Part 8
The object of using semi-lenses is to facilitate the union of the two pictures into one, by looking through the lens towards its edge, instead of through the centre, the image being thus refracted to a different position. This may be easily exemplified by looking at an object steadily through different parts of the same lens. After looking at it with the right eye through the centre, and whilst keeping the axis of the eye in the same direction, move the lens slowly towards the right, so as to bring the edge of the lens opposite the pupil. This movement of the lens towards the right hand will be accompanied by an apparent movement of the image towards the left, so as to bring it to a point between the two eyes. If the experiment be repeated with the left eye, the image will be removed towards the right hand; and thus, by looking at the two stereoscopic pictures through the thin parts of two lenses, the images are superposed and form a single one.
Sir David Brewster attached much importance to the semi-lenses, which have the effect of prisms in refracting the rays of light; but that form of lens is not essential to give apparent solidity to the images; and many of the commoner kind of instruments are now made with ordinary double-convex lenses, and without any partition. With the semi-lens, however, there is less difficulty in uniting the two pictures into one than when an ordinary lens is employed.
In taking photographic pictures for the Stereoscope with a single camera, it is necessary to alter the angle of the instrument after having taken one picture, to direct it to the same object in the angle of vision as seen by the other eye. This method of producing stereoscopic pictures with the same camera is very objectionable when any moving objects are in the field; for they will be in a different position in each, and sometimes disappear altogether from the second picture. The plan adopted by the best photographers is to have two cameras set at the requisite angle to each other, so that both pictures or portraits may be taken at the same time.
At the meeting of the British Association in 1853, M. Claudet endeavoured to establish some rules for the angle at which photographic pictures must be taken, in order to produce the best effect of relief and distance without exaggeration. He observed, that in looking at a single picture with two eyes, there is less relief and less distance than when looking at it with one eye, because in the latter case we have the same effect we are accustomed to feel when we look at the natural objects with one eye; while, if we look at the single picture with two eyes, we have on the two retinæ the same image with the same perspective, which is not natural, and the eyes have not to make the usual effort for altering their convergence according to the plane on which the object observed is situated. This inaction of the convergence of the eyes diminishes the illusion of the picture, because the same convergence for all the objects represented gives an idea that they are all placed on the same plane. The photographic image being the representation of two different perspectives, we must, when we look at them in the Stereoscope, as when looking at the natural objects themselves, converge, more or less, the axes of the eyes. Therefore we make the same effort, and have the same sensation in regarding the combined photographic pictures, as when we look at the objects represented.
Sir David Brewster has suggested various applications of the Stereoscope; viz., to painting, to sculpture and engineering, to natural history, to education, and to purposes of amusement. The latter is the principal purpose to which the instrument is at present applied; and some of the many ways in which it may contribute to delight the spectator are pointed out in Sir David Brewster's book.
"For the purpose of amusement," he observes, "the photographer might carry us even into the regions of the supernatural. His art enables him to give a spiritual appearance to one or more of his figures, and to exhibit them as 'thin air,' amid the solid realities of the stereoscopic picture. While a party are engaged with their whist or their gossip, a female figure appears in the midst of them with all the attributes of the supernatural. Her form is transparent; every object or person beyond her being seen in shadowy but distinct outline. She may occupy more than one place in the scene, and different portions of the group might be made to gaze upon one or other of the visions before them. In order to produce such a scene, the parties which are to compose the group must have their portraits nearly finished in the binocular camera, in the attitude which they may be supposed to assume if the vision were real. When the party have nearly sat the proper length of time, the female figure, suitably attired, walks quickly to the place assigned to her, and after standing a few seconds in the proper attitude, retires quickly, or takes as quickly a second, or even a third, place in the picture, if it is required, in each of which she remains a few seconds, so that her picture in these different positions may be taken with sufficient distinctness in the negative photograph. If these operations have been well performed, all the objects immediately behind the female figure, having been previous to her introduction impressed upon the negative surface, will be seen through her, and she will have the appearance of an aërial personage, unlike the other figures in the picture."
It is in the foregoing manner that the remarkable stereoscopic effect of "Sir David Brewster's ghost" is produced, a representation of which is given in the next page.
Sir David Brewster mentions many other curious applications of the Stereoscope, among which are the dioramic effects of pictures seen alternately by reflected and by transmitted light; a daylight view being apparently lighted up artificially in the night, by seeing it at one time with the light reflected from the surface, and then excluding the light from the front, and viewing it as a transparency.
One of the most interesting effects of the Stereoscope has been recently produced by Mr. De la Rue, who has contrived the means of giving apparent rotundity to the surface of the moon, as viewed through a powerful telescope. The disc of the full moon, however highly magnified, presents, as is well-known, the appearance of a flat surface, with the lights and shadows marked seemingly on a plane. Owing to the great distance of that luminary, there is no variation in its appearance, whether it be looked at with one eye or with the other, therefore it seems removed beyond the operation of the ordinary cause of stereoscopic effects. Nevertheless, Mr. De la Rue has taken photographs of the moon which, when placed in the Stereoscope, combine to form a solid-looking globe, on which all the lights and shadows are distinctly and beautifully delineated. He has produced this effect by taking his photographs at different periods of the year, when there is a slight variation in the direction of the moon's face to the earth; and by combining these separate photographs into one image in the Stereoscope, the form of the moon appears as convex as the surface of an artificial globe.
M. Claudet, who is one of the most successful photographers in the metropolis, has contrived an arrangement which he calls a "Stereomonoscope," by which the appearance of solidity is communicated to a single image formed on a screen of ground glass. The screen of ground glass has a black back, and is placed in the focus of a lens in an ordinary camera obscura, wherein the image may be seen by looking down upon it. The particles of the roughened glass reflect to each eye different parts of the image focused on the screen, and by this means a similar effect is produced as when two dissimilar pictures are looked at through a stereoscope instrument. One great advantage of this arrangement is that several persons may look at the image at the same time.
Mr. John Sang, of Kirkaldy, has very recently imparted stereoscopic effect to copies of paintings and engravings, the flat surfaces of which were previously thought to defy any such application of the Stereoscope. The means he employs of doing so are at present kept secret, but he has shown its practicability by copying, on wood engravings, Mr. George Cruikshank's series of "The Bottle." In some respects this process seems almost more wonderful than the original Stereoscope, for it gives solid form and apparent substantiality to the mere creations of the artist's pencil.
THE ELECTRIC TELEGRAPH.
No application of science has so completely realized the visions of fancy as the Electric Telegraph. So closely, indeed, does the real of the present day approach to the ideal of ages past, that it might be supposed the narratives in the tales of faëry land were true records of the inventions of former times, and that the combined efforts of inventive genius during the last half century were but imitations and reproductions of what had been successfully accomplished "once upon a time." There is also an intermediate period--between the indefinite of faëry tales and the positive of scientific history--in which sympathetic tablets and magical loadstones, scarcely less mythical, are stated to have been invented; and the individuals are named who thus paved the way for instantaneous communication between all parts of the world.
The Jesuits of the sixteenth and seventeenth centuries took the place of the magicians of the Middle Ages. In the seclusion of their monasteries, they speculated on the mysterious powers of Nature, then partially revealed to them, and shadowed forth images of their possible applications. It is to a vague speculation of this kind that we may attribute the notice given by Strada, in his "Prolusiones Academicæ," of the sympathetic magnetic needles, by which two friends at a distance were able to communicate; though the then fanciful idea has been literally realized. A still more extraordinary foreshadowing of one of the most recent improvements of the Electric Telegraph was the transference of written letters from one place to another by electric agency. This is said to have been accomplished by Kircher, who, in his "Prolusiones Magneticæ," describes, though very vaguely, the mode of operation. But even admitting that there were substantial foundations for these imaginary phantasms, that would not in the least detract from the merit of those who, following closely the footsteps of scientific discovery, have successfully applied the principles unfolded by the investigations of others, and by their own assiduous researches. Thus, whilst steam navigation was facilitating the means of intercourse over rivers and seas, and whilst railways and locomotive engines served to bring distant cities within a few hours' journey of each other, another source of power, infinitely more rapid in its action than steam, has been made to transmit intelligence from place to place, and from one country to another, with the speed of lightning.
The plan of making communications by signals has been in operation from time immemorial; the beacon lights on hills having served in ancient as well as in modern times to give warning of danger, or to announce tidings of joy. Such simple signals were not capable of much variety of expression; but even beacon lights might be made to indicate different kinds of intelligence, by multiplying the number of the fires, and by altering their relative positions. It was not, however, till the invention of telegraphs that anything approaching to the means of holding regular communication by signals was attained. The semaphore of the brothers Chappe, of France, invented by them in 1794, was the most perfect instrument of the kind, and was generally employed for telegraphic purposes, until it was supplanted by the Electric Telegraph.
The semaphore consisted of an upright post, having arms on each side, that could be readily extended, at any given angle. The extension of these arms on one side or the other, either separately or together, and at different angles, constituted a variety of signals sufficient for the purposes of communication. The semaphores, erected on elevated points, so as to be visible through telescopes, signalled intelligence slowly from one station to another, till it reached its ultimate destination; and thus--daylight and clear weather permitting--brief orders could be sent from the Admiralty to Portsmouth in the course of a few minutes. But the communication was liable to be interrupted by fogs, as well as by nightfall.
A remarkable instance of the imperfection of sight telegraphs occurred during the Peninsular War. A telegraphic despatch, received at the Admiralty from Portsmouth, announced--"Lord Wellington defeated;"--and then the communication was interrupted by a fog. This telegraphic message caused great consternation, and the utmost anxiety was experienced to learn the extent of the supposed disaster. When, however, the fog dispersed, the remainder of the message gave a completely opposite character to the news, which in its completed form ran thus: "Lord Wellington defeated the French," &c.
Some better means of transmitting important intelligence was evidently wanted; for not only was the semaphore liable to frequent interruptions by the weather, but its action was very slow, and the frequent repetitions from station to station increased the risk of blunders.
The instantaneous transmission of an electric shock suggested the means of communicating with greatly increased rapidity; and when it was ascertained, by experiments made by Dr. Watson at Shooter's Hill, in 1747, that the charge of a Leyden jar could be sent through a circuit of four miles, with velocity too great to be appreciable, the practicability of applying electricity for conveying intelligence became at once apparent.
Of the many means by which this object was attempted to be accomplished, it will be only possible, in this general survey, to notice those that mark the first steps of the invention, and the most important of those that have accompanied its progress to the present time.
The first method that suggested itself was to transmit signals by means of pith-ball electrometers. When, for instance, two pith-balls are suspended from a wire that is made to form part of an electric circuit, the electricity communicated to the balls causes them to diverge, and when the electricity in the wire is discharged, they immediately collapse. This action of pith-balls, when electrified, was the simplest mode known of making telegraphic signals, and it was accordingly adopted by several of the early inventors of Electric Telegraphs. The first person who proposed to apply it for that purpose was M. Lesage, of Geneva, in 1774. His plan was to form 24 electric circuits by as many separate wires, insulated from each other in glass tubes; and to place in the circuit, at each communicating station, an equal number of pith-ball electrometers. Each electrometer was to represent a letter of the alphabet, and they were to be brought into action by an excited glass rod. When a communication was to be made, the wires connected with the separate galvanometers were to be charged alternately with electricity by the excited rod of glass; and the person at the receiving station, by noticing which of the electrometers were successively put into action, could spell the words intended to be communicated.
By the means thus proposed, correspondence could have taken place at only short distances, for the charge of an excited glass rod would have been too feeble to produce any sensible effect on the electrometers had the length of the circuit been considerable. This difficulty might have been overcome by substituting the charge of a Leyden jar for the excited glass; but the more serious obstacle to the use of such a telegraph would have been the cost, and the difficulty of insulating the 24 wires required to work it.
Most of the early telegraphic inventors encumbered their inventions with the same obstacle, as they seemed to consider it necessary to have a separate circuit for each letter of the alphabet. It was not so however, with all; for M. Lomond, a Frenchman, who ranks second in the list of telegraphic inventors, modified the principle of M. Lesage, so as to enable him to work with only two wires and one electrometer at each station. With the experience since gained in the application of the needle telegraph, such an arrangement seems very simple, and we are inclined to wonder that it was not generally adopted, especially after M. Lomond had shown the way.
To produce all the requisite signals with a single pith-ball electrometer, it was necessary to vary the durations of each divergence, and to combine several to form a single symbol. Thus, suppose that a single divergence of the pith-balls for a second was understood to signify the letter _A_; one divergence, followed by an immediate collapse, by discharging the electricity, might signify _B_; two prolonged divergences might signify _C_, and two short ones _D_; and by thus increasing the number and varying the divergences of the two pith-balls, all the letters of the alphabet might be indicated.
A still more direct method of representing the letters of the alphabet was proposed by M. Reizen in 1794, by the application of the means frequently adopted for exhibiting the light of the electric spark. The charge of a Leyden jar was sent through strips of tin foil, pasted on to a flat piece of glass, so as to form several lines, joined at the ends alternately into a continuous circuit. Interruptions were made in the foil by cutting small portions away, at which points brilliant sparks appeared when the jar was discharged. As the interruptions were so contrived as to form letters, and the strips of tin foil were all arranged separately on a long pane of glass, any letter required could be distinctly made visible by discharging the jar through that particular circuit. To produce all the letters of the alphabet in this manner, a separate circuit was required for each.
Another plan, far less feasible, and scarcely deserving of notice, excepting for its peculiarity, was proposed in the following year by M. Cavallo, who suggested the setting fire to combustibles, or the explosion of detonating substances, as the means of signalling intelligence. About the same time several attempts were made by electricians in Spain to transmit signals by electricity, but their plans were not more practicable than those already mentioned, and depended for their effects on the discharge of Leyden jars.
The discovery of voltaic electricity at the beginning of the present century was an important step in the progress of the Electric Telegraph, though several years elapsed before the applicability of the discovery for that purpose became known; and it was not fully appreciated till within the last twenty years.
The electricity generated by the voltaic battery is far greater in quantity than the most powerful electrical machine can excite, whilst its intensity is so feeble that it cannot pass in a spark through the smallest interval of air. It presents, therefore, much less difficulty in the insulation of the wires than frictional electricity, whilst the rapidity of its transmission is for practical purposes equally efficient. The electricity generated by the voltaic battery being great in quantity and feeble in intensity, it is capable also of effecting chemical decomposition and of imparting magnetism, both of which properties have proved eminently useful in perfecting the Electric Telegraph.
The first application of voltaic electricity to telegraphic purposes was made by Mr. Soemmering in 1809. The signals of his telegraph consisted of the bubbles of gas arising from the decomposition of water, during the action of the electric current. His apparatus consisted of a small glass trough, filled with acidulated water, through the bottom part of which were introduced several gold wires corresponding to the letters of the alphabet. The instant that an electric current was sent through any two of the wires, by making connection with a voltaic battery at the transmitting instrument, bubbles of hydrogen gas rose from one of the gold wires, and bubbles of oxygen gas from another; and as the volume of hydrogen gas, liberated during the decomposition of water, exceeds by sixteen times that of the oxygen, it was easy to distinguish them. In this manner all the letters of the alphabet could be indicated by using 24 wires. The object of having gold wires in the decomposing trough was to prevent the oxidation of the metal; for had copper, or any other metal that combines with oxygen, been employed, the points of the wires would soon have become corroded.
This telegraph of Soemmering's, though not adapted for practical application in the form he presented it, on account of the number of wires required for the purpose, was nevertheless superior to any that had previously been invented; and by a little modification it might have been made a perfect instrument, capable of transmitting messages by means of only two wires. Such a modification of the instrument was proposed by M. Schweigger, twenty years afterwards; the only thing required being the adoption of a code of symbols, by means of which all the letters might be indicated by combinations of the four primary signals that are obtainable by two wires, as is at present done by the needle telegraph in common use. At that time, however, the discovery of the magnetic properties of the electric current, and other improvements in the means of communicating, superseded for some years the use of signals made by electro-chemical decomposition.
The next important step in the progress of telegraphic invention, after that of Mr. Soemmering, was made by Mr. Ronalds, who in 1816 succeeded in making a perfect apparatus, that transmitted every requisite signal with the use of only a single circuit. In the agent employed, however, there was a retrogression to frictional electricity and the pith-ball electrometer, for at that time the property which a voltaic current possesses of deflecting a magnetic needle had not been discovered.
Mr. Ronalds's plan was to have, at each communicating station, a good clock with a light paper disc fixed on to the seconds wheel, on which were marked all the letters of the alphabet, and the ten numerals. Only so much of this disc was exposed to view as to show a single letter at a time, through a small aperture, as the seconds wheel revolved. The clocks at the corresponding stations were set exactly together, so that the same letter was exposed to view at each instrument at the same instant. A pith-ball electrometer, connected in a single circuit with the transmitting station, was kept distended during the transmission of a message by charging the wire from an electrical machine; and when the letter required to be indicated appeared at the aperture of both instruments, the operator at the transmitting instrument instantly discharged the electricity of the wire by touching it, and thus caused the pith-balls to collapse. In this manner the person at the receiving station, by attentively watching the pith-balls, and noticing the letter that appeared at the instant of collapse, could read the messages signalled.