CHAPTER XIII.

VISION—THE EYE—THE STEREOSCOPE—SPECTRUM ANALYSIS—THE SPECTROSCOPE—THE TELESCOPE AND MICROSCOPE—PHOTOGRAPHY—DISSOLVING VIEWS—LUMINOUS PAINT.

The eye and vision are such important subjects to all of us that we may be excused for saying something more concerning phenomena connected with them, and the instruments we use for assisting them. We do not propose to write a treatise upon the physiology of vision, for we know the image in the eye is produced physically in the same manner as the image in a camera obscura. In the eye the sides of the box are represented by the sclerotic (see chap. x. fig. 95); the dark inner surface has its parallel in the pigment of the choroid; the opening in the box in the pupil of the eye; the convex lens in the crystalline and the cornea; and the retina receives the image. But why we see—beyond the fact that we do see—no one can explain. Science is dumb on the subject. Thought and consciousness elude our grasp; and, as Professor Tyndall says on this subject, “we stand face to face with the incomprehensible.”

But there are many interesting facts connected with our vision which may be plainly described. Some people are obliged to carry an object (or a book) to within ten inches of the eye to see it distinctly; and a person who does not possess convergent power of the eye will have to move it farther off, or use convex glasses; while a “near-sighted” person, whose eyes are too quickly convergent, will use concave glasses to bring the object near to the eye.

There is but one small place in the retina of the eye which admits of perfect vision. This, the most sensitive portion, is called the “yellow spot,” and vision becomes more and more indistinct from this point towards the circumference. This can be proved by any one; for in reading we are obliged to carry our eyes from word to word, and backwards and forwards along the lines of print. Another very important element in our vision is the contraction and enlargement of the iris around the pupil. In cases where strong light would only dazzle us the iris expands, and the pupil is contracted to a sufficient size to accommodate our vision. At night, or in a darkened room, the pupil is enlarged. This change will account for our not being immediately able to see objects when we have passed from darkness to great light, or vice versâ. The iris must have time to accommodate itself to the light.

Now, outside the small space of perfect vision above mentioned, there is a circle of considerable extent, called the “field of vision.” In man this field, when the eyes are fixed, subtends an angle of about 180°, because beyond that the rays cannot enter the pupil of the eye. But in the lower animals, the fish and birds,—notably the ostrich,—the field of vision is much more extensive, because the pupils are more prominent, or the eyes are set more towards the sides of the head. The ostrich can see behind him, and fish can see in any direction without apparent limit. Man can only see indistinctly; and though he can move his eyes rapidly, he can see distinctly but a small portion of any object at a time, yet he sees with both eyes simultaneously a single object, because the two lines of vision unite at a single point, and as the two images cover each other we perceive only one image. Beyond or within this point of meeting the vision is indistinct, but the angle of convergence is always varied according to the distance of the object. If we hold up a penholder in front of us, and in a line with any other defined object,—say an ink-bottle,—we can see the penholder distinctly, and the ink-bottle indistinctly, as two images. If we then look at the ink-bottle we shall see it single, while the penholder will appear double, but with imperfect outlines.

Fig. 147.—The Stereoscope.

Fig. 148.—Mode of taking photograph for Stereoscope.

Again, if we look at a box both eyes will see it equally well, but the right eye will see a little more on its right side, and the left eye on the left. It is on this principle that the Stereoscope is constructed. Sir Charles Wheatstone was the inventor, and the instrument may be thus described:—Two pictures are taken by photography—one as the landscape is seen by the right eye, the other as it is viewed by the left; the points of view thus differing slightly. When both eyes are simultaneously applied to the instrument the view is seen exactly as it would appear to the beholder at the actual place it represents. The views are taken singly; one side at one time, and another after, as in the camera (fig. 148). A is the first view; B is kept dark; C is the shutter for A. There are Reflecting and Refracting Stereoscopes. In the former a mirror reflects the image into each eye; in the latter the views are pasted on a card, side by side, and looked at through prismatic lenses. The principles of Binocular Vision have been applied to the Microscope.

In foregoing chapters we have given many examples with diagrams of the temporary impressions made upon the retina of the eye. It is a fact that a wheel revolving at a great rate will appear to be standing still when suddenly illuminated by a flash of lightning, because the eye has not time to take in the motion in the instant of time, for the spokes of the wheel are not moving fast enough to convey the impression of motion in that half second to the eye; yet the perfect outline of the wheel is distinctly visible.

Indeed, distinct vision can be exercised in a very small fraction of a second. It was calculated by Professor Rood, and proved by experiment, that forty billionths of a second is sufficient time for the eye to distinguish letters on a printed page. It this instance the illuminating power was an electric spark from a Leyden Jar.

We have remarked upon the distinctness with which we can see an object when we direct our gaze upon it, and this appears a self-evident proposition; but have any of our readers remarked the curious fact that when they want to see a faint and particular star in the sky it will at once disappear when they gaze at it? The best way to see such very faint orbs as this is to look away from them,—a little to one side or the other,—and then the tiny point will become visible again to the eye. There is also a degree of phosphorescence in the eye, which any one who receives a blow upon that organ will readily admit. Even a simple pressure on the closed lid will show us a circle of light and “colours like a peacock’s tail,” as the great Newton expressed it. There are many occasions in which light is perceived in the eye—generally the result of muscular action; and the Irish term to “knock fire out of my eye” is founded upon philosophical facts.

We are many of us aware of “spots” on our eyes when our digestion is out of order, and the inability of the eye to see figures distinctly in a faint light—within a proper seeing distance, too—has often given rise to the “ghost.” These shadowy forms are nothing more than affections of the eye, and, as well remarked in Brewster’s Letters on Natural Magic, “are always white because no other colour can be seen.” The light is not sufficiently strong to enable the person to see distinctly; and as the eye passes from side to side, and strives to take in the figure, it naturally seems shadowy and indistinct, and appears to move as our eyes move. “When the eye dimly descries an inanimate object whose different parts reflect different degrees of light, its brighter parts may enable the spectator to keep up a continued view of it; but the disappearance and reappearance of its fainter parts, and the change of shape which ensues, will necessarily give it the semblance of a living form; and if it occupies a position which is unapproachable, and where animate objects cannot find their way, the mind will soon transfer to it a supernatural existence. In like manner a human figure shadowed forth in a feeble twilight may undergo similar changes, and after being distinctly seen while it is in a situation favourable for receiving and reflecting light, it may suddenly disappear in a position before and within the reach of the observer’s eye; and if this evanescence takes place in a path or roadway where there is no sideway by which the figure could escape, it is not easy for an ordinary mind to efface the impression which it cannot fail to receive.” This will account for many so-called “ghosts.”

Accidental colours, or ocular spectra, are, so to speak, illusions, and differently-coloured objects will, when our gaze is turned from them, give us different “spectra” or images. For instance, a violet object will, when we turn to a sheet of white paper, give us a yellow “spectrum”; orange will be blue; black and white will change respectively; red will become a blue-green. From a very strong white light the accidental colours will vary.

Fig. 149.—The Solar Spectrum.

The Solar Spectrum is the name given to the coloured band formed by the decomposition of a beam of light into its elementary colours, of which there are seven. This is an easy experiment. A ray of light can be admitted into a darkened room through a hole in the shutter, and thus admitted will produce a white spot on the screen opposite, as at g in the diagram (fig. 149). If we interpose a prism—a triangular piece of glass—the “drop” of a chandelier will do—we cause it to diverge from its direct line, and it will produce a longer streak of light lower down. This streak will exhibit the prismatic colours, or the “colours of the rainbow”; viz., red (at the top), orange, yellow, green, blue, indigo (blue), and violet last. These are the colours of the Solar Spectrum. The white light is thus decomposed, and it is called mixed light, because of the seven rays of which it is composed. These rays can be again collected and returned to the white light by means of a convex lens.

“White light,” said Sir Isaac Newton, “is composed of rays differently refrangible,” and as we can obtain the colours of the rainbow from white light, we can, by painting them on a circular plate and turning it rapidly round, make the plate appear white. Thus we can prove that the seven colours make “white” when intermingled. But Newton (1675) did not arrive at the great importance of his experiment. He made a round hole in the shutter, and found that the various colours overlapped each other. But, in 1802, Dr. Wollaston improved on this experiment, and by admitting the light through a tiny slit in the wood, procured an almost perfect spectrum of “simple” colours, each one perfectly distinct and divided by black lines.

But twelve years later, Professor Fraunhofer made a chart of these lines, which are still known by his name. Only, instead of the 576 he discovered, there are now thousands known to us! To Fraunhofer’s telescope Mr. Simms added a collimating lens, and so the Spectroscope was begun; and now we use a number of prisms and almost perfect instruments, dispersing the light through each. We have here an illustration of a simple Spectroscope, which is much used for chemical analysis (fig. 150).

In the spectrum we have long and short waves of light, as we have long and short (high and low) waves in music, called notes. The long or low notes are as the red rays, the high notes as the blue waves of light. (Here we have another instance of the similarity between light and sound.) But suppose we shut out the daylight and substitute an artificial light. If we use a lamp burning alcohol with salt (chloride of sodium), the spectrum will only consist of two yellow bands, all the other colours being absent. With lithium we obtain only two, one orange and one red. From this we deduce the fact that different substances when burning produce different spectra; and although a solid may (and platinum will) give all seven colours in its spectrum, others, as we have seen, will only give us a few, the portion of the spectrum between the colours being black. Others are continuous, and transversed by “lines” or narrow spaces devoid of light; such is the spectrum of the sun, and by careful and attentive calculation and observation we can get an approximate idea of the matter surrounding the heavenly bodies.

Fig. 150.—The Spectroscope.

We have said there are lines crossing the spectrum transversely; these are called Fraunhofer’s lines, after the philosopher who studied them; they were, however, discovered by Wollaston. These lines are caused by light from the lower portion of the sun passing through the metallic vapours surrounding the orb in a state of incandescence, such as sodium, iron, etc. One of Fraunhofer’s lines, a black double line known as D in the yellow portion of the spectrum, was known to occupy the same place as a certain luminous line produced by sodium compounds in the flame of a spirit lamp. This gave rise to much consideration, and at length Kirchkoff proved that the sodium vapour which gives out yellow light can also absorb that light; and this fact, viz., that every substance, which at a certain temperature emits light of a certain refrangibility, possesses at that temperature the power to absorb that same light. So the black lines are now considered the reversal of luminous lines due to the incandescent vapours by which the sun is surrounded. Thus the presence of an element can be found from black or luminous lines, so the existence of terrestrial elements in celestial bodies has been discovered by means of preparing charts of the lines of the terrestrial elements, and comparing them with the lines of stella spectra.

We have supposed the beam of light to enter through a slit in the shutter, and fall upon a screen or sheet. The solar spectrum shown by the passage of the beam through a prism is roughly as below—

Fig. 151.—Example of the Spectrum.

Fraunhofer substituted a telescope for the lens and the screen, and called his instrument a Spectroscope. He then observed the lines, which are always in the same position in the solar spectrum. The principal of them he designated as A, B, C, D, E, F, G, H. The first three are in the red part of the spectrum; one in the yellow, then one in the green; F comes between green and blue, G in the indigo-blue, and H in the violet. But these by no means exhaust the lines now visible. Year by year the study of Spectrum Analysis has been perfected more and more, and now we are aware of more than three thousand “lines” existing in the solar spectrum. The spectra of the moon and planets contain similar dark lines as are seen in the solar spectrum, but the fixed stars show different lines. By spectrum analysis we know the various constituents of the sun’s atmosphere, and we can fix the result of our observations made by means of the Spectroscope in the photographic camera. By the more recent discoveries great studies have been made in “solar chemistry.”

What can we do with the Spectroscope, or rather, What can we not do? By Spectroscopy we can find out, and have already far advanced upon our path of discovery, “the measure of the sun’s rotation, the speed and direction of the fierce tornados which sweep over its surface, and give rise to the ‘maelstroms’ we term ‘sunspots,’ and the mighty alps of glowing gas that shoot far beyond the visible orb, ever changing their form and size; even the temperature and pressure of the several layers and their fluctuations are in process of being defined and determined.” This is what science is doing for us, and when we have actually succeeded in ascertaining the weather at various depths in the atmosphere of the sun, we shall be able to predict our own, which depends so much upon the sun. Last year (1880) Professor Adams, in his address to the British Association, showed that magnetic disturbances, identical in kind, took place at places widely apart simultaneously. He argues that the cause of these identical disturbances must be far removed from the earth.

“If,” he says, “we imagine the masses of iron, nickel, and magnesium in the sun to retain even in a slight degree their magnetic power in a gaseous state, we have a sufficient cause for all our magnetic changes. We know that masses of metal are ever boiling up from the lower and hotter levels of the sun’s atmosphere to the cooler upper regions, where they must again form clouds to throw out their light and heat, and to absorb the light and heat coming from the hotter lower regions; then they become condensed, and are drawn back again towards the body of the sun, so forming those remarkable dark spaces or sunspots by their down rush to their former levels. In these vast changes we have abundant cause for those magnetic changes which we observe at the same instant at distant points on the surface of the earth.” So we are indebted to the Spectroscope for many wonderful results—the constitution of the stars, whether they are solid or gaseous, and many other wonders.

The manner in which we have arrived at these startling conclusions is not difficult to be understood, but some little explanation will be necessary.

The existence of dark lines in the solar spectrum proves that certain rays of solar light are absent, or that there is less light. When we look through the prism we perceive the spaces or lines, and we can produce these ourselves by interposing some substance between the slit in the shutter before mentioned and the prism. The vapour of sodium will answer our purpose, and we shall find a dark line in the spectrum, the bright lines being absorbed by the vapour. We can subject a substance to any temperature we please, and into any condition—solid, liquid, or gaseous; we can also send the light the substance may give out through certain media, and we can photograph the spectrum given out under all conditions. The distance of the source of the light makes no difference. So whether it be the sun, or a far-distant star, we can tell by the light sent to us what the physical condition of the star may be. It was discovered in 1864 that the same metallic body may give different spectra; for instance, the spectrum might be a band of light,—like the rainbow,—or a few isolated colours; or again, certain detached lines in groups. The brightness of the spectrum lines will change with the depth of the light-giving source, or matter which produces it.

We have become aware by means of the Spectroscope that numerous metals known to us on earth are in combustion in the sun, and new ones have thus been discovered. In the immense ocean of gas surrounding the sun there are twenty-two elements as given by Mr. Lockyer, including iron, sodium, nickel, barium, zinc, lead, calcium, cobalt, hydrogen, potassium, cadmium, uranium, strontium, etc. Not only is the visible spectrum capable of minute examination, but, as in the case of the heat spectrum already mentioned when speaking of Calorescence, the light spectrum has been traced and photographed far beyond the dark space after the blue and violet rays, seven times longer than the visible solar spectrum—a spectrum of light invisible to our finite vision. Although a telescope has been invented for the examination of these “ultra violet” rays, no human eye can see them. But—and here science comes in—when a photographic plate is put in place of the eye, the tiniest star can be seen and defined. Even the Spectroscope at length fails, because light at such limits has been held to be “too coarse-grained for our purposes”! “Light,” says a writer on this subject “we can then no longer regard as made of smooth rays; we have to take into account—and to our annoyance—the fact that its ‘long levelled rules’ are rippled, and its texture, as it were, loose woven”!

Twenty years ago Professors Kirchkoff and Bunsen applied Fraunhofer’s method to the examination of coloured flames of various substances, and since then we have been continually investigating the subject; yet much remains to be learnt of Spectrum Analysis, and Spectroscopy has still much to reveal. From Newton’s time to the present our scientists have been slowly but surely examining with the Spectroscope the composition of spectra, and the Spectroscope is now the greatest assistant we possess.

“Spectrum Analysis, then, teaches us the great fact that solids and liquids give out continuous spectra, and vapours and gases give out discontinuous spectra instead of an unbroken light” (Lockyer). We have found out that the sunlight and moonlight are identical, that the moon gives us spectrum like a reflection of the former, but has no atmosphere, and that comets are but gases or vapours. The most minute particles of a grain of any substance can be detected to the millionth fraction. The 1/1000 of a grain of blood can be very readily distinguished in a stain after years have passed. The very year of a certain vintage of wine has been told by means of “absorption,” or the action of different bodies on light in the spectrum. It is now easy, “by means of the absorption of different vapours and different substances held in solution, to determine not only what the absorbers really are, but also to detect a minute quantity.” The application of this theory is due to Dr. Gladstone, who used hollow prisms filled with certain substances, and so thickened the “absorption lines.” By these lines, or bands, with the aid of the Spectrum Microscope, most wonderful discoveries have been made, and will continue to be made. We will close this portion of the subject with a brief description of the Spectroscope in principle.

The instrument consists of two telescopes arranged with two object-glasses on a stand (fig. 150). A narrow slit is put in place of the eye-piece of one, the arrangement admitting of the slit being made smaller or larger by means of screws. The glass to which the slit is attached is called the collimating lens. The light, at the end of the slit seen from the other telescope, being separated by the prisms between the two telescopes, will produce the spectrum. The Spectroscope is enclosed, so that no exterior light shall interfere with the spectra the student wishes to observe. This merely indicates the principle, not the details, of the Spectroscope, which vary in different instruments.

We may now pass from the Spectroscope to the Telescope and the Microscope, instruments to which we are most largely indebted for our knowledge of our surroundings in earth, air, and water.

The word Telescope is derived from the Greek tele, far, and skopein, to see; and the instrument is based upon the property possessed by a convex lens or concave mirror, of converging to a focus the rays of light falling on it from any object, and at that point or focus forming an image of the object. The following diagram will illustrate this. Let VW be a lens, and AB an object between the glass, and F the focus. The ray, Ac, is so refracted as to appear to come from a. The ray from b likewise appears in a similar way, and a magnified image, ab, is the result (fig. 152).

The ordinary Telescope consists of an object-glass and an eye-lens, with two intermediates to bring the object into an erect position. A lens brings it near to us, and a magnifier enlarges it for inspection. We will now give a short history of the Telescope and its improved construction.

Roger Bacon was supposed to have had some knowledge of the Telescope, for in 1551 it was written: “Great talke there is of a glass he made at Oxford, in which men see things that were don.” But a little later, Baptista Porta found out the power of the convex lens to bring objects “nearer.” It was, however, according to the old tale, quite by an accident that the Telescope was discovered about the year 1608.

Fig. 152.—Converging rays to a focus.

In Middleburg, in Holland, lived a spectacle-maker named Zachary Jansen, and his sons, when playing with the lenses in the shop, happened to fix two of them at the proper distance, and then to look through both. To the astonishment of the boys, they perceived an inverted image of the church weathercock much nearer and much larger than usual. They at once told their father what they had seen. He fixed the glasses in a tube, and having satisfied himself that his sons were correct, thought little more about the matter. This is the story as told, but there is little doubt that for the first Telescope the world was indebted either to Hans Lippersheim or Joseph Adriansz, the former a spectacle-maker of Middleburg; and in October 1608, Lippersheim presented to the Government three instruments, with which he “could see things at a distance.” Jansen came after this. The report of the invention soon spread, and Galileo, who was then in Venice, eagerly seized upon the idea, and returning to Padua with some lenses, he managed to construct a telescope, and began to study the heavens. This was in 1609. Galileo’s Tube became celebrated, and all the first telescopes were made with the concave eye-lens. Rheita, a monk, made a binocular telescope, as now used in our opera- and field-glasses approximately.

But the prismatic colours which showed themselves in the early telescopes were not got rid of, nor was it till 1729 that Hall, by studying the mechanism of the eye, managed a combination of lenses free from colour. Ten years before (in 1718) Hadley had established the Reflector Telescope; Herschel made his celebrated forty-foot “reflector” in 1789.

Fig. 153.—The Microscope.

However, to resume. In 1747, Euler declared that it was quite possible to construct an arrangement of lenses so as to obtain a colourless image, but he was at first challenged by John Dollond. The latter, however, was afterwards induced to make experiments with prisms of crown and flint glass. He then tried lenses, and with a concave lens of flint, and a convex lens of crown, he corrected the colours. The question of proper curvature was finally settled, and the “Achromatic” Telescope became an accomplished fact.

There are two classes of Telescopes—the reflecting and refracting. Lord Rosse’s is an instance of the former. Mr. Grubb’s immense instrument in Dublin is a refractor.

The Microscope has been also attributed to Zacharias Jansen, and Drebbel, in 1619, possessed the instrument in London, but it was of little or no use. The lens invented by Hall, as already mentioned, gave an impetus to the Microscope. In the simple Microscope the objects are seen directly through the lens or lenses acting as one. The compound instrument is composed of two lenses (or a number formed to do duty as two), an eye-lens, and an object-lens. Between these is a “stop” to restrain all light, except what is necessary to view the object distinctly. The large glass near the object bends the rays on to the eyeglass, and a perfect magnified image is perceived. We annex diagrams, from which the construction will be readily understood.

Fig. 154.—Image on the Retina.

We have in the previous chapter mentioned the effect of light upon the eye and its direction, and when an object is placed very near the eye we know it cannot be distinctly seen; a magnified image is thrown upon the retina, and the divergency of the rays prevents a clear image being perceived. But if a small lens of a short “focal length” be placed in front of the eye, having PQ for its focus, the rays of light will be parallel, or very nearly so, and will as such produce “distinct vision,” and the image will be magnified at pq. In the Compound Refracting Microscope, BAB is the convex lens, near which an object, PQ, is placed a little beyond its focal length. An inverted image, pq, will then be formed. This image is produced in the convex lens, bab′, and when the rays are reflected out they are parallel, and are distinctly seen. So the eye of the observer at the point E will see a magnified image of the object at PQ brought up to pq (fig. 155).

Fig. 155.—The Microscope lenses.

Sir Isaac Newton suggested the Reflecting Microscope, and Dr. Wollaston and Sir David Brewster improved the instrument called the “Periscopic Microscope,” in which two hemispherical lenses were cemented together by the plane surfaces, and having a “stop” between them to limit the aperture. Then the “Achromatic” instrument came into use, and since then the Microscope has gradually attained perfection.

Fig. 156.—Concave lens.

We have so frequently mentioned lenses that it may be as well to say something about them. Lenses may be spherical, double-convex, plane-convex, plane-concave, double-concave, and concave-convex. Convex lenses bring the parallel lines which strike them to a focus, as we see in the “burning-glass.” The concave or hollow lens appears as in fig. 156. The rays that follow it parallel to its axis are refracted, and as if they came from a point F in the diagram. But converging rays falling on it emerge in a parallel direction as above, or diverge as in fig. 158.

Fig. 157.
1. Focus of parallel rays. 2. Focus of divergent rays. 3. Focus of divergent rays brought forward by more convex lens.

The use of spectacles to long or short-sighted people is a necessity, and the lenses used vary. The eye has usually the capacity of suiting itself to viewing objects—its accommodation, as it is termed—near or far. But when the forepart of the eye is curved, and cannot adapt itself to distant objects, the person is said to be short-sighted. In long sight the axis of the eyeball is too short, and the focus falls beyond the retina; in short sight it is too long. In the diagrams herewith fig. 159 shows by the dotted lines the position of the retina in long sight, and fig. 160 in short sight, the clear lines showing in each case the perfectly-formed eye. For long sight and old sight the double-convex glass is used, for short sight the double-concave (fig. 162). We know the burning-glass gives us a small image of the sun as it converges the rays to its focus. But lenses will do more than this, and in the Photographic Camera we find great interest and amusement.

Fig. 158.—Diverging rays.

Photography (or writing by light) depends upon the property which certain preparations possess of being blackened by exposure to light while in contact with matter. By an achromatic arrangement of lenses the camera gives us a representation of the desired object Fig. 163 shows the image on the plate, and figs. 164 and 165 the arrangement of lenses.

Fig. 159.—Hypermetropia (long sight).. Fig. 160.—Myopia (short sight).

Fig. 161.—Concave and convex lenses.

Fig. 162.—Lenses for long and short sight.

To Porta, the Neapolitan physician, whose name we have already mentioned more than once, is due the first idea of the Photographic Camera. He found that if light was admitted through a small aperture, objects from which rays reached the hole would be reflected on the wall like a picture. To this fact we are indebted for the Camera Obscura, which receives the picture upon a plane surface by an arrangement of lenses. In fact, Porta nearly arrived at the Daguerreotype process. He thought he could teach people to draw by following the focussed picture with a crayon, but he could not conquer the aërial perspective.

So the camera languished till 1820, when Wedgwood and Sir Humphrey Davy attempted to obtain some views with nitrate of silver, but they became obliterated when exposed to the daylight.

Fig. 163.—The Camera.

As early as 1814, however, M. Niepce had made a series of experiments in photography, and subsequently having heard that M. Daguerre was turning his attention to the same subject, he communicated with him. In 1827 a paper was read before the Royal Society, and in 1829 a partnership deed was drawn up between Daguerre and Niepce for “copying engravings by photography.” Daguerre worked hard, and at length succeeded in obtaining a picture by a long process, to which, perhaps, some of our readers are indebted for their likenesses forty years ago. By means of iodine evaporated on a metal plate covered with “gold-yellow,” and exposing the plate then in a second box to mercurial vapour, he marked the image in the camera, and then he immersed the plate in hyposulphate of soda, and was able to expose the image obtained to daylight.

Fig. 164. Arrangement of lenses Fig. 165.

But the mode now in use is the “collodion” process. We have all seen the photographer pouring the iodized collodion on the plate, and letting the superfluous liquid drain from a corner of the glass. When it is dry the glass-plate is dipped into a solution of nitrate of silver, and then in a few minutes the glass is ready. The focus is then arranged, and the prepared plate conveyed in a special slide—to keep it from the light—to the camera. When the “patient” is ready, the covering of the lens is removed, and the light works the image into the sensitive plate. The impression is then “brought up,” and when developed is washed in water, and after by a solution which dissolves all the silver from the parts not darkened by the light. Thus the negative is obtained and printed from in the usual manner.

Instantaneous photography is now practised with great success. An express train, or the movements of a horse at full speed, can thus be taken in a second or less. These results are obtained by using prepared plates, and the “emulsion process,” as it is called, succeeds admirably. The mode of preparation is given in a late work upon the subject, and the photographic plates may also be obtained ready for use. Gelatine and water, mixed with bromide of ammonium, nitrate of silver, and carbonate of ammonium, mixed with certain proportions of water, form the “emulsion.” We need not go into all the details here. Information can easily be obtained from published works, and as the plates can be purchased by amateurs, they will find that the best way.

Aside from the art interest in the new plates is quite another, that springs from the fact that it is now possible to take pictures of men, animals, and machinery in rapid motion, thus enabling us to view them in a way that would be impossible with the unaided eye. The first experiments in this direction were applied to the movements of a horse moving at full speed. The pictures, taken in series, showed that he performed muscular actions that were not before comprehended or even imagined. These pictures at the time attracted great attention, and instantaneous pictures have been since taken of dancers in a ball-room, of vessels and steam-boats in rapid movement, of all kinds of animals in motion, and of machinery in operation. As the pictures represent the movements at one instant of time, they give, as it were, a fixed view of a motion, precisely as if it were suddenly arrested in full action. In the case of animals, the motions of the nostrils are represented in the most singular manner, and the spokes of a steam-boat’s paddle-wheel are shown apparently perfectly still while the spray and waves appear in active motion, or, rather, as they would look if they could be instantly frozen. It is clear the new process and pictures will open a wide and instructive field in art and in the study of mechanical action.

While on the subject of Photography we may mention a very ingenious little apparatus called a Scenograph, the invention of Dr. Candize. It is really a pocket-camera, and is so easily manipulated that it will be found a most pleasant and useful holiday companion. Any one may obtain good results with it, and friends of ours have had occasion to put it in practice during a series of excursions, when it was found to answer in every instance.

The Scenograph is something like a common Stereoscope in outward appearance, and would, perhaps, be at first regarded as a mere toy, did not a more intimate acquaintance prove it a great acquisition, particularly to explorers and tourists. The tripod stand which supports the apparatus is, when not in actual use in that capacity, a very excellent walking-stick, in which the two other “legs” are carried. The instrument, as will be perceived from the illustration (fig. 166), is very handy. It produces pictures about the “cabinet” size, and the whole is so arranged that it can be packed and carried in the pocket with ease.

Fig. 166.—The Scenograph.

Photography, as a rule, necessitates a dark room or cabinet, and many preparations as we all know—a “messing” about with chemicals and considerable practice before we can become proficient; so it is not surprising that few amateurs take to it—they prefer to purchase the pictures. But in the new apparatus of which we are speaking, the glass plates are already prepared to receive the image. It is not at all necessary for the operator to stain his fingers and knuckles and nails with nitrate of silver, or any other “chemicals” whatever. He just inserts the plate in the Scenograph, and then his apparatus being steadily set up, he removes the covering from the lens. To develop the image (in the dusk of the evening or by candle-light) it is necessary to put some drops of ammonia in a saucer, breathe upon the plate so as to soften the collodion, and hold it above the ammonia, and then, under the influence of the vapour, the picture will appear. After this simple operation the picture will be found fixed for a lengthened period—practically indefinitely. Thus on the return of the pedestrian he can reproduce, at small expense, a whole series of little pictures faithfully representing his holiday tour. The illustration shows a small apparatus by which on thin plates small photographs can be taken and fixed till it is found desirable to enlarge them.

The Photophone, one of the most recent contributions to science, is an instrument which, in combination with the telephone principle, makes it possible to convey sounds by means of a ray of light, and by means of a quivering beam “to produce articulate speech at a distance. The success of the Photophone depends upon a rare element, selenium, which has its “electrical resistance” affected by light. Professor Adams demonstrated that the resistance of selenium was reduced just in proportion as the intensity of the light which was acting upon it. Here was the key to the Photophone as thought out by Professor Bell. He fancied that he might by means of his telephone produce sound if he could vary the intensity of the beam of light upon the selenium, which he connected with his telephone and battery.

The Photophone consists of a transmitter for receiving the voice and conveying it along the beam of light, and a receiver for taking the light and converting it into sound—the receiver being the telephone. There is a small mirror (silvered mica has been used) suspended freely for vibration. A lens is used to transmit to this the beam of light, and this beam is again reflected by another lens to the receiver, which consists of a reflector which has a cell of selenium in its focus, connected, as already stated, with the telephone and battery. The speaker stands behind the mirror, and the sound of his voice against the reverse side makes it vibrate in unison with the sounds uttered. The movements cause a quivering in the reflected beam, and this in its changing intensity acts on the selenium, which changes its resistance accordingly, and through the telephone gives forth a sound!

This is the apparently complicated but really simple, and at the same time wonderful, invention of Professor Bell. By the Photophone not only sounds but movements can be converted into sound; even the burning of a candle can be heard! The Photophone is still capable of improvement, and has not as yet arrived at its full development, for it is stated it can be made quite independent of a battery or telephone.

There are many phenomena connected with the Polarization of Light. This requires some notice at our hands. We know that a ray of ordinary light is supposed to be caused by vibrations of the highly attenuated medium, æther. These vibrations occur across the direction of the ray; but when they occur only in one plane the light is said to be “polarized.” Polarization means possessing poles (like a magnet); the polarized rays have “sides,” as Newton said, or, as explained by Dr. Whewell, “opposite properties in opposite directions, so exactly equal as to be capable of accurately neutralizing each other.” There are some crystals which possess the property of “double refraction,” and thus a ray of common light passing through such a crystal is divided into two polarized rays, taking different directions. One is refracted according to the usual laws of refraction; the other is not, and the planes of polarization are at right angles. It is difficult within the limits of this chapter to explain the whole theory of Polarization. In order to account for certain phenomena in optics, philosophers have assumed that rays possess polarity; and polarized light is light which has had the property of Polarization conferred upon it by reflection, refraction, or absorption. Common light has been compared to a round ruler, and polarized light to a flat ribbon. Huygens found out, when engaged upon the investigation of double refraction, that the rays of light, divided by passing through a crystal (a rhomb) of Iceland spar, possessed certain qualities. When he passed them through a second rhomb, he found that the brightness, relatively, of the rays depended upon the position of the second prism, and in some positions one ray disappeared entirely. The light had been reduced to vibrations in one plane. In 1808, Malus, happening to direct a double refracting prism to the windows then reflecting the sunset, found that as he turned the prism round, the ordinary image of the window nearly disappeared in two opposite positions; and in two other positions, at right angles, the “extraordinary” image nearly vanished. So he found that polarization was produced by reflection as well as by transmission. The differences between common and polarized light have been summed up by Mr. Goddard as follows:—

Common LightPolarized Light
“Is capable of reflection at oblique angles of incidence in every position of the reflector.“Is capable of reflection at oblique angles only in certain positions of the reflector.
“Will pass through a bundle of plates of glass in any position in which they may be placed.“Will only pass through such glasses when they are in certain positions.
“Will pass through a plate of tourmaline, cut parallel to the axis of the crystal, in every position of the plate.”“Will only pass in certain positions, and in others will not pass at all.”

The bundle of glass plates or the tourmaline plate is thus the test for polarized light, and is termed an analyzer.

The arrangement called a “Nichol’s prism,” made by cutting a prism of Iceland spar and uniting the halves with a cement, so that only one polarized ray can pass through it, is termed a Polarizer. It only permits one of the two rays produced by “double refraction” to pass, and the ray (as said above) will contain none but transverse vibrations. Polarized light will produce beautiful colours. The whole subject is very interesting to the scientist, but rather a difficult one for the general reader to understand.

Amongst the uses to which light has been put is that of a milk-tester. The Lactoscope will show the quantity of butter contained in a certain quantity of milk, by diluting it till it displays a certain degree of transparency. There is another method, by the transmission of light.

The first test is obtained by means of a glass tube about nine inches long, closed at one end, and containing a small porcelain rod marked with black lines. A small quantity of milk is measured and placed in the tube. The black lines cannot at first be seen through the tube, but by adding water the milk is rendered transparent, and the black lines become visible. The surface of milk in the tube, by a graduated scale upon it, shows the percentage of butter.

Fig. 167.—Cut card figures.

The second method is not so simple. A short tube of tin, blackened on the inside, and supported upright, has an opening on one side, and opposite this, inside the tube, is a mirror placed at an angle of 45°. “By placing a lighted candle at a known distance opposite the opening, its light is reflected in the mirror and thrown upward through the tube. On top of the tube is placed a round vessel of glass or metal, closed at the bottom by a sheet of clear glass. The vessel is closed at the top by a cover having an opening in the centre, in which slides up and down a small tube closed at the bottom with glass, and having an eye-piece at the top. The milk to be tested is placed in this vessel on the top of the tin tube, so that the light of the candle reflected from the mirror passes upward through the milk. Then, by looking through the sliding tube and moving it up and down, a point may be found where the image of the candle in the mirror can be seen through the milk. This device depends, as will be seen, on observing the light transmitted through a film of milk, and the thickness of the film is the measure of the value of the milk. The movable tube contains a graduated scale, and by comparison of this with a printed table, the percentage of butter in the milk may be ascertained.”

In concluding this chapter we give a few hints for some pleasant relaxation for young people, which has many a time created amusement. The experiment consists in cutting out in paper or cardboard certain portions of a face or figure, as per the illustration herewith. Fig. 167 gives the card as cut with the scissors, and the two subsequent faces are the result of the same held at a less or greater distance from a screen. The illustrations (fig. 168) will assist those who wish to amuse children by making rabbits, etc., on the wall. The shadows will be seen perfectly thrown if the hands be carefully fixed near a good light.

Fig. 168.—Hand-shadows on the wall.

We are all so familiar with the “Magic Lantern,” and the apparatus for dissolving views by an arrangement of lenses and manipulation of slides, that we need do no more than refer to them.

Fig. 169.—Dissolving views apparatus.

The various ghost illusions, etc., produced by indirect mirrors, have already been referred to, the ghost being merely the reflection of an individual seen through a sheet of glass between the spectators and the stage. The strong light throws a reflection from a parallel mirror lower down, and this reflected image can be made to appear amongst the real actors who are behind the plate-glass in full view of the audience, who are, however, ignorant of the existence of the glass screen.

For the winter evenings one may easily procure an apparatus for dissolving views by the oxy-hydrogen light. One, as shown in the illustration herewith (fig. 169), will answer every purpose, and by this double arrangement phantasmagoria may be produced, or a fairy tale may be illustrated. The effect of gradually-approaching night may be given to the picture by means of a special glass in the lower lanthorn. The apparatus is exhibited by means of a Drummond light, and is very simple, although a certain supply of gas is necessary for the performance. But this can be easily procured by an indiarubber tube, or in a bag supplied for the purpose. Almost any objects can be used, photographs, etc., etc., and many very comical arrangements can be made.

We have lately been reading a curious method of obtaining light from oyster-shells in a Trans-Atlantic magazine. We give an extract wherewith to close this chapter. The compound is “luminous paint.”

“It has been known that certain compounds of lime and sulphur had the property of absorbing light, and giving it out again when placed in the dark. A simple way to do this is to expose clean oyster-shells to a red heat for half an hour. When cold, the best pieces are picked out and packed with alternate layers of sulphur in a crucible, and exposed to a red heat for an hour. When cold, the mass is broken up, and the whitest pieces are placed in a clean glass bottle. On exposing the bottle to bright sunshine during the day, it is found that at night its contents will give out a pale light in the dark. Such a bottle filled more than a hundred years ago still gives out light when exposed to the sun, proving the persistency of the property of reproducing light. Very many experiments have been more recently made in this direction, and the light-giving property greatly enhanced. The chemicals, ground to a flour, may now be mixed with oils or water for paints, may be powdered on hot glass, and glass covered with a film of clear glass, or mixed with celluloid, papier-maché, or other plastic materials. As a paint, it may be applied to a diver’s dress, to cards, clock dials, sign-boards, and other surfaces exposed to sunlight during the day; the paint gives out a pale violet light at night sufficient to enable the objects to be readily seen in the dark. If the object covered with the prepared paint is not exposed to the sun, or if the light fades in the dark, a short piece of magnesium wire burned before it serves to restore the light-giving property.”

CHAPTER XIV.
SPECTRAL ILLUSIONS.

A SPECTRE VISIBLE—CURIOUS ILLUSIONS—GHOSTS.

We have already given numerous examples of the effects produced by impressions on the retina by mechanical appliances. We can now in a short chapter speak of the cause of many spectral illusions, commonly supposed to be “ghosts” or “spirits.” That there are many “well-authenticated ‘ghost stories’” no one can doubt who has read the literature of the day; and we ourselves do not in any way desire to throw any doubt upon the existence of certain so-called “ghosts.” That appearances of some kind or another are seen by people we know. We ourselves have seen such, but we cannot say we believe in the popular ghost.

In ancient times mirrors were much employed by the so-called magicians, and in our day many wonderful ghost effects have been shown at the (late) Polytechnic Institution. Some people are believers in table-turning and spiritualism, and mesmerists still attract large audiences, and appear to possess extraordinary power over some individuals. But apparitions have been seen by people eminently worthy of credit. The experience of the learned Doctor, which appeared some months ago in the Athenæum, is a case in point. This narrative is concise and clear. The spectre was there. How did it get there? Was the “appearance” objective or subjective? Let us give an extract from the Reverend Doctor’s narrative, and comment upon it afterwards. We may premise that Dr. Jessopp had gone over to Lord Orford’s (Mannington Hall), and at eleven o’clock was busy writing in the library, and was “the only person downstairs.” We will give this ghost story in the Doctor’s own words. After taking up a certain volume—time about 1 a.m.:—

“I had been engaged on it about half an hour, and was beginning to think my work was drawing to a close, when, as I was actually writing, I saw a large white hand within a foot of my elbow. Turning my head, there sat a figure of a somewhat large man with his back to the fire, bending slightly over the table, and apparently examining the pile of books that I had been at work upon.”... After describing the appearance of the nocturnal visitor, Dr. Jessopp proceeds:—

“There he sat, and I was fascinated; afraid not of his staying, but lest he should go. Stopping in my writing I lifted my left hand from the paper, stretched it out to the pile of books, and moved the top one—my arm passed in front of the figure, and it vanished.”... Shortly after the figure appeared again, and “I was penning a sentence to address to him, when I discovered I did not dare to speak. I was afraid of the sound of my own voice! There he sat, and there sat I. I turned my head and finished writing. Having finished my task, I shut the book, and threw it on the table; it made a slight noise as it fell;—the figure vanished.”

Now here we have a perfectly plain narrative, clear and full. A ghost appeared; he is described distinctly. How can we account for the apparition? In the first place, someone might have played a trick, but that idea was put aside by Dr. Wilks, who attempted to explain the appearances. He went fully into the question, and as it bears upon our explanation of the reality of Spectral Illusions, we may condense his evidence. It will of course be conceded that all the usual objects seen by people are material, and the image of what we look at is formed upon the retina in the manner already explained. But all images upon the retina are not immediately observed; the impression may, to a certain extent, remain. Words are often impressed upon the brain,—words which we in our sober senses would never think of repeating,—and yet when we are delirious we give vent to these expressions, of whose very nature and meaning we are perfectly unconscious. It is, according to our reference (Dr. Wilks), “quite possible for the perceptive part of the brain to be thrown into an active condition quite independent of the normal stimulus conducted to it from the retina.” If, under these circumstances, an object be viewed independently, and, as it were, unconsciously, it is merely, we believe, a parallel to the impression of words before noted. Sound and light are governed by the same laws. In fevers we fancy we see all kinds of things which have no existence. In dreams we hear noises; and many a time people dreaming have been awakened by the report of a gun, or the ringing of a bell which had no material origin,—the nerves were excited, the “perceptive centre” of the brain was moved.

But if sight and hearing thus have their origin from the brain and not from without, there must have been some predisposing cause, some excitement to induce such a condition of things. “The impressions become abnormal and subjective,—the normal condition being objective,—the impression is received from without, and impressed upon the eye.

Now, let us consider the “ghost”! Lately there have been many instances brought forward of “spiritual” appearances, but we think nobody has ever seen a “material” ghost; yet on the other hand none of us have any knowledge of anything in the likeness of a ghost, or that has not a material basis which can bring forward an image on the retina! Therefore we are brought to the conclusion that apparitions are spectres emanating from within the brain, not from any outward manifestation, because it is within the experience of everybody that in bad health, or disordered digestive functions, images are produced in the brain and nerves of the eye.

These remarks have perhaps been made before in one form or other, but as much popular interest is always awakened by the supernatural, or what is supposed to be supernatural, we may go a little farther, and inquire how it was that the ghost seen by Dr. Jessopp disappeared when he raised his arm. Would any ghost be afraid of the Doctor extending his hand? The fact no doubt occurred as related. The explanation is that the narrator had been much impressed by a certain picture, which a correspondent soon identified as a portrait of “Parsons, the Jesuit Father.” The description given is that of the priest who was described by the Doctor in one of his books. The association of ideas in the library of a Norfolk house connected with the Walpoles, with whom Parsons had been a leader, gave rise, during a period of “forty winks” at midnight, to the spectre.

In the interesting letters written upon “Natural Magic” by Sir David Brewster, the subject of Spectral Illusions is treated at some length, and with undoubted authority. Sir David thought the subject worth discussing with reference to the illusions or spectres mentioned by Dr. Hibbert. Sir David Brewster gives his own experiences which occurred while he was staying at the house of a lady in the country.

The illusions appear to have affected her ear as well as the eye. We shall see in the next chapter how intimately sound and light are connected, and how the eyes and ears are equally impressed, though in a different way, by the vibration of particles. The lady referred to was about to go upstairs to dress for dinner one afternoon, when she heard her husband’s voice calling to her by name. She opened the door, and nobody was outside; and when she returned for a moment to the fire she heard the voice again calling, “Come to me; come, come away,” in a somewhat impatient tone. She immediately went in search of her husband, but he did not come in till half an hour afterwards, and of course said he had not called, and told her where he had been at the time—some distance away. This happened on the 26th December, 1830, but a more alarming occurrence took place four days after.

About the same time in the afternoon of the 30th December, the lady came into the drawing-room, and to her great astonishment she perceived her husband standing with his back to the fireplace. She had seen him go out walking a short time previously, and was naturally surprised to find he had returned so soon. He looked at her very thoughtfully, and made no answer. She sat down close beside him at the fire, and as he still gazed upon her she said, “Why don’t you say something!” The figure immediately moved away towards the window at the farther end of the room, still gazing at her, “and it passed so close that she was struck by the circumstance of hearing no step nor sound, nor feeling her dress brushed against, nor even any agitation of the air.” Although convinced this was not her husband, the lady never fancied there was anything supernatural in the appearance of the figure. Subsequently she was convinced that it was a spectral illusion, although she could not see through the figure which appeared as substantial as the reality.

Were it advisable, we could multiply instances. In the Edinburgh Journal of Science these, and many more instances of spectral illusions were narrated by the husband of the lady. She frequently beheld deceased relatives or absent friends, and described their dress and general appearance very minutely. On one occasion she perceived a coach full of skeletons drive up to the door, and noticed spectral dogs and cats (her own pets’ likenesses) in the room. There can be no doubt upon these points; the appearances were manifest and distinct. They were seen in the presence of other people, in solitude, and in the society of her husband. The lady was in delicate health, and very sensitive. The spectres appeared in daylight as well as in the dark, or by candle-light.

Let us now, guided by what we have already written, and by Sir David Brewster’s experience, endeavour to give a rational explanation of these illusions. “The mind’s eye is really the body’s eye, and the retina is the common tablet upon which both classes of impressions are painted, and by means of which they receive their visual existence according to the same optical laws.”

“In the healthy state of mind and body the relative intensity of the two classes of impressions on the retina are nicely adjusted—the bodily and mental are balanced. The latter are feeble and transient, and in ordinary temperaments are never capable of disturbing or effacing the direct images of visible objects.... The mind cannot perform two different functions at the same instant, and the direction of its attention to one of the two classes of impressions necessarily produces the extinction of the other; but so rapid is the exercise of mental power, that the alternate appearance and disappearance of the two contending impressions is no more recognized than the successive observations of external objects during the twinkling of the eyelids.”

We have before illustrated, by means of the pen and the ink-bottle, how one object is lost sight of when the other is attentively regarded, and a material picture or scene may be equally lost sight of, and a mental picture take its place in the eye, when we recall places or people we have seen or remembered.

We have all heard numerous anecdotes of what is termed “absent-mindedness.” Some people are quite absorbed in study, and can see or hear no one in the room when deeply occupied. We may be satisfied then that “pictures of the mind and spectral illusions are equally impressions upon the retina, and only differ in the degree of vividness with which they are seen.” If we press our eyes the phosphorescence becomes apparent, and we can make a picture of the sun or a lamp visible for a long time to our closed eyes if we stare at either object for a few seconds, and shut our lids. So by increasing the sensibility of the retina we can obtain the image, and alter its colour by pressure on the eye.

It is well known that poisons will affect sight, and belladonna applied to the eyes will so affect them as to render the sight nil, by enlargement of the “pupil.” If one is out of health there is practically a poisoning of the system, and when we have a “bilious headache” we see colours and stars because there is a pressure upon the blood-vessels of the eye. The effects of a disordered stomach, induced by drinking too much, are well known; objects are seen double, and most ghosts may be traced to a disordered state of health of mind or body, brought on by excitement or fatigue. We could relate a series of ghost stories,—some in our own experience, for we have seen a ghost equally with our neighbours,—but this is not the place for them. Although many apparently incontrovertible assertions are made, and many spectres have been produced to adorn a tale, we must put on record our own opinion, that every one could be traced to mental impression or bodily affection had we only the key to the life and circumstances of the ghost-seer. Many celebrated conjurers will convince us almost against our reason that our pocket-handkerchief is in the orange just cut up. They will bring live rabbits from our coat-pockets or vests, and pigeons from our opera-hats. These are equally illusions. We know what can be done with mirrors. We have seen ghosts at the Polytechnic, but we must put down all apparitions as the result of mental or bodily, even unconscious impressions upon the retina of the eye. There are numerous illusions, such as the Fata Morgana, the Spectre of the Brocken, etc., which are due to a peculiar state of the atmosphere, and to the unequal reflection and refraction of light. Those, and many other optical phenomena, will, with phenomena of heat and sound, be treated under Meteorology, when we will consider the rainbow and the aurora, with many other atmospheric effects.

CHAPTER XV
ACOUSTICS.

THE EAR, AND HEARING—PHYSIOLOGY OF HEARING AND SOUND—SOUND AS COMPARED WITH LIGHT—WHAT IS SOUND?—VELOCITY OF SOUND—CONDUCTIBILITY—THE HARMONOGRAPH.

Before entering upon the science of Acoustics, a short description of the ear, and the mode in which sound is conveyed to our brain, will be no doubt acceptable to our readers. The study of the organs of hearing is not an easy one; although we can see the exterior portion, the interior and delicate membranes are hidden from us in the very hardest bone of the body—the petrous bone, the temporal and rock-like bone of the head.

Fig. 170- 1. Temple bone. 2. Outer surface of temple. 3. Upper wall of bony part of hearing canal. 4. Ligature holding “hammer” bone to roof of drum cavity. 5. Roof to drum cavity. 6. Semicircular canals. 7. Anvil bone. 8. Hammer bone. 9. Stirrup bone. 10. Cochlea. 11. Drum-head cut across. 12. Isthmus of Eustachian tube. 13. Mouth of tube in the throat. 14. Auditory canal. 15. Lower wall of canal. 16. Lower wall of cartilaginous part of canal. 17. Wax glands. 18. Lobule. 19. Upper wall of cartilaginous portion of canal. 20. Mouth of auditory canal. 21. Anti-tragus.

The ear (external) is composed of the auricle, the visible ear, the auditory canal, and the drum-head, or membra tympani. The tympanum, or “drum,” is situated between the external and the internal portions of the ear. This part is the “middle ear,” and is an air cavity, and through it pass two nerves, one to the face, and the other to the tongue. The internal ear is called the “labyrinth,” from its intricate structure. We give an illustration of the auditory apparatus of man (fig. 170).

The auricle, or exterior ear, is also represented, but we need not go into any minute description of the parts. We will just name them (fig. 171).

Sound is the motion imparted to the auditory nerve, and we shall see in a moment how sound is produced. The undulations enter the auditory canal, having been taken up by the auricle; the waves or vibrations move at the rate of 1,100 feet a second, and reach the drum-head, which has motion imparted to it. This motion or oscillation is imparted to other portions, and through the liquid in the labyrinth. The impressions of the sound wave are conveyed to the nerve, and this perception of the movement in the water of the labyrinth by the nerve threads and the brain causes what we term “hearing.”

Fig. 171.—1. Pit of anti-helix. 2, 6, 10. Curved edge of the auricle. 3. Mouth of auditory canal. 4. Tragus. 5. Lobe. 7. Anti-helix. 8. Concha. 9. Anti-tragus.

Let us now endeavour to explain what sound is, and how it arises. There are some curious parallels between sound and light. When speaking of light we mentioned some of the analogies between sound and light, and as we proceed to consider sound, we will not lose sight of the light we have just passed by.

Sound is the influence of air in motion upon the hearing or auditory nerves. Light, as we have seen, is the ether in motion, the vibrations striking the nerves of the eye.

There are musical and unmusical sounds. The former are audible when the vibrations of the air reach our nerves at regular intervals. Unmusical sounds, or irregular vibrations, create noise. Now, musical tones bear the same relation to the ear as colours do to the eye. We must have a certain number of vibrations of ether to give us a certain colour (vide table). “About four hundred and fifty billion impulses in a second” give red light. The violet rays require nearly double. So we obtain colours by the different rate of the impingement of impulses on the retina. The eyes, as we have already learned, cannot receive any more rapidly-recurring impressions than those producing violet, although as proved, the spectrum is by no means exhausted, even if they are invisible. In the consideration of Calorescence we pointed this out. These invisible rays work great chemical changes when they get beyond violet, and are shown to be heat. So the rays which do not reach the velocity of red rays are also heat, which is the effect of motion.

Thus we have Heat, Light, and Sound, all the ascertained results of vibratory motion. The stillness of the ether around us is known as “Darkness”; the stillness of the air is “Silence”; the comparative absence of heat, or molecular motion of bodies is “Cold”!

In the first part we showed how coins impart motion to each other.

VELOCITY OF LIGHT WAVES.
According to Sir J. Herschel.
Colour of the
Spectrum.		No. of Undulations
in an inch.		No. of Undulations
in a second.
Extreme Red37,640458,000,000,000,000
Red39,180477,000,000,000,000
Intermediate40,720495,000,000,000,000
Orange41,610506,000,000,000,000
Intermediate42,510517,000,000,000,000
Yellow44,000535,000,000,000,000
Intermediate45,600555,000,000,000,000
Green47,460577,000,000,000,000
Intermediate49,320600,000,000,000,000
Blue51,110622,000,000,000,000
Intermediate52,910644,000,000,000,000
Indigo54,070658,000,000,000,000
Intermediate55,240672,000,000,000,000
Violet57,490699,000,000,000,000
Extreme Violet59,750727,000,000,000,000

When an impulse was given the motion was carried from coin to coin, and at length the last one in the row flew out. This is the case with sound. The air molecules strike one upon another, and the wave of “sound” reaches the tympanum, and thus the impression is conveyed to the brain. We say we hear—but why we hear, in what manner the movement of certain particles affects our consciousness, we cannot determine.

That the air is absolutely necessary to enable us to hear can readily be proved. The experiment has frequently been made; place a bell under the receiver of an air-pump, and we can hear it ring. But if we exhaust the air the sound will get fainter and fainter. Similarly, as many of us have experienced upon high mountains, sounds are less marked. Sound diminishes in its intensity, just as heat and light do. Sound is reflected and refracted, as are light and radiant heat. We have already shown the effect of reflectors upon heat. Sound is caught and reflected in the same way as light from mirrors, or as the heat waves in the reflectors. We have what we term “sounding boards” in pulpits, and speaking tubes will carry sound for us without loss of power. Echoes are merely reflected sounds.

The velocity of sound is accepted as 1,100 feet in a second, which is far inferior to the velocity of light. Fogs will retard sound, while water will carry it. Those who have ever rowed upon a lake will remember how easily the sound of their voices reached from boat to boat, and Dr. Hutton says that at Chelsea, on the Thames, he heard a person reading from a distance of a hundred and forty feet. Some extraordinary instances could be deduced of the enormous distances sound is said to have travelled. Guns have been heard at eighty miles distant, and the noise of a battle between the English and Dutch, in 1672, was heard even in Wales, a distance of two hundred miles from the scene of action.

Sound always travels with uniform velocity in the air in the same temperature. But sound! What is the cause of it? How does it arise? These questions can now be fully answered with reference to the foregoing observations. Phenomena of vibration render themselves visible by light, heat, and sound, and to arrive at some definite ideas of sound vibrations we may compare them to waves, such as may be produced by throwing a stone into a pond.

There are, so to speak, “standing” waves and “progressive” waves. The former can be produced (for instance) by thrumming a fiddle-string, and when the equilibrium of the cord is disturbed, the position of the equilibrium is passed simultaneously by the string-waves. In water the waves or vibrating points pass the position of equilibrium in succession.

Waves consist of elevations and depressions alternately, and when we obtain two “systems” of waves by throwing two stones into water, we can observe some curious effects. It can be seen how one series of depressions will come in contact with the other series of depressions, and the elevations will likewise unite with the result of longer depressions and elevations respectively; or it may very well be that elevation will meet depression, and then the so-called “interference” of waves will produce points of repose. These points are termed nodes. The waves of the string proceed in the plane of its axis; water waves extend in circles which increase in circumference.

The progression or propagation of sound may be said to begin when some tiny globule of matter expands in the air. The air particles strike one against the other, and so the motion is communicated to the air waves, which in time reach the ear. But the velocity of the sound is not equal in all substances. Air will convey it around our earth at the rate of 765 miles an hour, or 1,090 feet in a second. That is, we may accept such rate as correct at a temperature of 32° Fahr., and at a pressure of thirty inches, and the velocity increases almost exactly one foot per second for each degree of temperature above 32°. Therefore on an average, and speaking in “round numbers,” the estimate of 1,100 feet in a second may be accepted as correct. In hydrogen gas the rate is much higher. Through water again it is different, and still faster through iron, glass, and wood, as will be seen in the following table:—

TAKING AIR AS 1.
Whalebone6⅔
Tin7½
Silver9
Walnut10⅔
Brass10⅔
Oak10⅔
Earthen pipes11
Copper12
Pear-wood12½
Ebony14⅔
Cherry15
Willow16
Glass16⅔
Iron or Steel16⅔
Deal18

So there is a considerable difference in the velocities of sound through the solid substances quoted, but these figures cannot be taken as exact, as different samples may give different results. In wires and bells the bodies themselves produce the sounds we hear. In wind instruments and the voice the air is the cause of the sound.

The very deepest notes are produced by the fewest vibrations. Fourteen or fifteen vibrations will give us a very low note, if not the very lowest. The pipe of sixteen feet, closed at its upper end, will produce sound waves of thirty-two feet. High notes can be formed from vibrations up to 48,000 in a second. Beyond these limits the ear cannot accept a musical sound.

Fig. 172.—The vibration of strings.

We will explain the phenomenon of the vibration of strings by means of the illustration. In the cut we find a string or wire, which can be lengthened or shortened at pleasure by a movable bridge, and stretched by weights attached to the end (fig. 172).

We can now easily perceive that the shorter and thinner the string is, and the tighter it is, the number of vibrations will be greater and greater. The density of it is also to be considered, and when these conditions are in the smallest proportion then the tone will be highest. The depth will naturally increase with the thickness, density, and length, and with a decreasing tension. But we have strings of same thickness stretched to different degrees of tension, and thus producing different notes. Some strings are covered with wire to increase their gravity, and thus to produce low notes.

When a number of separate sounds succeed each other in very rapid course they produce a sound, but to appear as one sound to the ear they must amount to fifteen or sixteen vibrations every second. The particles of matter in the air form a connected system, and till they are disturbed they remain in equilibrium; but the moment they are in any way thrown out of this state they vibrate as the pendulum vibrates. The particles thus strike each other, and impart a motion to the elastic medium air, so a sound comes to us.

The intensity of sounds gets less the farther it goes from us, or the loudness of sound is less the greater its distance. The law is, that in an unvarying medium the loudness varies inversely as the square of the distance. But Poisson has shown that when air-strata, differing in density, are existing between the ear and the source of the sound, the intensity or loudness with which it is heard depends only on the density of the air at the place the sound originated. This fact has been substantiated by balloonists who heard a railway whistle quite distinctly when they were nearly 20,000 feet above the ground. It therefore follows that sound can be heard in a balloon equally well as on the earth at certain given distances. But as the density of the air diminishes the sound becomes fainter, as has been proved by the bell rung in the receiver of an air-pump. The velocity of sound, to a certain extent, depends upon its intensity, as Earnshaw sought to prove; for he instanced a fact that in the Arctic regions, where sound can be heard for an immense distance, in consequence of the still and homogeneous air, the report of a cannon two miles and a half away was heard before the loud command to “fire,” which must have preceded the discharge. Another instance showing the difference in hearing through mixed and homogeneous media may be referred to. In the war with America, when the English and their foes were on opposite sides of a stream, an American was seen to beat his drum, but no sound came across. “A coating of soft snow and a thick atmosphere absorbed the noise.” Glazed, or hard snow, would have a contrary effect. Reynault also experimentally verified his theory, that sound when passing through a space of nearly 8,000 feet lost velocity as its intensity diminished, and in that distance between its arrival at 4,000 feet and at 7,500 feet, the sound velocity diminished by 2·2 feet per second. He also tried to demonstrate that sound velocity depended upon its pitch, and that lower notes travelled with the greater speed.

The reflection and refraction of sound follows the same fundamental laws as the reflection and refraction of light. The reflection of sound is termed an Echo, which is familiar to all tourists in Switzerland and Ireland particularly. There are several very remarkable echoes in the world: at Woodstock, and at the Sicilian cathedral of Gergenti, where the confessions poured forth near the door to priestly ears were heard by a man concealed behind the high altar at the opposite end. It is curious that such a spot should have been accidentally chosen for the Confessional. The whispering gallery in St. Paul’s is another instance of the echo.

Echoes are produced by the reflection of sound waves from a plane or even surface. A wall, or even a cloud, will produce echoes. Thunder is echoed from the clouds. (The celebrated echo of “Paddy Blake,” at Killarney, which, when you say “How do you do,” is reported to reply, “Very well, thank you,” can scarcely be quoted as a scientific illustration.) And the hills of Killarney contain an echo, and the bugle sounds are beautifully repeated. In the cases of ordinary echo, when the speaker waits for the answer, he must place himself opposite the rock. If he stand at the side the echo will reply to another person in a corresponding place on the farther side, for the voice then strikes the rock at an angle, and the angle of reflection is the same, as in the case of light.

But if it should happen that there are a number of reflecting surfaces the echo will be repeated over and over again, as at the Lakes of Killarney. The Woodstock Echo, already referred to, and mentioned by several writers, repeats seventeen syllables by day, and twenty by night. In Shipley there is even a greater repetition. Of course the echo is fainter, because the waves are weaker if the reflecting surface be flat. But, as in the case of the mirrors reflecting light, a circular or concave surface will increase the intensity. A watch placed in one mirror will be heard ticking in the other focus. Whispering galleries carry sound by means of the curved surface. Sir John Herschel mentions an echo in the Menai Suspension Bridge. The blow of a hammer on one of the main piers will produce the sound from each of the crossbeams supporting the roadway, and from the opposite pier 576 feet distant, as well as many other repetitions.

Refraction of sound is caused by a wave of sound meeting another medium of different density, just as a beam of light is refracted from water. One sound wave imparts its motion to the new medium, and the new wave travels in a different direction. This change is refraction. The sound waves are refracted in different directions, according to the velocity it can acquire in the medium. If a sound pass from water into air it will be bent towards the perpendicular, because sound can travel faster in water than in air. If it pass from air into water its force will cause it to assume a less perpendicular direction, there being greater velocity in water. The velocity in air is only 1,100 feet in a second in our atmosphere. In water sound travels 4,700 feet in the same time. When the wave of sound falls upon a medium parallel to the refracting surface there is, however, no refraction—only a change of velocity, not direction.

When sound waves are prevented from dispersing the voice can be carried a great distance. Speaking tubes and trumpets, as well as ear trumpets, are examples of this principle, and of the reflection of sound.

There are many very interesting experiments in connection with Acoustics, some of which we will now impart to our readers. We shall then find many ingenious inventions to examine,—the Audiphone, Telephone, Megaphone, and Phonograph, which will occupy a separate chapter. We now resume.

Amongst the experiments usually included in the course of professors and lecturers who have a complete apparatus at their command, and which at first appear very complicated and difficult, there are some which can be performed with every-day articles at hand. There is no experiment in acoustics more interesting than that of M. Lissajons, which consists, as is well known to our scientists, of projecting upon a table or other surface, with the aid of oxy-hydrogen light, the vibratory curves traced by one of the prongs of a tuning-fork. We can perform without difficulty a very similar experiment with the humble assistance of the common knitting-needle.

Fix the flexible steel needle firmly in a cork, which will give it sufficient support; fasten then at the upper extremity a small ball of sealing wax, or a piece of paper about the size of a large pea. If the cork in which the needle is fixed be held firmly in one hand, and you cause the needle to vibrate by striking it, and then letting it sway of itself, or with a pretty strong blow with a piece of wood, you will perceive the little pellet of wax or paper describe an ellipse more or less elongated, or even a circle will be described if the vibrations be frequent. The effect is much enhanced if the experiment be performed beneath a lamp, so that plenty of light may fall upon the vibrating needle. In this case, the persistence of impressions upon the retina admits of one seeing the vibrating circle in successive positions, and we may almost fancy when the needle is struck with sufficient force, that an elongated conical glass, like the old form of champagne glass, is rising from the cork, as shown in the illustration annexed (fig. 173).

Fig. 173.—Experiment showing vibration of sound waves.

Acoustics may be studied in the same way as other branches of physical science. We will describe an interesting experiment, which gives a very good idea of the transmission of sounds through solid bodies. A silver spoon is fastened to a thread, the ends of which are thrust into both ears, as shown in fig. 174; we then slightly swing the spoon until we make it touch the edge of the table; the transmission of sound is in consequence so intense that we are ready to believe we are listening to the double diapason of an organ. This experiment explains perfectly the transmission of spoken words by means of the string of a telephone, another contrivance which any one may make for himself without any trouble whatever. Two round pieces of cardboard are fitted to two cylinders of tin-plate, as large round as a lamp-glass, and four-and-a-half inches in length. If the two rounds of cardboard are connected by a long string of sixteen to eighteen yards, we can transmit sounds from one end to the other of this long cord; the speaker pronouncing the words into the first cylinder, and the listener placing his ear against the other. It is easy to demonstrate that sound takes a certain time to pass from one point to another. When one sees in the distance a carpenter driving in a stake, we find that the sound produced by the blow of the hammer against the wood only reaches the ear a few seconds after the contact of the two objects. We see the flash at the firing of a gun, before hearing the sound of the report—of course on the condition that we are at a fairly considerable distance, as already remarked upon.

Fig. 174.—Conductibility of sound by solid bodies.

Fig. 175.—Musical glasses.

We can show the production of the Gamut by cutting little pieces of wood of different sizes, which one throws on to a table; the sounds produced vary according to the size of the different pieces. The same effect may be obtained much better by means of goblets more or less filled with water; they are struck with a short rod, and emit a sound which can be modified by pouring in a greater or less quantity of water; if the performer is gifted with a musical ear, he can obtain, by a little arrangement, a perfect Gamut by means of seven glasses which each give a note (fig. 175). A piece of music may be fairly rendered in this manner, for the musical glasses frequently produce a very pure silvery sound. We will complete the elementary principles of acoustics by describing a very curious apparatus invented by M. Tisley, the Harmonograph. This instrument, which we can easily describe, is a most interesting object of study. The Harmonograph belongs to mechanics in principle, and to the science of acoustics in application. We will first examine the apparatus itself. It is composed of two pendulums, A and B (fig. 176), fixed to suspensions. Pendulum B supports a circular plate, P, on which we may place a small sheet of paper, as shown in the illustration. This paper is fixed by means of small brass clips. Pendulum A supports a horizontal bar, at the extremity of which is a glass tube, T, terminating at its lower extremity with a capillary opening; this tube is filled with aniline ink, and just rests on the sheet of paper; the support and the tube are balanced by a counterpoise on the right. The two pendulums, A and B, are weighted with round pieces of lead, which can be moved at pleasure, so that various oscillations may be obtained. The ratio between the oscillations of the two pendulums may be exactly regulated by means of pendulum A carrying a small additional weight, the height of which may be regulated by means of a screw and a small windlass. If we give to pendulum A a slight movement of oscillation, the point of tube T traces a straight line on the paper placed in P; but if we move pendulum B, the paper also is displaced, and the point of tube T will trace curves, the shape of which varies with the nature of the movement of pendulum B, the relation between the oscillations of the two pendulums, etc. If the pendulums oscillate without any friction the curve will be clear, and the point will pass indefinitely over the same track, but when the oscillations diminish, the curve also diminishes in size, still preserving its form, and tending to a point corresponding with the position of repose of the two pendulums. The result is therefore that the curves traced by the apparatus, of which we produce three specimens (figs. 177, 178, 179), are traced in a continuous stroke, commencing with the part of the greatest amplitude.

Fig. 176.—M. Tisley’s Harmonograph.

By changing the relation and phases of the oscillations we obtain curves of infinitely varied aspect. M. Tisley has a collection of more than three thousand curves, which we have had occasion to glance over, in which we failed to meet with two corresponding figures. The ratio between these curves corresponds with some special class, of which the analyst may define the general characters, but which is outside our present subject. By giving the plate P a rotatory movement, we obtain spiral curves of a very curious effect, but the apparatus is more complicated. Considered from this point of view it constitutes an interesting mechanical apparatus, showing the combination of oscillations, and resolving certain questions of pure mechanics. From the point of view of acoustics it constitutes a less curious object of study. The experiments of M. Lissajons have proved that the vibrations of diapasons are oscillations similar to, though much more rapid than those of the pendulum. We can therefore with this apparatus reproduce all the experiments of M. Lissajons, with this difference, that the movements being slower are easier to study. When the ratio between the number of vibrations—we purposely use the term vibration instead of the term oscillation—is a whole number, we obtain figs. 177 and 178. If the ratio is not exact, we obtain fig. 179, which is rather irregular in appearance, corresponding to the distortions noticeable in M. Lissajon’s experiments. Fig. 178 has been traced in the exact ratio 2:3; fig. 177 in the ratio 1:2; and fig. 179 corresponds to the ratio 1:2 and a small fraction, which causes the irregularity of the figure.

Fig. 177.—Ratio 1:2. Fig. 178.—Ratio 2:3.

Fig. 179.—Ratio 1:2 and a fraction.

Fig. 180. Construction of the Harmonograph. Fig. 181.

Fig. 182.—Method of constructing an Harmonograph.

Fig. 183.—The apparatus completed.

In considering the harmony of figs. 177 and 178,—the first of which corresponds to the octave, the second to the fifth, whilst fig. 179 corresponds to the disagreeable interval of the ninth,—one is almost tempted to put a certain faith in the fundamental law of simple ratios as the basis of harmony. At first sight this appears beyond doubt, but perhaps musicians would be hardly content with the explanation. M. Tisley’s Harmonograph, it will be seen, is a rather complicated apparatus; and I will now explain how it may be constructed by means of a few pieces of wood. I endeavoured to construct as simple an apparatus as possible, and with the commonest materials, feeling that it is the best means of showing how it is possible for everybody to reproduce these charming curves of musical intervals. Also I completely excluded the employment of metals, and I constructed my apparatus entirely with pieces of wooden rulers, and old cigar boxes. I set to work in the following manner: on the two consecutive sides of a drawing board I fixed four small pieces of wood (fig. 180), side by side in twos, having at the end a small piece of tin-plate forming a groove (fig. 181). In these grooves nails are placed which support the pendulums. The piece of wood is placed on the corner of the table, so that the pendulums which oscillate in two planes at right angles, are in two planes that are sensibly parallel to the sides of the table. The pendulums are made of a thin lath, with two small pieces of wood fixed to them containing some very pointed nails, on which the pendulum oscillates. Fig. 182 gives an illustration. The pendulums have a pin fixed in vertically, which passes through a piece of wood, and by means of a hinge connects the upper ends of the two pendulums. This contrivance of the pin is very useful, and if care is taken to make the hole through the hinge in the form of a double cone, as shown in fig. 182, c, it makes a perfect joint, which allows the piece of wood to be freely moved.

Fig. 184.—Details of mechanism.

To complete the apparatus, the heads of the two pendulums are united by the hinge, at the bend of which a slender glass tube is fixed, which traces the curves. The hinge is given in fig. 184, and to its two extremities are adjusted the two pins of the pendulum (fig. 183). The pendulums are encircled with round pieces of lead, which can be fixed at any height by means of a screw.

CHAPTER XVI.
ACOUSTICS (Continued).

THE TOPOPHONE—THE MEGAPHONE—THE AUTOPHONE—THE AUDIPHONE—THE TELEPHONE—THE PHONOGRAPH—THE MICROPHONE.

We propose in this chapter to give as shortly as possible a description of the various instruments lately come into use, by means of which, and electricity, sounds can be carried from place to place, and their locality identified. It is only within the last few years that these wonderful inventions have come into use, and in a measure superseded the at one time invincible electric telegraph. The Telephone is now in daily use in London and other places, and its novelty, if not all its capability, has been discounted. The Phonograph has also been frequently seen. So we will on this occasion commence with the Topophone, a rather novel instrument.

As the name indicates, the Topophone is an apparatus for discovering the position of a sound, from the Greek words signifying a “place” and “sound.” The sources of sound can be found by it, and indeed this is its actual and practical use. It is claimed for this new apparatus that it stands in the same relation to the sailor as his old and trusty friends, the compass and sextant. These in navigation inform the steersman as to his course, and tells him his position by observation. The Topophone will tell him whence a sound arises, its origin wherever it may be; and this in a fog is no mean advantage. Suppose a ship to be approaching a dangerous coast in a fog. We are all aware how deceptive sounds are when heard through such a medium. We cannot tell from what precise direction the horn, whistle, or bell is sounding. The Topophone will give us the exact spot, and we can then, from our general knowledge of the locality, work our vessel up the river, or into the harbour, in safety.

The Topophone was invented in 1880, by Professor Alfred Mayer, an American, and is based upon the well-known theory of sound waves. These, as we have already explained, exist in the air; and if the theory of sound waves has perfected the Topophone, we can fairly say that it has confirmed the supposed form of the sound waves. “Sound,” says the inventor of the apparatus, “is supposed to be a particle continually expanding in the air, composed of a wave produced by compression, and followed by rarefaction. A continuous sound is a series of these particles or globules spreading and expanding as the water-rings in a pond.” This much will be at once perceived.

Now, suppose a person up to his shoulders in a pond of water, and someone throws a stone into it. If that person extend his arms and hands at right angles facing the sound, each hand would touch the edge of a ripple as it came towards him across the pond. He would then be facing the source of the ripples or waves, and look along a radius of the circle formed by the waves. But if he please, he can move his body so that both hands shall touch the same wave at the same time, or he might turn away from the source, and only one hand would touch the wave. But when both hands are actually washed by the same circular ripple he must be facing the source of it. Any position in which his fingers did not touch the ripple almost at the same instant, would not be facing the source of the wave ripples. So by turning and extending his hands, he could with his eyes shut find out whether he was or was not facing the original source of the waves.

This applied to sound waves in the air is the whole theory of the Topophone, which, however, depends for its usefulness upon the same note being sounded by all horns and whistles. One note must be better than all the others, and that note, probably C (treble), caused by about two hundred and sixty vibrations per second, has been found most applicable. If all whistles and horns can by law be compelled to adjust themselves to this note, then the Topophone will be a real and lasting benefit.

Let us now look at the apparatus itself.

It being conceded that the resonators are in the same key as the Foghorn,—and this is necessary,—they are placed upon the deck of the vessel. An ear-tube of indiarubber is carried from each of these “resonators” into the cabin. These tubes unite and again separate, ending in small pieces ready to be fitted to the ears. The apparatus is fixed on deck, and the arrangement which supports it passes into the cabin, and can be turned about in any direction. Of course in this case a dial point is necessary to indicate the direction in which the instrument is turned. If the machine be worn on the shoulders of the officer of the watch he can move as he pleases, and wants no indicator.

The Topophone when used is so constructed, that when a horn is heard, and when the listener is facing the sound, he can hear nothing! When not facing the origin of the sound he can hear the horn very well, but the moment the resonators receive the sound together as they face the source, a very low murmur is heard, or perhaps no sound at all.—Why?

A certain pitch of tone is composed of vibrations or waves of equal length. In all waves there is a hollow and a crest. One neutralizes the other. The hollow of a sound wave meeting the crest of another wave “interferes” to produce silence, stillness, a dead level. So in “light”; two rays will produce darkness. We will endeavour to explain this.

If we have two equal strings, each performing an equal number of vibrations in a second, they will produce equal sound waves, and the sound produced by both together will be uninterrupted, and twice as loud as one of them. But if one string vibrate, say one hundred times, and the other one hundred and one times in a second, they will not be in unison, and one will gain upon the other string, till after it has got to fifty vibrations it will be half a vibration ahead. At that moment they will neutralize each other, and silence will ensue for an appreciable time.

Fig. 185.—The Megaphone.

In the case of light suppose a red ray strikes the eye, and another red ray to come upon it from somewhere else. If the difference between its distance and the other point from the spot in the retina on which the first ray fell, is the 258/1000 part of an inch, or exactly twice, thrice, four times as much, etc., that distance, the light will be seen twice as strong. But if the difference in the distances between the points whence the light comes be only one-half the 258/1000 part of an inch, or 1½, 2½, 3½, or 4½ times that distance, one light will extinguish the other, and darkness will be the result. Now this is precisely what happens in the case of the Topophone. To return to our simile of water waves. If two stones be cast into a pond, and two equal and similar waves produced, and if they reach a certain place at the same moment, they will make one large wave. But if one followed the other a little, so that the hollow of one coincided with the crest of the other, and vice versâ, the waves would obliterate each other, and a dead level would result. One tube of the Topophone is half a wave length longer than the other, and when the resonators are in a line and receive the wave at the same time, one ear hears the elevation of the sound wave, and the other the depression,—the sound is neutralized, and comparative, if not actual, silence results. The sailor knows in what direction the land lies, and can calculate his distance, or anchor if he please.

If amongst our readers there be any who wish to make for themselves an acoustic signalling apparatus there is physically nothing to prevent them from constructing such an instrument as that shown in the annexed woodcut (fig. 185). It is founded upon the speaking-trumpet principle, which is supposed to have been originated by Samuel Markland, in 1670.

Kircher, in his “Ars magna et umbra,” and in his “Phonurgia,” mentions a kind of speaking-trumpet, or porte voix, of gigantic dimensions, and called the “Horn of Alexander.” According to Kircher, the instrument was used by Alexander the Great to summon his soldiers from a distance of ten miles. The diameter of the circumference was about eight feet, and Kircher conjectured that the instrument was mounted upon three supports.

During the last century, a German professor, named Huth, made a model of the horn, and found it answered every purpose of a speaking-trumpet with most powerful results, but we beg leave to doubt whether the instrument really carried the voice to any very great distance.

The Acoustic Cornet, which is the counterpart of the speaking-trumpet, has been made in many different forms during the last two centuries, but none of them to the present time consist of anything more intricate than a simple tube with a mouthpiece and bell-shaped orifice.

Professor Edison, however, in his researches regarding the conveyance of sounds, has made numerous and interesting experiments. On one occasion, with his Megaphone he carried on a conversation at a distance of nearly two miles, without any other assistance from instruments except a few little cornets of cardboard. These constitute the Megaphone, which may be regarded as a curiosity, considering the effects produced by such simple means. The illustration (fig. 185) represents the instrument which is (or was lately) fixed upon the balcony of Mr. Edison’s house. At a mile-and-a-half distant from the house, at a spot indicated by the two birds in the picture, another instrument was fixed, and conversation was carried on with ease.

Perhaps the present opportunity will be the most convenient to speak of the Autophone, although it is more a musical than an acoustic instrument. Until lately Barbary organs and piano organs have been the only means by which poor people have been able to hear any music, and that not of a very elevated class. Besides, there is a good deal of expense connected with the possession of an organ. But the Americans, with a view to popularize music, have invented the Autophone, which is simply a mechanical accordeon, manufactured by the Autophone Company, of Ithaca, New York.

The principle of the instrument is represented in fig. 186, and is extremely simple. An upright frame carries within it on one side a bellows, and on the other a flexible air chamber, which serves as a reservoir.

The upper portion contains a set of stops like an accordeon, but the escape of the air through the small vibrating plates can only take place by the upper surface of the frame work, upon which slides a thin plate of Bristol board pierced with holes at convenient distances, and set in motion by the mechanism shown in the annexed diagram (fig. 187).

Fig. 186.—The Autophone.

The figure represents an axle furnished with a series of “washers,” which, acting upon the plate, cause it to move round. It is the bellows movement that turns the axle by the aid of two “catches,” B and C, which work upon a toothed wheel fixed upon it.

The “catch” B moves the paper on which the tune is “perforated,” when the bellows is empty, the other catch when it is distended; but a counter catch, D, represented by the dotted lines in the illustration, is so arranged that the paper cannot pass on except the tooth of the catch D is opposite a hole pierced upon the plate above. In the contrary case there is no movement of the paper during the dilatation of the bellows. The effect of this very ingenious arrangement is to give to the “musical” band of “board” an irregular movement, but it economises it in the case of sustained notes. The whole action of the instrument depends upon the correct working of the bellows.

The effect, from an artistic point of view, certainly leaves something to be desired, but the instrument is cheap, and not cumbersome, and the slips of paper upon which the music is “cut out” can be made by machinery, and consequently are not dear. So far, the Autophone is fitted for popular favour and use, and may supersede the barrel organ.

Fig. 187.—Detail of the Autophone.

The Audiphone is an instrument to conduct sound to the ear, to supplement it when temporary or partial deafness has occurred. Very likely many of our readers have observed ladies carrying large black fans on occasions,—at church, or lecture, or theatre,—and wondered why, perhaps. Those “fans” are Audiphones. The instrument is made of vulcanized rubber, and consists of a long flexible disc supported by a handle. To the upper edge of the “fan” are attached cords, which pass through a clip on the handle. If the person who wishes to hear by means of the Audiphone will hold the fan against the upper teeth,—the convex side of the fan outward,—he or she will hear distinctly, for the vibrations of sound are collected and strike upon the teeth and bones, and act upon the auditory nerves from within, precisely as the vibrations act from without through the auricle. We need hardly add that if the ear be injured the Audiphone will be of no use. A writer says: “From personal observation with the Audiphone it appears to convey the sonorous vibrations to the ear through the teeth, just as a long wooden rod held in the teeth will convey the vibrations of the sounding-board of a piano, though the piano is in another room and out of hearing by the ear. In using the Audiphone during conversation there is no movement or vibration felt by the teeth; in listening to a piano there is a very faint sensation as if the Audiphone vibrated slightly, while with the handle of the Audiphone resting on the sounding-board of the piano the vibrations are so violent as to be painful to the teeth. By closing the ears a person with even acute hearing can observe the admirable manner in which the instrument conveys spoken words to the ear. The Audiphone will prove to be of great value to deaf mutes, as it enables them to hear their own voices, and thus to train them to express words, while, before, they could only make inarticulate sounds.”

We have a variation of this instrument which has been introduced employing a diaphragm held in a telephone mouthpiece, and free to vibrate under the influence of sounds. This is connected by a string to a bit of wood that may be held in the teeth. In use the hearer places the wood between his teeth, the string is drawn tight, and the speaker speaks through the telephone mouthpiece, the vibrations of the diaphragm being then conveyed to the teeth through the stretched string. This apparatus works very successfully, and ladies use it, but it is not so convenient for general use as the Audiphone.

Fig. 188.—The Telephone.

The Telephone is now in daily use in London, and is by no means strange to the majority of our countrymen, still some description of it will probably be acceptable, and a glance at its history may prove interesting.

Fig. 189.—The “receiving” apparatus.

In speaking of the Telephone, we must not lose sight of the facts before mentioned, as regards our sense of hearing, and the manner in which the ear acts by the series of bones termed the hammer, the anvil, and stirrup. In the process of reproduction of tone in the magnetic instruments, the mechanism of the human ear was, to a certain extent, imitated, and a diaphragm, by vibrations, generates and controls an electric current.

Professor Wheatstone was the first person to employ the electric wire for the transmission of sounds, but Professor Philip Reiss, of Friedrichsdorf, was the first to make the experiment of producing musical sounds at a distance. His first instrument was of a most primitive nature; subsequently he produced an instrument of which fig. 188 is the Telephone, fig. 189 the “receiver.”

In fig. 188, it will be seen that there is an aperture on the top and one at the side; the latter is the mouthpiece. The top aperture is covered with a membrane which is stretched very tightly. When a person speaks or sings into the mouthpiece his voice is at once concentrated upon the tight membrane, which it causes to vibrate in a manner corresponding with the vibrations of the voice. There are two binding screws, one at each side. To the centre of the tight membrane a piece of platinum is fixed, and this is connected with the binding screw on one side, in which a wire from the battery is fixed. On the membrane is a tripod, the feet of which (two) rest in metal cups, one of them being in a mercury cup connected with the binding screw at the opposite side to that already mentioned. The third “foot”—a platinum point—is on the platinum in the centre of the membrane or top, and moves with it. Every time the membrane is stretched by a vibration the platinum point is touched, and the closed circuit is broken by the return of each vibration.

Fig. 190.—Bell’s first Telephone (Transmitter).
a. Electro-magnet. b. Diaphragm. c. Collar. d. Collar and tube. f. Screw. g. Mouthpiece. h. Battery. i. Wire from battery to coil. k. Telegraph wire. l. Through binding screw. m. Pillar holding magnet.

The receiving instrument (fig. 189) consists of a coil enclosing an iron rod, and fixed upon a hollow sounding box. It is founded upon a fact discovered by Professor Henry, that iron bars when magnetized by an electric current become a little longer, and at the interruption of the current resume their former length. Thus in the receiver the iron will become alternately longer and shorter in accordance with the vibrations of the membrane in the box far away, and so the longitudinal vibrations of the bar of iron will be communicated to the sounding box, and become perfectly audible. This instrument, however, could only produce the “pitch” of sound, “not different degrees of intensity, or other qualities of tones.” It merely sang with its own little trumpet whatever was sung into it; for all the waves were produced by an electric current of a certain and uniform strength, and therefore the sound waves were of the same size.

But in 1874, Mr. Elisha Gray, of Chicago, improved Reiss’ instrument, and discovered a method by which the intensity or loudness of tones, as well as their “pitch,” were transmitted and reproduced. In this method he employed electrical vibrations of varying strength and rapidity, and so was enabled to reproduce a tune. Subsequently he conceived the notion of controlling the vibrations by means of a diaphragm, which responded to every known sound, and by this he managed to transmit speech in an articulate manner.

Fig. 191.—Bell’s Telephone (Receiver).

In 1876, Professor Graham Bell sent a Telephone to the Centennial Exhibition at Philadelphia. Mr. Bell, according to the report, managed to produce a variation of strength of current in exact proportion to the particle of air moved by the sound. A piece of iron attached to a membrane, and moved to and fro in proximity to an electro magnet, proved successful. The battery and wire of the electro magnet are in circuit with the telegraph wire, and the wire of another electro magnet at the receiving station. This second magnet has a solid bar of iron for core, which is connected at one end, by a thick disc of iron, to an iron tube surrounding the coil and bar. The free circular end of the tube constitutes one pole of the electro magnet, and the adjacent free end of the bar core the other. A thin circular iron disc held pressed against the end of the tube by the electro-magnetic attraction, and free to vibrate through a very small space without touching the central pole, constitutes the sounder by which the electric effect is reconverted into sound. The accompanying illustrations (figs. 190, 191) show Mr. Bell’s Telephone as described.

The Telephone, subsequently simplified by Professor Bell, is shown in the two following illustrations (figs. 192, 193). The voice strikes against the diaphragm, and it begins to vibrate. The sound is not conveyed by the wire; the motion is communicated, and the vibrations become sound waves again. The Telephone consists of a cylindrical magnet encircled at one end by a bobbin, on which is wound a quantity of fine insulated copper wire. The magnet and coil are contained in a wooden case, the ends of the coil being soldered to thick copper wire, which traverse the “wooden envelope,” and terminate in the binding screws. In front of the magnet is a thin circular iron plate, in which is the mouthpiece. The drawings will explain the instrument.

Fig. 192.—External appearance of Bell Telephone.

Fig. 193.—a. Bobbin of coil wire round magnet. b. Diaphragm. c. Mouthpiece. d. Permanent magnet. e. Wires to binding screws. f. Both wires as one for convenience. g. Adjusting screw-holding magnet.

Mr. Edison also invented a Telephone like Gray’s, and made the discovery, that when properly prepared, carbon would change its resistance with pressure, and that the ratio of these changes corresponded with the pressure. This solved the problem of the production of speech. The carbon is placed between two plates of platinum connected in the circuit and near the diaphragm, and the carbon receives the pressure from it by means of the mouthpiece.

When we come to Magnetism and Electricity we may have something more to say respecting the mysteries of the Telephone and its later developments. At present we are only concerned with it as a sound conveyer, and it answers its purpose admirably, although somewhat liable to attract other sounds or vibrations from neighbouring wires.

The Phonograph, a mechanical invention of Mr. Edison, does not make use of electricity, although the vibratory motion of the diaphragm is utilized. It, in a simple form, consists of a diaphragm so arranged as to operate upon a small stylus, placed just opposite and below the centre, and a brass cylinder, six or eight inches long, by three or four in diameter, mounted upon a horizontal axis, extending each way beyond its ends for a distance about its own length.

“A spiral groove is cut in the circumference of the cylinder, from one end to the other, each spiral of the groove being separated from its neighbour by about one-tenth of an inch. The shaft or axis is also cut by a screw thread corresponding to the spiral groove of the cylinder, and works in screw bearings; consequently when the cylinder is caused to revolve, by means of a crank that is fitted to the axis for this purpose, it receives a forward or backward movement of about one-tenth of an inch for every turn of the same, the direction, of course, depending upon the way the crank is turned. The diaphragm is firmly supported by an upright casting capable of adjustment, and so arranged that it may be removed altogether when necessary. When in use, however, it is clamped in a fixed position above or in front of the cylinder, thus bringing the stylus always opposite the groove as the cylinder is turned. A small, flat spring attached to the casting extends underneath the diaphragm as far as its centre and carries the stylus, and between the diaphragm and spring a small piece of india-rubber is placed to modify the action, it having been found that better results are obtained by this means than when the stylus is rigidly attached to the diaphragm itself.

Fig. 194.—Mode of using the Telephone.

“The action of the apparatus will now be readily understood from what follows. The cylinder is first very smoothly covered with tin-foil, and the diaphragm securely fastened in place by clamping its support to the base of the instrument. When this has been properly done, the stylus should lightly press against that part of the foil over the groove. The crank is now turned, while, at the same time, someone speaks into the mouthpiece of the instrument, which will cause the diaphragm to vibrate, and as the vibrations of the latter correspond with the movements of the air producing them, the soft and yielding foil will become marked along the line of the groove by a series of indentations of different depths, varying with the amplitude of the vibrations of the diaphragm; or in other words, with the inflections or modulations of the speaker’s voice. These inflections may therefore be looked upon as a sort of visible speech, which, in fact, they really are. If now the diaphragm is removed, by loosening the clamp, and the cylinder then turned back to the starting point, we have only to replace the diaphragm and turn in the same direction as at first, to hear repeated all that has been spoken into the mouthpiece of the apparatus; the stylus, by this means, being caused to traverse its former path, and consequently, rising and falling with the depressions in the foil, its motion is communicated to the diaphragm, and thence through the intervening air to the ear, where the sensation of sound is produced.

Fig. 195.—Bell’s long-distance Telephone
a. Compound magnet. d. Diaphragm. e. Speaking tube. f. Telegraph wire. g. Line to earth. b, c. Small spaces.

“As the faithful reproduction of a sound is in reality nothing more than a reproduction of similar acoustic vibrations in a given time, it at once becomes evident that the cylinder should be made to revolve with absolute uniformity at all times, otherwise a difference more or less marked between the original sound and the reproduction will become manifest. To secure this uniformity of motion, and produce a practically working machine for automatically recording speeches, vocal and instrumental music, and perfectly reproducing the same, the inventor devised an apparatus in which a plate replaces the cylinder. This plate, which is ten inches in diameter, has a volute spiral groove cut in its surface on both sides from its centre to within one inch of its outer edge; an arm guided by the spiral upon the under side of the plate carries a diaphragm and mouthpiece at its extreme end. If the arm be placed near the centre of the plate and the latter rotated, the motion will cause the arm to follow the spiral outward to the edge. A spring and train of wheel-work regulated by a friction governor serves to give uniform motion to the plate. The sheet upon which the record is made is of tin-foil. This is fastened to a paper frame, made by cutting a nine-inch disc from a square piece of paper of the same dimensions as the plate. Four pins upon the plate pass through corresponding eyelet-holes punched in the four corners of the paper, when the latter is laid upon it, and thus secure accurate registration, while a clamping-frame hinged to the plate fastens the foil and its paper frame securely to the latter. The mechanism is so arranged that the plate may be started and stopped instantly, or its motion reversed at will, thus giving the greatest convenience to both speaker and copyist.

“The articulation and quality of the Phonograph, although not yet perfect, is full as good as the Telephone was. The instrument, when perfected and moved by clock-work, will undoubtedly reproduce every condition of the human voice, including the whole world of expression in speech and song, and will be used universally.

“The sheet of tin-foil or other plastic material receiving the impressions of sound, will be stereotyped or electrotyped so as to be multiplied and made durable; or the cylinder will be made of a material plastic when used, and hardening afterward. Thin sheets of papier maché, or of various substances which soften by heat, would be of this character. Having provided thus for the durability of the Phonograph plate, it will be very easy to make it separable from the cylinder producing it, and attachable to a corresponding cylinder anywhere and at any time. There will doubtless be a standard of diameter and pitch of screw for Phonograph cylinders. Friends at a distance will then send to each other Phonograph letters, which will talk at any time in the friend’s voice when put upon the instrument.” (Scribner.)

The Microphone (an outcome of the Telephone) was discovered by Professor Hughes, of London. It is an instrument which in its main features consists of a carbon “pencil,” so suspended that one end rests upon a carbon “die.” The instrument being connected with a Telephone by the circuit wires, will reproduce faint sounds very distinctly. Once a Microphone was put into a preacher’s pulpit, and joined to a private telegraph wire which led to a gentleman’s house. The owner was thus enabled to hear the sermon. So long as it is thus connected every minute sound, even a fly’s footstep, will be faithfully reproduced.