first instruments that were constructed, the objects were fixed in the field of view, therefore scarcely any change of pattern was obtainable. It was not until some time afterwards that the idea occurred to Sir David Brewster of producing endless changes of the figures, by making the objects movable in a cell of glass at the end of the instrument. He afterwards introduced other improvements in the Kaleidoscope, for extending its range of objects, forvarying the angles of inclination, and for projecting the figures on a screen. In the instrument, as ordinarily made, the objects to be seen properly must be placed close to the end of the reflectors; but by the addition to the instrument of a tube containing a lens, the rays from distant objects are brought to a focus near the mirrors, and the image formed there is repeated by the reflectors in the same manner as a solid object.
The projection of the figures on a screen, by an apparatus similar to a magic lantern, gives great additional pleasure to the effects of the Kaleidoscope, as the figures are not only seen by several persons at the same time, but they are presented in a magnified form. The projection of the figures also increases the use of the instrument in designing patterns, for which purpose it has been employed with great advantage.
A patent for the Kaleidoscope was taken out in 1817, but the high prices charged by the opticians who were authorized by the inventor to sell the instrument, and the facility with which it could be made, occasioned a general violation of the patent right, and it was not long before the claim of Sir David Brewster, as the original inventor, was disputed. In the indignant vindication of his claim, he observes:—"There never was a popular invention which the labours of envious individuals did not attempt to trace to some remote period;" and the Kaleidoscope was not an exception. It was found that Kircher had described the effects of repeated reflections as far back as 1630; and that Mr. Bradleyhad, in 1717, made a philosophical toy, consisting of two small mirrors, that opened like a book, which, when partially opened, repeated the reflections of objects placed near it in the same manner as the Kaleidoscope. But this instrument was so different in its construction, and in the effects it produced, from the Kaleidoscope, that Sir David Brewster's claim to be the inventor may be freely admitted. The fact that it took the world by surprise, and created a sensation greater than any other invention had done before, is sufficient to establish its title as an original invention.
There are several ways of illustrating the retention by the retina of the eye of the images of objects after they have been withdrawn from sight, but none is so curious as the philosophical toy called the Magic Disc, which, from the optical principles involved in its extraordinary effects, deserves to be noticed as one of the remarkable inventions of the present century.
One of the most striking methods of exhibiting the retentive property of the retina, before the invention of the Magic Disc, was to paint different objects at the back and on the front of a card, and by then giving rapid rotation to the card, both objects were seen together. Thus, when the figure of a bird is painted on one side, and an empty cage on the other, by rapidly turning the card, the bird appears to be in the cage. In the Magic Disc the objects are painted on the same side of a circular piece of card-board, and both are exposed to view during their rapid rotation.
The disc is divided into eight or ten compartments, in each one of which the same figures are repeated, though the positions of one or more of them are changed. A favourite subject represented is a clownleaping over the back of a pantaloon, which affords a simple illustration of the apparent relative movements of two bodies, and will serve to explain how the effect is produced.
The instrument consists of a disc of stiff card-board, about nine inches diameter, mounted on a horizontal pivot in the centre, on which it may be freely turned. Between each of the compartments of the disc there is an elongated aperture, about one inch long and a quarter of an inch wide, for the eye to look through. Suppose the disc to be divided into eight compartments, by radial lines. In the compartment No. 1, the pantaloon is represented in a stooping posture,and the clown is on the ground ready to make a spring. In No. 2 the pantaloon is in the same attitude, but the clown has commenced his leap, and is raised a little way from the ground. In the third division he is shown still higher in the air; and in the fourth he is mounted above the shoulders of pantaloon, who retains the same posture as at first. The fifth compartment represents the clown as having jumped over pantaloon's head, and coming down to the ground; and in each succeeding division his farther descent is shown, till, in No. 8, he has reached the ground again, and is ready to recommence the leap.
When the disc is turned rapidly round on its pivot, the figures painted upon it are mingled together, and present a confused medley of lines and colours, in which no object can be distinctly defined. This mingling of the objects is caused by the retention of the images by the retina, so that if the eye be directed to any point, the impression of the lines and colours that pass rapidly before it is not effaced before another and another appear to produce fresh impressions, and they mingle together in confusion. If, for instance, there were a circle formed of dots marked on the disc, the impression of each dot on the retina would be prolonged; and as, by the rotation, other dots would come into the field of view before the impression of the first was removed, it would form an unbroken ring. But if the disc were screened from sight, at intervals of nearly equal duration to that of the continuous impression, so as to efface the image of one dot before the rays of another were admitted to theeye, then the ring would be seen to be composed of dots, as distinctly as when the disc was stationary.
The effect of screening the objects from the eye at short intervals is produced by looking with one eye through the openings at the image of the disc, reflected from a mirror. The figures are then seen only when the apertures come opposite the eye; but as the impression of one view remains till it is renewed by the light admitted through the next aperture, there is continuous vision of the objects painted on the disc.
It is thus that the figures of pantaloon and clown become visible, and their apparent relative movements are occasioned. For instance; each time that the impression of the figure of the pantaloon is renewed, he is seen in the same place and in the same attitude; therefore he appears to be stationary, though the successive pictures that compose his figure to the eye are in rapid rotary motion. The figure of the clown, however, is seen in a different position each time that he comes into view, therefore he appears to be in motion relatively to pantaloon, though stationary as regards his absolute position on the disc.
The same effect would be produced if the disc, during its rotation, were seen by successive electric sparks. The electric spark is so momentary in its duration, that the most rapidly moving objects appear stationary; therefore each spark would show a seemingly stationary disc, on which the figure of the clown would appear in different relative positions; and the illusion would be as perfect as when the rays of light are interrupted at intervals.
The electric spark is so instantaneous that a cannon ball might be seen in its rapid flight, if illuminated by a flash of lightning, and would seem to be stationary. Professor Faraday mentioned, in one of his lectures, the extraordinary appearance which a man, who was jumping over a stile, presented when seen by lightning on a dark night. The man seemed to be resting horizontally in the air, with one hand touching the stile.
The duration of the impression of an object on the retina is capable of illustration by means of the Magic Disc in a great variety of designs, each one of which may represent many movements. The turning of the wheels of machinery, the tossing of balls, the dancing figures of men and women may thus be shown, the designs for which afford ample scope for exercising the pencil of an ingenious artist.
Those who are old enough to remember the Regent's Park before there were any houses northward of the New Road, may recollect that among the first buildings erected, on what is now called Park Square, was a strange-looking, partly semi-circular erection, provided with ample lighting space, which attracted great attention during its progress, and was the cause of much speculation as to its probable purpose. That building was intended for the exhibition of the Diorama.
M. Daguerre, the inventor of the Daguerreotype, had, in conjunction with M. Bouton, a short time previously opened a similar exhibition in Paris, where the beauty of the paintings, aided by the extraordinary effects of newly contrived dispositions of the light, had excited a great sensation. The Diorama was opened in London on the 6th of October, 1823, and for a long time it was equally popular in this metropolis.
The visitors, after passing through a gloomy anteroom, were ushered into a circular chamber, apparently quite dark. One or two small shrouded lampsplaced on the floor served dimly to light the way to a few descending steps, and the voice of an invisible guide gave directions to walk forward. The eye soon became sufficiently accustomed to the darkness to distinguish the objects around, and to perceive that there were several persons seated on benches opposite an open space, resembling a large window. Through the window was seen the interior of a cathedral, undergoing partial repair, with the figures of two or three workmen resting from their labour. The pillars, the arches, the stone floor and steps, stained with damp, and the planks of wood strewn on the ground, all seemed to stand out in bold relief, so solidly as not to admit a doubt of their substantiality, whilst the floor extended to the distant pillars, temptingly inviting the tread of exploring footsteps. Few could be persuaded that what they saw was a mere painting on a flat surface. This impression was strengthened by perceiving the light and shadows change, as if clouds were passing over the sun, the rays of which occasionally shone through the painted windows, casting coloured shadows on the floor. Then shortly the brightness would disappear, and the former gloom again obscure the objects that had been momentarily illuminated. The illusion was rendered more perfect by the excellence of the painting, and by the sensitive condition of the eye in the darkness of the surrounding chamber. Whilst gazing in wrapt admiration at the architectural beauties of the cathedral, the spectator's attention was disturbed by sounds underground. He became conscious that the scene before him was slowlymoving away, and he obtained a glimpse of another and very different prospect, which gradually advanced until it was completely developed, and the cathedral had disappeared. What he now saw was a valley, surrounded by high mountains capped with snow. This mountain valley seemed scarcely less real than the arched roof and columns of the cathedral, whilst a foaming cascade, dashing down the rocks, and the sound of rushing waters, added to the illusion. After looking for some time at this beautiful valley, the clouds were seen to gather on the mountain tops, and a storm impended. A gleam of sun-light, still resting on the edge of the clouds, exhibited a strange contrast between the silvery brightness and the dense black vapour that shrouded the hills, and could almost be felt. It was but a passing thunderstorm. Presently the dark clouds rose from the valley, and dispersed; the sun again shone on cottage, vineyard, and mountain, charming the spectator as much by the beauty of the scene as he was astonished by the wonderful change.
Such was the Diorama as it was first exhibited in London to admiring crowds. In subsequent years greater changes were made in the variations of light and shade; and by the introduction of mechanical contrivances, with more or less success, the magical effects were increased, without, however, adding to the apparent reality of the objects. A church or cathedral was always the subject of one view, and sometimes of both. The interior of an empty church would be shown by evening twilight. The shades ofevening gradually darkened into the obscurity of night, and then the glimmer of candles would be seen spreading more and more widely, until the church was lighted up, and it was occupied by a crowded congregation at midnight mass. Some views represented the exterior of a ruin or of a cathedral after sunset, and as night advanced, the stars twinkled in the blue sky, and the moon rose and threw its silvery light on water, buildings, and clouds, contrasting in some cases with the red glare of lamps from the windows of houses and shops. The disc of the moon exactly resembled that of the real luminary, and all around being so dark, the rays from its surface cast shadows of intervening objects. In one picture a still more astonishing appearance was produced, by the change of the interior of a beautifully painted and decorated church into a mass of charred ruins.
The means principally adopted for the production of these magical changes in a painting on a flat surface, and for giving such seeming reality to the objects represented, were for some time kept secret; nor do we think they are even yet much known. As in many other clever inventions, the effects are produced in a very simple manner. The picture is painted on both sides of a transparent screen, and the change of scene is occasioned almost entirely by exhibiting the picture at one time by reflected light, from the surface nearest the spectator, and afterwards by transmitted light, after excluding the light from the front.
Let us take for illustration the interior of a church,at first seen empty, and afterwards filled with people, and illuminated by candles. The empty church is painted on the front on fine canvas or silk, in transparent colours, and at the back are the figures and candles, and other objects intended to appear with them. The arrangements for illuminating the picture are so contrived, that the light may be thrown entirely on the front or on the back, or partly on both. When the light is on the front, the empty church only is visible. It is then gradually darkened, and the back of the picture is illuminated, by which means the figures and candles are seen; and the form of the building being preserved, the same church, which was before empty, becomes occupied by a crowded congregation.
It may be mentioned, as an illustration of the perfect illusion of the Diorama, that a lady who on one occasion accompanied the author to the exhibition, was so fully convinced that the church represented was real, that she asked to be conducted down the steps to walk in the building.