Chapter D: D, an apparatus for holding a small tube, that the spectrum given by (3)

But for the art of photography the stereoscope would not thus be ready to minister to our enjoyment, for no pictures wrought by man’s handiwork could approach the requisite accuracy which the two stereoscopic pictures must possess. All attempts to produce such pictures by engraving or lithography have failed, except only in the case of linear geometrical designs, such as representations of crystals. A very useful and suggestive application of the stereoscope has been made to the illustration of a treatise on _solid geometry_, where the lines representing the planes, being drawn in proper perspective, the reader by placing a simple stereoscope over the plates sees the planes stand out in relief before him, and the multitude of lines, angles, &c., which in a simple drawing might be distracting even for a practised geometrician, assume a clear and definite form. The difference between the two retinal pictures of objects is so slight, that when the objects are at a little distance, ordinary observation fails to discover it without the aid of special instruments; and an inspection of the pair of photographs in a stereoscopic slide will convince any one that, even in these, close and careful observation is required to perceive the difference.

Some of the principles of stereoscopic drawings may be seen exemplified by the pair we give in Fig. 245. With this figure the reader may attempt the experiment of seeing the stereoscopic effect without the stereoscope. When he has succeeded in doing this, or when he fuses the images together by placing a simple stereoscope over the page, he will find the result very singular; for he will receive the impression of a solid crystal of some dark polished substance—black lead, for instance—placed on a surface of the same material. The edges of the solid will appear to have a certain lustre, such as one sees on the edges of a real crystal. The reason of this impression being produced by two drawings, one of which is formed by black lines on a white ground, while the other has white lines on a black ground, is probably due to the circumstance that we very often see in nature the _lustrous_ edges of an object with one eye only. That is, one eye is in the path of the rays which are regularly reflected from the object, while the other is not,—a fact which may be verified in an instant by looking first with one eye and then with the other, at a polished pencil, or similar object, when placed in a certain position.

There is a kind of modification of the reflecting stereoscope, known under the name of the _pseudoscope_, which is highly instructive, as showing how much our notions of the solidity of objects are due to the differences of the retinal images. In the pseudoscope the rays reach the eyes after passing through rectangular prisms in such a manner that objects on the right appear on the left, and objects on the left appear on the right; but the images agree by reason of the symmetry of the reflection, although the image of the objects that without the instrument would be formed in the right eye is, by the action of the prisms, formed in the left eye, and _vice versâ_. The impressions produced are very curious: convex bodies appear concave—a coin, for example, seems to have the image hollowed out, a pencil appears a cylindrical cavity, a globe seems a concave hemisphere, and objects near at hand appear distant, and so on. These illusions are, however, easily dispelled by any circumstance which brings before the mind our knowledge of the actual forms, and by a mental effort it is possible to perceive the actual forms even with the pseudoscope, and indeed to revert alternately, with the same object, from convexity to concavity. This last effect is very curious, for the object appears to abruptly change its form, becoming alternately hollow and projecting, according as the mind dwells upon the one notion or the other; but the experiment is attended with a feeling of effort, which is very fatiguing to the eyes.

Professor Helmholtz has contrived another very curious instrument, depending on the same principles as the stereoscope. He terms it the _telestereoscope_, and while the effect of the pseudoscope is to reverse the relief of objects, the telestereoscope merely exaggerates this relief; hence this instrument is well adapted for making those objects which from their distance present no stereoscopic effect, stand out in relief. The distance between our eyes is not sufficiently great to give us sensibly different views of very distant objects, and what the telestereoscope does is virtually to separate our eyes to a greater distance. Fig. 246 is a horizontal section of the instrument. L and R represent the position of the eyes of the spectator; _a, b_, are two plane mirrors at 45° to his line of sight; A, B, are two larger plane mirrors, respectively nearly parallel to the former. _c d a_ L and _f g b_ R show the paths of rays from distant objects, and it is obvious that the right eye will obtain a view of the objects identical with that which would be presented to an eye at R´, while the left eye has similarly the picture of the objects as seen from the point L´. The four mirrors are mounted in a box, and means are provided for adjusting the positions of the larger mirrors, as may be required. With this instrument the distant objects in a landscape—a range of mountains, for example—which present to the naked eye little or no appearance of relief, have their projections and hollows revealed in the most curious manner.

FIG. 246.—_The Telestereoscope._
]

It is upon a similar principle that stereoscopic views of some of the celestial bodies have been obtained. Admirable stereoscopic slides of the moon have been produced by photographing her at different times, when the illumination of the surface is the same, but when, in consequence of her _libration_, somewhat different views of our satellite are presented to us. Two such photographs, properly combined in the stereoscope, give not only the spherical form in full relief, but all the details of the surface: the mountains, craters, valleys, and plains are seen in their true relative projection.

The telestereoscope may be inverted, so to speak, and its effect reversed; for an arrangement of mirrors similarly disposed, but on such a scale as will permit the eyes to be respectively in the lines _c d_ and _f g_, would reflect from objects in the direction L R rays which would have but little of the difference of direction to which the stereoscopic effect is due. Hence solid objects viewed with such an instrument appear exactly like flat pictures, the effect being far more marked than in simply viewing them with one eye.

An ingenious method of exhibiting a stereoscopic effect to an audience has been contrived by Rollmann. He draws on a black ground two linear stereoscopic designs—that for the left eye with red lines, that for the right eye with blue. Each individual in the audience is provided with a piece of blue glass and a piece of red: he places the red glass before the left eye, the blue glass before the right: each eye thus receives only the picture intended for it, for the blue lines cannot be seen through the red glass, or the red lines through the blue glass. The diagrams may, of course, be projected on a screen by a magic lantern, in which case the circumstances are even more favourable. Duboscq has arranged a kind of opera-glass, so that a person may view appropriate designs on the large scale, and arrangements have been also contrived by which the stereoscopic effect may be seen in moving figures.

Every student of this interesting subject should examine a few stereoscopic images produced by simple lines representing geometrical figures, or the photographs of the model of a crystal, as these exhibit in the most striking manner the conditions requisite for the production of stereoscopic effects. A person having a little skill in perspective and geometry might construct the two stereoscopic images of a body defined by straight lines, but the drawings must be executed with extreme exactitude, for the least deviation would produce the most marked effect in the stereoscopic appearance. The production of stereoscopic photographs now forms a considerable branch of industrial art. At first, these photographs were made by taking the two different views with the same camera at two operations. But there were difficulties in obtaining uniformity of depth in the impressions, and the change in the shadows produced by the earth’s rotation showed itself—although the interval between the two exposures might not exceed three or four minutes. The increased shadows in such cases show themselves in the stereoscope, like dark screens suspended in the air. It was Sir David Brewster who, in 1849, first proposed the plan now universally adopted, of producing the views simultaneously by twin cameras forming their images on different parts of the same sensitive plate, the centres of the lenses being placed at the same distance apart as a man’s eyes, that is, from 2½ to 3 in. This is, of course, the only manner in which instantaneous views can be secured. Helmholtz, however, advocates the photographs of remote objects being taken at a much greater distance apart, for they otherwise present little appearance of relief. By selecting from an assortment of slides, two views of the Wetterhorn, taken from different points in the Grindelwald valley, and combining these in the stereoscope, he found that a far more distinct idea of the modelling of the mountain could be thus obtained than even a spectator of the actual scene would receive by viewing the mountain from any one point. Such a mode of combining the photographs would produce in the stereoscope the same effect as the telestereoscope would in the landscape, but the effect would be caused to a proportionately far higher degree.

The date of Wheatstone’s first publication regarding the stereoscope was 1833; but a complete description and theory of the instrument was not published until five years afterwards. Brewster first made public, in 1843, his invention of the stereoscope with lenses, which is now so familiar to us, and few scientific instruments have become so quickly and extensively popular; certainly no other simple and inexpensive instrument has contributed so largely to the amusement and instruction of our domestic circles. And, to the philosopher who studies the nature of our perceptions, the stereoscope has been even more instructive, for, instead of vague surmises, it provided him with the solid ground of experiment on which to found his theories. The literature of this one subject—stereoscopic effect—is extensive enough to occupy a tolerably long book-shelf. It dates from 300 B.C., when Euclid touched upon the subject in his Optics; and after a lapse of more than eighteen centuries it was taken up by Baptista Porta, in 1583; but the whole development of this subject belongs almost entirely to the last half-century.

FIG. 247.
]

The part which the muscles of the eyes take in our perceptions of form has been already alluded to, and it may be interesting to illustrate this point by a curious example or two of illusions arising from their movements. If our reader will glance at Fig. 247, he will see that the lines, _a b_ and _c d_, appear to be farther apart towards the centre than at the ends, while _f g_ and _h i_, on the other hand, appear nearest together in the middle. He will hardly be convinced that in each case the lines are quite parallel until he has actually measured the distances. A still more striking example of the same kind of illusion is shown by Fig. 248, due to Zöllner. This appears a sort of pattern, in which the broad bands are not upright, but sloping alternately to the right and left, and with the spaces between the lines wider at one end than the other. The lines in the figure are, however, strictly parallel. The illusion by which they appear divergent and convergent is still more strongly felt when the book is held so that the wider bands are inclined at an angle of 45° to the horizon. There is another illusion here with reference to the short lines, which will appear to be opposite to the white spaces on the other side of the long lines to which they are attached. That these illusions are really due to movements of the eyes may be proved by viewing the designs in any manner which entirely prevents the movement, as by fixing the gaze on one spot in the case of Fig. 247, when the illusion will vanish; but this plan is not so easily applied to Fig. 248. A convincing proof, however, will be found in the appearance of these figures when they are viewed by the instantaneous light of the electric spark, as when a Leyden jar is discharged in a dark room. The reader viewing the figures, held near the place where the spark appears, will see them distinctly without the illusions as to the non-parallelism of the lines. In the absence of an electrical machine, or coil and jar, the reader may have an opportunity of seeing the figures by flashes of lightning at night, when the result will be the same.

FIG. 248.
]

There is a property of the eye which has led to the production of many amusing and curious illusions. This property in itself is no new discovery, for its presence and effects must have been noticed ages ago. The property in question is illustrated when we twirl round a stick or cord, burning with a red glow at the end. We seem to trace a _circle_ of fire; but as the glowing spark cannot be in more than one point of the circle at once, it is plain that the impression produced on the eye must remain until the spark has completed its journey round the circle, and reaching each point successively renews the luminous impression. Like other subjects relating to vision, this phenomenon has been carefully examined in recent times, and its laws accurately determined.

The fact which is obvious from such an experiment, may be thus stated: Visual impressions repeated with sufficient rapidity produce the effect of objects continually present. This persistence of the visual impressions is easily made the subject of experiment by means of rapidly rotating discs; and in the common toy called a “colour top” we have a ready means of verifying some of the conclusions of science on this subject. Some very interesting results may be obtained by an apparatus as simple as this, regarding the laws of the phenomenon we are considering, and the effects of various mixtures of tints and colours. The well-known toy, the thaumatrope, depends on the same principle. In this a piece of cardboard is painted on one side, with a bird, for example, and on the other side with a cage: when the cardboard is twirled round very rapidly by means of a cord fixed at opposite points of its length, both bird and cage become visible at once, and the bird appears in the cage.

FIG. 249.
]

FIG. 250.
]

FIG. 251.
]

A still more ingenious application of this principle we owe to Plateau, who described it in 1833, under the title of the _phenakistiscope_; and also to Stampfer, who independently devised the same arrangement about the same time, and named it the _stroboscopic disc_. The reader may, at almost any toy-shop, purchase one of them, provided with a number of amusing figures; or he may easily construct for himself one which will exemplify the principle. He requires no other materials than a piece of cardboard, and his only tools may be a sharp penknife, a pair of compasses, and a flat ruler. Let him draw on his cardboard a circle of 8 in. diameter, and divide its circumference by eight equidistant points. From these radii should be drawn with the point of the compasses, and equal distances from the centre marked off upon them, to fix the centres of the small circles, which must all have exactly the same size (say, 1 in. in diameter) and be marked by a distinct line. In these are to be marked the hand of a clock-face in the positions shown in Fig. 249; and finally, in the direction of the radii, narrow slips are to be cut out of the cardboard as shown. If a pin be put through the centre of the disc, attaching it thus to the flat end of a cork, so that it can freely rotate in its own plane, and the disc be turned rapidly round, as in Fig. 250, in front of a looking-glass, while the spectator looks through the slits, he will see the hand on the little dial apparently turning round, with rather a jerky movement it is true, somewhat like the dead-beat seconds-hand that is sometimes seen on clocks. The illusion is best when the slits are so narrow that only one of the several images is visible by reflection, namely, that which is adjacent to the slit. Thus, as the disc rotates, each little circle is visible for an instant as the slit passes in front of the spectator’s eye; and if the rotation be sufficiently rapid, the impression of the disc is permanent, as it is constantly being renewed by the successive circles, while, on the contrary, the hands, having different positions, produce images in different positions, giving the appearance of a jerky rotation. The instruments sold in the shops have sometimes a thin metallic disc with the slits in it, and a series of designs printed in smaller paper discs. The paper discs may be screwed on the other disc as required, and a button on a pulley with an endless band is provided for producing the rotation more conveniently. Fig. 251 shows one of the pictures for a disc with twelve slits, and the effect produced by it is that of a dancing figure.

Another arrangement for showing the same illusion has lately become a very popular toy, and quite deservedly so, for it has the advantages of requiring no looking-glass, and of making the effect visible to a number of persons at the same time. This apparatus, which has been termed the Zoetrope, consists simply of a cylindrical box, like a drum with the upper end cut off. It is mounted on a pivot, which permits its revolving rapidly about its vertical axis when touched by the finger. The cylinder has a number of equidistant vertical slits round the upper part of its circumference. The figures which produce the illusion are printed on a slip of paper, which is placed in the lower part of the drum, and when this is in rapid rotation, and the figures are viewed through the slits, the illusion is produced in exactly the same manner as in the revolving disc.

FIG. 251_a_.—_Edison’s Kinetographic Theatre._
]

At the end of the article on the phonograph in a subsequent page, the reader will find a remark as to the effect that might be produced by a combination of that instrument with instantaneous and simultaneous photographs of some famous speaker. This combination has now been accomplished by the great inventive genius to whom we are indebted for the phonograph. Mr. Edison has done this so effectively that he may be said to have given life to the _zoetrope_ by the perfection in which the ocular illusion is produced together with the audible manifestations that keep time with it. The amount of thought and ingenuity expended on this new contrivance, which Edison has called the _kinetoscope_, will scarcely be appreciated by anyone who has not given some consideration to the many practical difficulties that have been overcome. No wonder that the announcement made at the beginning of 1892 should have been received with incredulity, for it was to the effect that Edison had contrived some happy combination of photography and electricity by which a man (presumably one who could afford to pay for luxuries) might sit in his own room and see the moving forms of the actors in an opera projected on a screen before his eyes, while at the same time he would hear their voices singing. Every movement, every change of expression, every glance of the eye, and, in fact, all that was visible to the spectator in front of the stage would appear on the screen, while not a note of vocalist, or chord of orchestra, would fail to reach the ear. And all this was to be evoked at will, and repeated as often as desired, not, therefore, of course, as a presentation of what was taking place at the time, but as a reproduction of some previous performance. This wonderful result has virtually been attained by the application of delicate and ingenious machinery designed to make the phonograph and the camera work synchronously. The first part of the problem was the production of a succession of so-called instantaneous photographs at an extremely rapid rate. In the actual apparatus forty-six photographs are taken every second, a feat which would beforehand be thought impracticable. This is accomplished by making use of a band of sensitive celluloid film, which alone admits of being moved and stopped with the desired rapidity. The movement is imparted by an electric motor, and the arrangement is such that for each exposure the film is held stationary for 9/10ths of 1/46th of a second, during which the lens is uncovered, then for the remaining ⅒th it is covered, while at the same time the film is jerked forward so as to expose a fresh surface to receive a new impression. Obviously the mass moved and stopped with this rapidity (which without the stoppages is at the rate of 26 miles an hour) must be small, and it is found that photographs about 1 in. in diameter cannot be much exceeded in view of this condition. The lens has to be entirely stopped or screened during the tenth of the short interval (1/460th of a second) in which the onward movement of the film is taking place, and it has to be practically open during the remaining 9/10ths of the interval (9/460ths of one second) in which the film is held stationary in order to receive the photographic image. These alternations of movement and stoppage must take place with the utmost regularity, and Edison has used a beautifully regulated electro-motor as the active power, which also simultaneously moves a phonograph so that sights and sounds shall proceed in step, for it is thus they have to be reproduced. This is done by developing the band of film, and from it printing photographic _positives_ on a similar band, whose images are successively projected on a screen by means of a lantern with a step by step movement, exactly the same as that by which the original photographs are taken, while the phonographic cylinder is so timed as to give off to a loud-speaking instrument the sounds that accompanied the photographs. A description of the ingenious mechanism by which all this is accomplished is not suitable for these pages, for it is the result, rather than the details of apparatuses, that interest the general reader. In a simpler form of kinetoscope the positive images on the band of film are viewed directly by single observers, each looking through magnifying glasses; in this a disc with 46 slits revolves, and in its passage, as each slit momentarily permits a view of the image, an electric flash simultaneously lights it up. The same principle is, of course, used in the screen projections. From the very great number of impressions made on the eye in one second, there is none of that jerky movement that is observable in the older appliances. Mr. Edison has found it necessary to provide a special stage, or rather small theatre, in which the actors of the little dramas may be photographed with every advantage in the way of lighting, &c. Fig. 251_a_ shows this kinetographic theatre with the electric camera in action. The subjects reproducible in the kinetoscope include the most rapid movements, such as quick dances, blacksmiths hammering on an anvil, &c., or incidents of ordinary life involving much gesture and change of facial expression, and nothing can be more amusing than to see all these shown to the life by the images on the screen, or by the pictures viewed through the lens, especially if at the same time the phonograph is made to emit the corresponding sounds.

FIG. 252.—_Portrait of Sir W. Thomson._[5]
]

Footnote 5:

Now Lord Kelvin.

ELECTRICITY.

About sixty years ago a popular book was published having for its theme the advantages which would flow from the general diffusion of scientific knowledge. Great prominence was, of course, given to the utility of science in its direct application to useful arts, and many scientific inventions conducing to the general well-being of society were duly enumerated. Under the head of electricity, however, the writer of that book mentioned but few cases in which this mysterious agent aided in the accomplishment of any useful end. The meagre list he gives of the instances in which he says “_even_ electricity and galvanism might be rendered subservient to the operations of art,” comprises only orreries and models of cornmills and pumps turned by electricity, the designed splitting of a stone by lightning, and the suggestion of Davy that the upper sheathing of ships should be fastened with copper instead of iron nails, with a hint that the same principle might be extended in its application. At the present day the applications of electricity are so numerous and important, that even a brief account of them would more than fill the present volume. Electricity is the moving power of the most remarkable and distinguishing invention of the age—the telegraph; it is the energy employed for ingeniously measuring small intervals of time in chronoscopes, for controlling time-pieces, and for firing mines and torpedoes; it is the handmaid of art in electro-plating and in the reproduction of engraved plates, blocks, letterpress, and metal work; it is the familiar spirit invoked by the chemist to effect marvellous transformations, combinations, and decompositions; it is a therapeutic agent of the greatest value in the hands of the skilful physician. Such an extension of the practical applications of electricity as we have indicated implies a corresponding development of the science itself; and, indeed, the history of electricity during the present century is a continuous record of brilliant discoveries made by men of rare and commanding genius—such as Davy, Ampère, and Faraday. To give a complete account of these discoveries would be to write a treatise on the science; and although the subject is extremely attractive, we must pass over many discoveries which have a high scientific interest, and present to the reader so much of this recently developed science as will enable him to comprehend the principles of a few of its more striking applications.

The science of electricity presents some features which mark it with special characters as distinguished from other branches of knowledge. In mechanics and pneumatics and acoustics we have little difficulty in picturing in our minds the nature of the actions which are concerned in the phenomena. We can also extend ideas derived from ordinary experience to embrace the more recondite operations to which heat and light may be due, and, by conceptions of vibrating particles and undulatory ether, obtain a mental grasp of these subtile agents. But with regard to electricity no such conceptions have yet been framed—no hypothesis has yet been advanced which satisfactorily explains the inner nature of electrical action, or gives us a mental picture of any pulsations, rotations, or other motions of particles, material or ethereal, that may represent all the phenomena. Incapable as we are of framing a distinct conception of the real nature of electricity, there are few natural agents with whose ways we are so well acquainted as electricity. The _laws_ of its action are as well known as those of gravitation, and they are far better known than those which govern chemical phenomena or the still more complex processes of organic life.

Definite as are the laws of electricity, there is no branch of natural or physical science on which the ideas of people in general are so vague. Spectators of the effects of this wonderful energy—as seen violently and destructively in the thunderstorm, and silently and harmlessly in the Aurora—knowing vaguely something of its powers in traversing the densest materials, in giving convulsive shocks, and in affecting substances of all kinds—the multitude regard electricity with a certain awe, and are always ready to attribute to its agency any effect which appears mysterious or inexplicable. The popular ignorance on this subject is largely taken advantage of by impostors and charlatans of every kind. Electric and magnetic nostrums of every form, electric elixirs, galvanic hair-washes, magnetized flannels, polarized tooth-brushes, and voltaic nightcaps appear to find a ready sale, which speaks unmistakably of the less than half-knowledge which is possessed by the public concerning even the elements of electrical science.

Electricity has also a special position with regard to its intimate connection with almost every other form of natural energy. Evolved by mechanical actions, by heat, by movements of magnets, and by chemical actions, it is capable in its turn of reproducing any of these. It plays an important, but as yet an undefined, part in the physiological actions constantly going on in the organized body, and is, in fact, all-pervading in its influence over all matter, organic and inorganic—a secret power strangely but universally concerned in all the operations of nature. We are compelled to regard electricity not as a kind of force acting upon otherwise inert matter, but rather as an affection or condition of which every kind of matter is capable, although we are still unable to form a conjecture of the precise nature of the action.

We have now to address ourselves to the task of unfolding so much of the science as will enable the reader to understand the leading principles of such important applications as electro-plating, illumination, and the telegraph; and this will necessarily include an account of the grand discovery of the identity, or at least intimate connection, of magnetism and electricity.

_ELEMENTARY PHENOMENA OF MAGNETISM AND ELECTRICITY._

The distinctive property of a magnet is, as everybody knows, to attract pieces of iron, and this property having been observed by the ancients in a certain ore of iron which was found near the city of Magnesia, in Asia Minor, the property itself came to be called Magnetism. A bar of steel, if rubbed with the natural magnet or loadstone, acquires the same property, and if the bar be suspended horizontally or poised on a pivot, it will settle only in one definite direction, which in this country is nearly north and south. If a narrow magnetized bar be plunged into iron filings, it will be found that these are attracted chiefly by the ends of the bar, and not at all by the centre. It appears as if the magnetic power were concentrated in the extremities of the bar, and these are termed its poles, the pole at the end of the bar which points to the north is called the _north pole_ of the magnet, and the other is named the _south pole_. If a north pole of one magnet be presented to the north pole of another, they will repel each other, and the same repulsion will take place between the south poles, whereas the north pole of one magnet attracts the south pole of another. In other words, poles of the same name repel each other, but poles of opposite names attract each other, or still more concisely, _like poles repel, unlike poles attract each other_.

Magnetism acts through intervening non-magnetic matter with undiminished energy. Thus, the attractions and repulsions of magnetic poles manifest themselves just as strongly when the poles are separated by a stratum of wood or stone as when merely air intervenes, and the attraction of small pieces of iron by a magnet takes place through the interposed palm of one’s hand without diminution. A delicately suspended needle in even a remote apartment of a large building moves whenever a cart passes in the street. It is almost too well known to require mention here, that iron and steel are the only common substances which are capable of plainly exhibiting magnetic forces, and, indeed, there are no known substances capable of so powerful a magnetization as these. But the difference in the magnetic behaviour of iron and steel is not so well understood, and it is a point of importance for our subject, and connected with a fundamental law which governs all magnetic manifestations. A piece of pure iron is very readily cut with a file, whereas a piece of steel may be so hard that the file makes no impression upon it whatever; and hence a piece of pure iron, or rather iron holding no carbon in combination, and possessed of no steely quality, is often spoken of as _soft iron_. When a piece of soft iron is placed near the pole of a magnet, the iron becomes, for the time, a magnet. If iron filings be sprinkled over it, they will arrange themselves about the parts of the iron respectively nearest and farthest from the magnet, thus showing that the piece of soft iron has acquired magnetic poles. It will be found on examining these poles that the one nearest the magnet is of the contrary name to the pole of the magnet, and the farthest is of the same name. The conversion of the soft iron into a magnet by the influence of a magnetic pole is termed _induction_. It need hardly be said that the inductive effect is more powerful in proportion to the shortness of the distance separating the piece of soft iron from the magnetic pole, and, of course, the effect is at its maximum when there is actual contact. Induction thus explains, by aid of the law of the poles, the attraction which a magnet exercises over pieces of iron, for it is plain that the inductive influence is accompanied by attraction between the two contiguous oppositely-named poles of the magnet, and of the piece of iron. But attraction is not the only force, for the pole developed at the farthest portion of the piece of iron being of the same name as the inducing pole, these will be mutually repulsive. The attractive force will, however, be more powerful on account of the shorter distance at which it is exerted, and will predominate over the repulsive force, particularly at short distances, because then the difference will be relatively greater. At distances from the inducing pole relatively great to the distance between the two poles of the piece of iron, the difference may be so small that its effect in attracting the piece of soft iron will be imperceptible, and then the piece of iron acted on by two (nearly) equal parallel forces, will be subject to what is termed in mechanics a _couple_, the only effect of which is to turn the body into such a position that the opposing forces act along the same line. The definite direction assumed by a freely suspended needle may be explained by supposing that the earth itself is a magnet having a _south_ pole in the _northern_ hemisphere, and a _north_ pole in the _southern_ hemisphere, the line joining these poles being shorter than the axis of the earth, and not quite coinciding with it in position; and the fact of the needle being turned round but not bodily attracted is then easily accounted for, the attractive and repulsive forces being reduced to a _couple_ in the manner just explained.

If the attempt be made to turn a piece of steel into a magnet, by the induction of a magnetic pole, the same results will be obtained as in the case of soft iron, but in a much feebler degree, and with this difference: the piece of steel does not lose its magnetism when the inducing magnet is withdrawn, whereas in the case of the soft iron every trace of magnetism vanishes the instant the inducing pole is removed. And if the pole of the magnet be not only put in contact with one end of the piece of steel, but rubbed on it, the piece will acquire permanent and powerful magnetism. Hence it will be noticed that a piece of soft iron can by the mere approximation of a magnetic pole be converted in an instant into a magnet, and by the removal of the magnet can as instantly be deprived of its magnetism, and made to revert into its ordinary condition; while steel is not so readily magnetized, but retains its magnetism permanently.

FIG. 253.—_A simple Electroscope._
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The elementary phenomena of electricity are extremely simple and easy of demonstration, and as the whole science rests upon inferences derived from these, the reader would do well to perform the following simple experiments for himself. Apparatus is represented in Fig. 253, but the only essential portion is a straw, B, suspended from any convenient support by a very fine filament of _white silk_. To one or both ends of the straw a little disc of gilt paper, or a small ball of elder-pith or of cork, should be attached, so that the straw may be balanced horizontally. Now rub on a piece of woollen cloth a bit of sealing-wax, or a stick of sulphur, or a piece of amber, or a penholder, paper-knife, or comb made of ebonite, and immediately present the substance to the ball at the end of the straw. It will be first attracted to the rubbed surface, but after coming into contact with it, repulsion will be manifested and the ball will separate, and may be chased round the circle by following it with the excited body. The attraction of light bodies by amber after it has been rubbed appears to be the one solitary electrical observation recorded by the ancients, but it has given its name to the science, ελεκτρον being the Greek name for amber. The cause, then, of this property is named _electricity_, and bodies which exhibit it are said to be _electrified_. The reader will remark that these words _explain_ nothing: they are used merely to _express_ a certain state of matter and the entirely unknown cause of that state. Let the pith or cork ball at the end of the straw be again charged with electricity, by bringing it into contact with a piece of sealing-wax or ebonite which has just been electrified by friction. In this condition it will, as we have just seen, be repelled by the substance which charged it, and on trial it will be found to be repelled also by all the substances we have named, after they have been excited by friction. But if, while still charged with the electricity communicated to it by contact with sealing-wax, sulphur, ebonite, or amber, we present to it a warm and dry glass tube which has just been rubbed with dry silk, we shall find that the ball will be strongly attracted. After contact with the glass, repulsion will take place, and the ball will refuse again to come into contact with the excited glass. In this condition, however, it will be immediately attracted by rubbed sealing-wax or ebonite, and so on alternately: the ball when repelled by the wax is attracted by the glass, and when repelled by the glass is attracted by the wax.

These simple experiments prove that, whatever electricity may be, there are two kinds of it, or, at least, it manifests two opposite sets of forces. The electricity evolved by the friction of glass with silk was formerly called _vitreous_ electricity, and that shown by excited resin, sealing-wax, amber, &c., was named _resinous_ electricity. These names have now been respectively replaced by the terms _positive_ and _negative_. It must be understood that these terms imply no actual excess or defect, but are purely distinguishing terms, just as we speak of the _up_ and _down_ line of a railway, without implying an inclination in one direction or the other. A fact of great importance in electrical theory is discovered when the substances in which electricity is developed are carefully examined: it is found that one kind is never produced without the other simultaneously appearing. Thus, the silk which has been used for rubbing the glass in the above experiments will be found to exhibit the same electricity as sealing-wax or ebonite. And, further, the _quantities_ of positive and negative electricity evolved are always found to be equal, or equivalent to each other; that is, if they are put together they completely neutralize or destroy each other’s effects. We have used the word “quantity,” implying that electricity can be measured. No doubt, whatever electricity may be, there may be more or less of it; but can we measure an imponderable, invisible, impalpable thing, incapable of isolation? What we really measure when we say that we measure electricity is the attractive or repulsive force: we balance this against some other force (that of gravitation, for example), and we say, so much weight lifted represents so much electricity.

If we try to electrify a piece of metal by holding it in the hand and rubbing it against woollen cloth, silk, or other substance, we shall fail in the attempt: no signs of electricity will thus be shown by the metal. Hence bodies were formerly divided into two classes—those which could be electrified by friction, and those which could not. It was afterwards found, however, that there was no real ground for this division, but that, on the contrary, _no two bodies can be rubbed together, even if they are made of the same substances, without positive electricity appearing in one, and an equivalent quantity of negative electricity in the other_. The real difference between bodies which prevents the manifestation of electricity in many cases depends upon the fact that electricity is able to traverse some substances with great facility, while others prevent its passage. Thus, if we suspend horizontally a hempen cord by white silk attached to the ceiling, so that the hempen cord comes in contact with nothing but the silk, we shall find, on presenting a piece of excited ebonite to one end of the cord, that electric attraction of light bodies will be manifested at the other. If a silk cord be substituted for the hempen one, no such effect will be observed. The hemp is, therefore, said to be a _conductor_, and the silk a _non-conductor_. Again, if we substitute for one of the silk threads suspending the cord a piece of twine, or a wire, we shall fail to obtain any electric manifestations at the remote end, because the electricity will be carried off into the earth by the conducting powers of these substances. On the other hand, filaments of glass or ebonite may be used, instead of the silk, with the same effect: they do not allow the electricity to run through them to the ground, and are therefore termed, like the silk, _insulators_ of electricity. The distinction of bodies into conductors on the one hand, and into non-conductors or insulators on the other, is of paramount importance in the science and in all its applications. This distinction, however, is not an absolute one: there is no substance so perfect an insulator that it will not permit any electricity to pass, and there is no conductor so perfect that it does not offer resistance to the passage. Substances may be arranged in a list which presents a gradation from the best conductor to the best insulator. The metals are by far the best conductors, but there is great relative diversity in their conductive power. Silver, copper, and gold are much the best conductors among the metals, iron offering eight times, and quicksilver fifty times, the resistance of silver. Coke, charcoal, aqueous solutions, water, vegetables, animals, and steam are all more or less conductors, while among the substances called insulators may be named, in order of increasing insulating power, india-rubber, porcelain, leather, paper, wool, silk, mica, glass, wax, sulphur, resins, amber, gum-lac, gutta-percha, and ebonite. It will now be obvious why the electricity developed by the friction of a piece of metal fails to manifest itself under ordinary circumstances, as, for instance, when held in the hand: the metal and the body being both conductors, the electricity escapes. But if the piece of metal be held by an insulating handle of glass or ebonite, the electrified condition may easily be observed.

_THEORY OF ELECTRICITY._

The few elementary facts which have been pointed out are absolutely necessary for the foundation of what is sometimes termed the theory of electricity, but which is properly no theory,—at least, not a theory in the same sense as gravitation is a theory explaining the motions of the planets, or even in the sense in which the hypothesis of the ether and its movements explains the phenomena of light. It is absolutely necessary to have a conception of some kind which may serve to connect in our minds the various phenomena of electricity, if it were only to enable us the more easily to talk about them. In default of any supposition which will shadow forth what actually occurs in these phenomena, we have recourse to what has been aptly termed _a representative fiction_: we picture to ourselves the actions as due to _imaginary fluids_—fluids which we know _do not exist_, but are as much creations of the mind as Macbeth’s air-drawn dagger; not, however, like his “false creation,” proceeding from “the heat-oppressed brain,” but intellectual fictions, consciously and designedly adopted for the purpose of enabling us the better to think of the facts, to readily co-ordinate them, and to express them in simple and convenient language. Non-scientific persons hearing this language usually mistake its purport, and imagine that the actual existence of an “electric fluid” is acknowledged. The accounts which appear in the newspapers of the damage done by thunderstorms are often amusing from the objectivity which the reporter attributes to the “electric fluid.” It is described, perhaps, as “entering the building,” “passing down the chimney,” then “proceeding across the floor,” “rushing down the gas-pipes,” “forcing its way through a crevice, and then streaming down the wall,” &c., in terms which imply the utmost confidence of belief in the existence of the “fluid.” With this intimation that the hypothesis of electric fluids is merely, then, a “_façon de parler_,” the reader will not be misled by the following brief explanation of the elementary facts in the language of the theory.

In the natural state all bodies contain an indefinite quantity of an imponderable subtile matter, which may be called “neutral electric fluid.” This fluid is formed by a combination of two different kinds of particles, positive and negative, which are present in equal quantities in bodies not electrified; but when there is in any body an excess of one kind of particles, that body is charged accordingly with positive or negative electricity. Both fluids traverse with the greatest rapidity certain substances termed _conductors_; but they are retained amongst the molecules of _insulating_ substances, which prevent their movement from point to point. When one body is rubbed against another, the neutral electric fluid is decomposed—the positive particles go to one body, the negative with which these positive particles were before united pass to the other body. The particles of the same name repel each other, but particles of opposite names attract each other; and it is this attraction which is overcome when the electricities are separated by friction or in any other manner.

It will be observed that the above is nothing but the statement of the elementary facts in the language of the hypothesis. This system of the two fluids readily lends itself to the explanation of nearly all the phenomena presented in what is termed _static electricity_—that is, in those phenomena where the actions are conceivably due to a more or less permanent separation of the fluids. The grand discoveries in electricity turn, however, upon quite another condition, namely, one in which the two hypothetical fluids must be imagined as constantly combining, and here the utility of the hypothesis is less marked. Inasmuch, however, as there can be no doubt regarding the identity of the agent operating in the two sets of circumstances, the facts of _dynamical electricity_ must still be expressed in the same language, with the aid of any additional conceptions which may give us more grasp of the subject.

_ELECTRIC INDUCTION._

In all electrical phenomena an inductive action occurs, which resembles that which we have already indicated with regard to magnetism. Thus, if we take an insulated metallic conductor in the uncharged state, and bring it near an electrified body, we shall find that the conductor, while still at a considerable distance, will give signs of an electrical charge. Suppose we have a cylindrical conductor, and that we present one end of it to the electrified body, but at such a distance that no spark shall pass, we shall find, if the charge on the electrified body be strong and the conductor be brought sufficiently near, that on bringing the finger near the insulated cylinder, a spark passes. While the cylinder continues in the same position with regard to the electrified body, no further sparks can be drawn from it; but if the distance between the two bodies be increased, the insulated cylinder will be found to have another charge of electricity, which will again produce a spark. And by repeating these movements we may obtain as many sparks as we desire by these mechanical actions, without in the least drawing upon the charge on the original electrified body. The electrophorus is a device for obtaining electricity by this plan, and several rotatory electrical machines have lately been invented which yield large supplies of electricity by a similar inductive action.

FIG. 254.

_The Gold-leaf Electroscope._
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It is found that in such a case as that we have above supposed, if the electrified body is charged with positive electricity, the uncharged conductor brought near it has its electricities separated—the negative attracted and held by the attraction of the positive charge in the parts of the cylinder nearest the inducing body; while the corresponding quantity of positive electricity is driven towards the most remote parts of the insulated conductor. It is this last which gives the spark in the first case, and if it be not thus withdrawn from the conductor, it re-combines with the negative electricity when the conductor is withdrawn from the neighbourhood of the electrified body, and the conductor then reverts to the natural or unelectrified state. But the contact of a conducting body with the conductor while it is under the influence of the electrified body withdraws only positive electricity, the negative—being held, as it were, by the attraction of the positive electricity of the charged body—is not thus removed, and in this condition it is sometimes called _disguised_ or _dissimulated_ electricity—a term the propriety of which is doubtful. The excess of negative “fluid” which the conductor thus acquires shows itself, however, only when the inducing body has been withdrawn. Precisely similar effects will take place, _mutatis mutandis_, if the electrified body has a negative charge. A demonstration of inductive effects is readily afforded in the action of the gold-leaf _electroscope_, Fig. 254, in which two strips of gold-leaf are suspended within a glass case from wire passing through the top, and terminated in a metal plate. This instrument is often used for showing the existence of very small electric charges. Let a stick of sealing-wax be rubbed and held, say, a foot or more from the plate of the electroscope, the leaves will diverge with negative electricity. The sealing-wax being retained in the same position, touch the plate for an instant with the finger. This will remove the negative charge, but the _positive_ electricity will be retained on the plate by the attraction of the negative of the sealing-wax. Now remove the sealing-wax, when the dissimulated charge will spread itself over the whole insulated metallic portion of the electroscope, and the leaves will diverge with a strong charge of _positive_ electricity. If an excited glass tube is brought near the electroscope, the leaves will now diverge still more; if the sealing-wax is replaced in its former position, the leaves will collapse. In all these cases the electrified body parts with none of its own electricity by developing electrical effects in the neighbouring bodies.