The introduction of the ribbon film in or about 1888 opened much greater possibilities to the living picture than would ever have existed had the glass plate been retained. It was now comparatively easy to take a long series of pictures; and accordingly we find Messrs. Friese-Greene and Evans exhibiting in 1890 a camera capable of securing three hundred exposures in half a minute, or ten per second.

The next apparatus to be specially mentioned is Edison’s Kinetoscope, which he first exhibited in England in 1894. As early as 1887 Mr. Edison had tried to produce animated pictures in a manner analogous to the making of a sound-record on a phonograph (see p. 56). He wrapped round a cylinder a sheet of sensitized celluloid which was covered, after numerous exposures, by a spiral line of tiny negatives. The positives made from these were illuminated in turn by flashes of electric light. This method was, however, entirely abandoned in the perfected kinetoscope, an instrument for viewing pictures the size of a postage stamp, carried on a continuously moving celluloid film between the eye of the observer and a small electric lamp. The pictures passed the point of inspection at the rate of forty-six per second (a rate hitherto never approached), and as each picture was properly centred a slit in a rapidly revolving shutter made it visible for a very small fraction of a second. Holes punched at regular intervals along each side of the film engaged with studs on a wheel, and insured a regular motion of the pictures. This principle of a perforated film has been used by nearly all subsequent manufacturers of animatographs.

To secure forty-six negatives per second Edison invented a special exposure device. Each negative would have but one-forty-sixth of a second to itself, and that must include the time during which the fresh surface of film was being brought into position before the lens. He therefore introduced an intermittent gearing, which jerked the film forwards forty-six times per second, but allowed it to remain stationary for nine-tenths of the period allotted to each picture. During the time of movement the lens was covered by the shutter. This principle of exposure has also been largely adopted by other inventors. By its means weak negatives are avoided, while pictures projected on to a screen gain greatly in brilliancy and steadiness.

The capabilities of a long flexible film-band having been shown by Edison, he was not long without imitators. Phantoscopes, Bioscopes, Photoscopes, and many other instruments followed in quick succession. In 1895 Messrs. Lumière scored a great success with their Cinematograph, which they exhibited at Marseilles and Paris; throwing the living picture as we now know it on to a screen for a large company to see. This camera-lantern opens the era of commercial animated-photography. The number of patents taken out since 1895 in connection with living-picture machines is sufficient proof that inventors have either found in this particular branch of photography a peculiar fascination, or have anticipated from it a substantial profit.

A company known as the Mutoscope and Biograph Company has been formed for the sole object of working the manufacture and exhibition of the living picture on a great commercial scale. The present company is American, but there are subsidiary allied companies in many parts of the world, including the British Isles, France, Italy, Belgium, Germany, Austria, India, Australia, South Africa. The part that the company has played in the development of animated photography will be easily understood from the short account that follows.

The company controls three machines, the Mutograph, or camera for making negatives; the Biograph, or lantern for throwing pictures on to the screen; and the Mutoscope, a familiar apparatus in which the same pictures may be seen in a different fashion on the payment of a penny.

Externally the Mutograph is remarkable for its size, which makes it a giant of its kind. The complete apparatus weighs, with its accumulators, several hundreds of pounds. It takes a very large picture, as animatograph pictures go—two by two-and-a-half inches, which, besides giving increased detail, require less severe magnification than is usual with other films. The camera can make up to a hundred exposures per second, in which time twenty-two feet of film will have passed before the lens.

The film is so heavy that were it arrested bodily during each exposure and then jerked forward again, it might be injured. The mechanism of the mutograph, driven at regular speed, by an electric motor, has been so arranged as to halt only that part of the film which is being exposed, the rest moving forward continuously. The exposed portion, together with the next surface, which has accumulated in a loop behind it, is dragged on by two rollers that are in contact with the film during part only of their revolutions. Thus the jerky motion is confined to but a few inches of the film, and even at the highest speeds the camera is peculiarly free from vibration.

An exposed mutograph film is wound for development round a skeleton reel, three feet in diameter and seven long, which rotates in a shallow trough containing the developing solution. Development complete, the reel is lifted from its supports and suspended over a succession of other troughs for washing, fixing, and final washing. When dry the negative film is passed through a special printing frame in contact with another film, which receives the positive image for the biograph. The difficulty of handling such films will be appreciated to a certain extent even by those whose experience is confined to the snaky behaviour of a short Kodak reel during development.

The Mutoscope Company’s organisation is as perfect as its machinery. It has representatives in all parts of the world. Wherever stirring events are taking place, whether in peace or war, a mutograph operator will soon be on the spot with his heavy apparatus to secure pictures for world-wide exhibition. It need hardly be said that great obstacles, human and physical, have often to be overcome before a film can be exposed; and considerable personal danger encountered. We read that an operator, despatched to Cuba during the Spanish-American War was left three days and nights without food or water to guard his precious instruments, the party that had landed him having suddenly put to sea on sighting a Spanish cruiser. Another is reported to have had a narrow escape from being captured at sea by the Spaniards after a hot chase. It is also on record that a mutograph set up in Atlantic City to take a procession of fire-engines was charged and shattered by one of the engines; that the operators were flung into the crowd: and that nevertheless the box containing the exposed films was uninjured, and on development yielded a very sensational series of pictures lasting to the moment of collision.

The Mutoscope Company owns several thousand series of views, none probably more valuable than those of his Holiness the Pope, who graciously gave Mr. W. K. Dickson five special sittings, during which no less than 17,000 negatives were made, each one of great interest to millions of people throughout the world.

The company spares neither time nor money in its endeavour to supply the public with what will prove acceptable. A year’s output runs into a couple of hundred miles of film. As much as 700 feet is sometimes expended on a single series, which may be worth anything up to £1000.

The energy displayed by the operators is often marvellous. To take instances. The Derby of 1898 was run at 3.20 p.m. At ten o’clock the race was run again by Biograph on the great sheet at the Palace Theatre. On the home-coming of Lord Kitchener from the Soudan Campaign, a series of photographs was taken at Dover in the afternoon and exhibited the same evening! Or again, to consider a wider sphere of action, the Jubilee Procession of 1897 was watched in New York ten days after the event; two days later in Chicago; and in three more the films were attracting large audiences in San Francisco, 5000 miles from the actual scene of the procession!

One may easily weary of a series of single views passed slowly through a magic-lantern at a lecture or entertainment. But when the Biograph is flashing its records at lightning speed there is no cause for dullness. It is impossible to escape from the fascination of movement. A single photograph gives the impression of mere resemblance to the original; but a series, each reinforcing the signification of the last, breathes life into the dead image, and deludes us into the belief that we see, not the representation of a thing, but the thing itself. The bill of fare provided by the Biograph Company is varied enough to suit the most fastidious taste. Now it is the great Naval Review off Spithead, or President Faure shooting pheasants on his preserves near Paris. A moment’s pause and then the magnificent Falls of Niagara foam across the sheet; Maxim guns fire harmlessly; panoramic scenes taken from locomotives running at high velocity unfold themselves to the delighted spectators, who feel as if they really were speeding over open country, among towering rocks, or plunging into the darkness of a tunnel. Here is an express approaching with all the quiver and fuss of real motion, so faithfully rendered that it seems as if a catastrophe were imminent; when, snap! we are transported a hundred miles to watch it glide into a station. The doors open, passengers step out and shake hands with friends, porters bustle about after luggage, doors are slammed again, the guard waves his flag, and the carriages move slowly out of the picture. Then our attention is switched away to the 10-inch disappearing gun, landing and firing at Sandy Hook. And next, as though to show that nothing is beneath the notice of the biograph, we are perhaps introduced to a family of small pigs feeding from a trough with porcine earnestness and want of manners.

It must not be thought that the Living Picture caters for mere entertainment only. It serves some very practical and useful ends. By its aid the movements of machinery and the human muscles may be studied in detail, to aid a mechanical or medical education. It furnishes art schools with all the poses of a living model. Less serious pursuits, such as dancing, boxing, wrestling and all athletic sports and exercise, will find a use for it. As an advertising medium it stands unrivalled, and we shall owe it a deep debt of gratitude if it ultimately supplants the flaring posters that disfigure our towns and desecrate our landscapes. Not so long since, the directors of the Norddeutscher-Lloyd Steamship Company hired the biograph at the Palace Theatre, London, to demonstrate to anybody who cared to witness a very interesting exhibition that their line of vessels should always be used for a journey between England and America.

The Living Picture has even been impressed into the service of the British Empire to promote emigration to the Colonies. Three years ago Mr. Freer exhibited at the Imperial Institute and in other places in England a series of films representing the 1897 harvest in Manitoba. Would-be emigrants were able to satisfy themselves that the great Canadian plains were fruitful not only on paper. For could they not see with their own eyes the stately procession of automatic “binders” reaping, binding, and delivering sheaves of wheat, and puffing engines threshing out the grain ready for market? A far preferable method this to the bogus descriptions of land companies such as lured poor Chuzzlewit and Mark Tapley into the deadly swamps of “Eden.”

Again, what more calculated to recruit boys for our warships than the fine Polytechnic exhibition known as “Our Navy”? What words, spoken or printed, could have the effect of a series of vivid scenes truthfully rendered, of drills on board ship, the manning and firing of big guns, the limbering-up of smaller guns, the discharge of torpedoes, the headlong rush of the “destroyers”?

The Mutoscope, to which reference has been made above, may be found in most places of public entertainment, in refreshment bars, on piers, in exhibitions, on promenades. A penny dropped into a slot releases a handle, the turning of which brings a series of pictures under inspection. The pictures, enlarged from mutograph films, are mounted in consecutive order round a cylinder, standing out like the leaves of a book. When the cylinder is revolved by means of the handle the picture cards are snapped past the eye, giving an effect similar to the lifelike projections on a biograph screen. From 900 to 1000 pictures are mounted on a cylinder.

The advantages of the mutoscope—its convenient size, its simplicity, and the ease with which its contents may be changed to illustrate the topics and events of the day—have made the animated photograph extremely popular. It does for vision what the phonograph does for sound. In a short time we shall doubtless be provided with handy machines combining the two functions and giving us double value for our penny.

The real importance and value of animated photography will be more easily estimated a few years hence than to-day, when it is still more or less of a novelty. The multiplication of illustrated newspapers and magazines points to a general desire for pictorial matter to help down the daily, weekly, or monthly budget of news, even if the illustrations be imaginative products of Fleet Street rather than faithful to fact. The reliable living picture (we expect the “set-scene”) which “holds up a mirror to nature,” will be a companion rather than a rival of journalism, following hard on the description in print of an event that has taken place under the eye of the recording camera. The zest with which we have watched during the last two years biographic views of the embarkation and disembarkation of troops, of the transport of big guns through drifts and difficult country, and of the other circumstances of war, is largely due to the descriptions we have already read of the things that we see on the screen. And, on the other hand, the impression left by a series of animated views will dwell in our memories long after the contents of the newspaper columns have become confused and jumbled. It is therefore especially to be hoped that photographic records will be kept of historic events, such as the Jubilee, the Queen’s Funeral, King Edward’s Coronation, so that future generations may, by the turning of a handle, be brought face to face with the great doings of a bygone age.

THE GREAT PARIS TELESCOPE

A telescope so powerful that it brings the moon apparently to within thirty-five miles of the earth; so long that many a cricketer could not throw a ball from one end of it to the other; so heavy that it would by itself make a respectable load for a goods train; so expensive that astronomically-inclined millionaires might well hesitate to order a similar one for their private use.

Such is the huge Paris telescope that in 1900 delighted thousands of visitors in the French Exposition, where, among the many wonderful sights to be seen on all sides, it probably attracted more notice than any other exhibit. This triumph of scientific engineering and dogged perseverance in the face of great difficulties owes its being to a suggestion made in 1894 to a group of French astronomers by M. Deloncle. He proposed to bring astronomy to the front at the coming Exposition, and to effect this by building a refracting telescope that in size and power should completely eclipse all existing instruments and add a new chapter to the “story of the heavens.”

To the mind unversed in astronomy the telescope appeals by the magnitude of its dimensions, in the same way as do the Forth Bridge, the Eiffel Tower, the Big Wheel, the statue of Liberty near New York harbour, the Pyramids, and most human-made “biggest on records.”

At the time of M. Deloncle’s proposal the largest refracting telescope was the Yerkes’ at William’s Bay, Wisconsin, with an object-glass forty inches in diameter; and next to it the 36-inch Lick instrument on Mount Hamilton, California, built by Messrs. Alvan Clark of Cambridgeport, Massachusetts. Among reflecting telescopes the prior place is still held by Lord Rosse’s, set up on the lawn of Birr Castle half a century ago. Its speculum, or mirror, weighing three tons, lies at the lower end of a tube six feet across and sixty feet long. This huge reflector, being mounted in meridian, moves only in a vertical direction. A refracting telescope is one of the ordinary pocket type, having an object-lens at one end and an eyepiece at the other. A reflector, on the other hand, has no object-lens, its place being taken by a mirror that gathers the rays entering the tube and reflects them back into the eyepiece, which is situated nearer the mouth end of the tube than the mirror itself.

Each system has its peculiar disadvantages. In reflectors the image is more or less distorted by “spherical aberration.” In refractors the image is approximately perfect in shape, but liable to “chromatic aberration,” a phenomenon especially noticeable in cheap telescopes and field-glasses, which often show objects fringed with some of the colours of the spectrum. This defect arises from the different refrangibility of different light rays. Thus, violet rays come to a focus at a shorter distance from the lens than red rays, and when one set is in focus to the eye the other must be out of focus. In carefully-made and expensive instruments compound lenses are used, which by the employment of different kinds of glass bring all the colours to practically the same focus, and so do away with chromatic aberration.

To reduce colour troubles to a minimum M. Deloncle proposed that the object-lens should have a focal distance of about two hundred feet, since a long focus is more easily corrected than a short one, and a diameter of over fifty-nine inches. The need for so huge a lens arises out of the optical principles of a refractor. The rays from an object—a star, for instance—strike the object-glass at the near end, and are bent by it into a converging beam, till they all meet at the focus. Behind the focus they again separate, and are caught by the eyepiece, which reduces them to a parallel beam small enough to enter the pupil. We thus see that though the unaided eye gathers only the few rays that fall directly from the object on to the pupil, when helped by the telescope it receives the concentrated rays falling on the whole area of the object-glass; and it would be sensible of a greatly increased brightness had not this light to be redistributed over the image, which is the object magnified by the eyepiece. Assuming the aperture of the pupil to be one-tenth of an inch, and the object to be magnified a hundred times, the object-lens should have a hundred times the diameter of the pupil to render the image as bright as the object itself. If the lens be five instead of ten inches across, a great loss of light results, as in the high powers of a microscope, and the image loses in distinctness what it gains in size.

As M. Deloncle meant his telescope to beat all records in respect of magnification, he had no choice but to make a lens that should give proportionate illumination, and itself be of unprecedented size.

At first M. Deloncle met with considerable opposition and ridicule. Such a scheme as his was declared to be beyond accomplishment. But in spite of many prophecies of ultimate failure he set to work, entrusting the construction of the various portions of his colossal telescope to well-tried experts. To M. Gautier was given the task of making all the mechanical parts of the apparatus; to M. Mantois the casting of the giant lenses; to M. Despret the casting of the huge mirror, to which reference will be made immediately.

The first difficulty to be encountered arose from the sheer size of the instrument. It was evidently impossible to mount such a leviathan in the ordinary way. A tube, 180 feet long, could not be made rigid enough to move about and yet permit careful observation of the stars. Even supposing that it were satisfactorily mounted on an “equatorial foot” like smaller glasses, how could it be protected from wind and weather? To cover it, a mighty dome, two hundred feet or more in diameter, would be required; a dome exceeding by over seventy feet the cupola of St. Peter’s, Rome; and this dome must revolve easily on its base at a pace of about fifty feet an hour, so that the telescope might follow the motion of the heavenly bodies.

The constructors therefore decided to abandon any idea of making a telescope that could be moved about and pointed in any desired direction. The alternative course open to them was to fix the telescope itself rigidly in position, and to bring the stars within its field by means of a mirror mounted on a massive iron frame—the two together technically called a siderostat. The mirror and its support would be driven by clockwork at the proper sidereal rate. The siderostat principle had been employed as early as the eighteenth century, and perfected in recent years by Léon Foucault, so that in having recourse to it the builders of the telescope were not committing themselves to any untried device.

In days when the handling of masses of iron, and the erection of huge metal constructions have become matters of everyday engineering life, no peculiar difficulty presented itself in connection with the metal-work of the telescope. The greatest possible care was of course observed in every particular. All joints and bearings were adjusted with an extraordinary accuracy; and all the cylindrical moving parts of the siderostat verified till they did not vary from perfect cylindricity by so much as one twenty-five-thousandth of an inch!

The tube of the telescope, 180 feet long, consisted of twenty-four sections, fifty-nine inches in diameter, bolted together and supported on seven massive iron pillars. It weighed twenty-one tons. The siderostat, twenty-seven feet high, and as many in length, weighed forty-five tons. The lower portion, which was fixed firmly on a bed of concrete, had on the top a tank filled with quicksilver, in which the mirror and its frame floated. The quicksilver supported nine-tenths of the weight, the rest being taken by the levers used to move the mirror. Though the total weight of the mirror and frame was thirteen tons, the quicksilver offered so little resistance that a pull of a few pounds sufficed to rotate the entire mass.

The real romance of the construction of this huge telescope centres on the making of the lenses and mirror. First-class lenses for all photographic and optical purposes command a very high price on account of the care and labour that has to be expended on their production; the value of the glass being trifling by comparison. Few, if any, trades require greater mechanical skill than that of lensmaking; the larger the lens the greater the difficulties it presents, first in the casting, then in the grinding, last of all in the polishing. The presence of a single air-bubble in the molten glass, the slightest irregularity of surface in the polishing may utterly destroy the value of a lens otherwise worth several thousands of pounds.

The object-glass of the great telescope was cast by M. Mantois, famous as the manufacturer of large lenses. The glass used was boiled and reboiled many times to get rid of all bubbles. Then it was run into a mould and allowed to cool very gradually. A whole month elapsed before the breaking of a mould, when the lens often proved to be cracked on the surface, owing to the exterior having cooled faster than the interior and parted company with it. At last, however, a perfect cast resulted.

M. Despret undertook the even more formidable task of casting the mirror at his works at Jeumont, North France. A special furnace and oven, capable of containing over fifteen tons of molten glass, had to be constructed. The mirror, 6-1/2 feet in diameter and eleven inches thick, absorbed 3-3/4 tons of liquid glass; and so great was the difficulty of cooling it gradually, that out of the twenty casts eighteen were failures.

The rough lenses and mirror having been ground to approximate correctness in the ordinary way, there arose the question of polishing, which is generally done by one of the most sensitive and perfect instruments existing-the human hand. In this case, owing to the enormous size of the objects to be treated, hand work would not do. The mere hot touch of a workman would raise on the glass a tiny protuberance, which would be worn level with the rest of the surface by the polisher, and on the cooling of the part would leave a depression, only 1-75,000 of an inch deep, perhaps, but sufficient to produce distortion, and require that the lens should be ground down again, and the whole surface polished afresh.

M. Gautier therefore polished by machinery. It proved a very difficult process altogether, on account of frictional heating, the rise of temperature in the polishing room, and the presence of dust. To insure success it was found necessary to warm all the polishing machinery, and to keep it at a fixed temperature.

At the end of almost a year the polishing was finished, after the lenses and mirror had been subjected to the most searching tests, able to detect irregularities not exceeding 1-250,000 of an inch. M. Gautier applied to the mirror M. Foucault’s test, which is worth mentioning. A point of light thrown by the mirror is focused through a telescope. The eyepiece is then moved inwards and outwards so as to throw the point out of focus. If the point becomes a luminous circle surrounded by concentric rings, the surface throwing the light point is perfectly plane or smooth. If, however, a pushing-in shows a vertical flattening of the point, and a pulling-out a horizontal flattening, that part is concave; if the reverse happens, convexity is the cause.

For the removal of the mirror from Jeumont to Paris a special train was engaged, and precautions were taken rivalling those by which travelling Royalty is guarded. The train ran at night without stopping, and at a constant pace, so that the vibration of the glass atoms might not vary. On arriving at Paris, the mirror was transferred to a ponderous waggon, and escorted by a body of men to the Exposition buildings. The huge object-lens received equally careful treatment.

The telescope was housed at the Exhibition in a long gallery pointing due north and south, the siderostat at the north end. At the other, the eyepiece, end, a large amphitheatre accommodated the public assembled to watch the projection of stellar or lunar images on to a screen thirty feet high, while a lecturer explained what was visible from time to time. The images of the sun and moon as they appeared at the primary focus in the eyepiece measured from twenty-one to twenty-two inches in diameter, and the screen projections were magnified from these about thirty times superficially.

The eyepiece section consisted of a short tube, of the same breadth as the main tube, resting on four wheels that travelled along rails. Special gearing moved this truck-like construction backwards and forwards to bring a sharp focus into the eyepiece or on to a photographic plate. Focusing was thus easy enough when once the desired object came in view; but the observer being unable to control the siderostat, 250 feet distant, had to telephone directions to an assistant stationed near the mirror whenever he wished to examine an object not in the field of vision.

By the courtesy of the proprietors of the Strand Magazine we are allowed to quote M. Deloncle’s own words describing his emotions on his first view through the giant telescope:—

“As is invariably the case, whenever an innovation that sets at nought old-established theories is brought forward, the prophecies of failure were many and loud, and I had more than a suspicion that my success would cause less satisfaction to others than to myself. Better than any one else I myself was cognisant of the unpropitious conditions in which my instrument had to work. The proximity of the river, the dust raised by hundreds of thousands of trampling feet, the trepidation of the soil, the working of the machinery, the changes of temperature, the glare from the thousands of electric lamps in close proximity—each of these circumstances, and many others of a more technical nature, which it would be tedious to enumerate, but which were no less important, would have been more than sufficient to make any astronomer despair of success even in observatories where all the surroundings are chosen with the utmost care.

“In regions pure of calm and serene air large new instruments take months, more often years, to regulate properly.

“In spite of everything, however, I still felt confident. Our calculations had been gone over again and again, and I could see nothing that in my opinion warranted the worst apprehensions of my kind critics.

“It was with ill-restrained impatience that I waited for the first night when the moon should show herself in a suitable position for being observed; but the night arrived in due course.

“Everything was in readiness. The movable portion of the roof of the building had been slid back, and the mirror of the siderostat stood bared to the sky.

“In the dark, square chamber at the other end of the instrument, 200 feet away, into which the eyepiece of the instrument opened, I had taken my station with two or three friends. An attendant at the telephone stood waiting at my elbow to transmit my orders to his colleague in charge of the levers that regulated the siderostat and its mirror.

“The moon had risen now, and her silvery glory shone and sparkled in the mirror.

“‘A right declension,’ I ordered.

“The telephone bell rang in reply. ‘Slowly, still slower; now to the left—enough; again a right declension—slower; stop now—very, very slowly.’

“On the ground-glass before our eyes the moon’s image crept up from one corner until it had overspread the glass completely. And there we stood in the centre of Paris, examining the surface of our satellite with all its craters and valleys and bleak desolation.

“I had won the day.”

PHOTOGRAPHING THE INVISIBLE.

Most of us are able to recognise when we see them shadowgraphs taken by the aid of the now famous X-rays. They generally represent some part of the structure of men, beasts, birds, or fishes. Very dark patches show the position of the bones, large and small; lighter patches the more solid muscles clinging to the bony framework; and outside these again are shadowy tracts corresponding to the thinnest and most transparent portions of the fleshy envelope.

In an age fruitful as this in scientific marvels, it often takes some considerable time for the public to grasp the full importance of a fresh discovery. But when, in 1896, it was announced that Professor Röntgen of Würzburg had actually taken photographs of the internal organs of still living creatures, and penetrated metal and other opaque substances with a new kind of ray, great interest was manifested throughout the civilised world. On the one hand the “new photography” seemed to upset popular ideas of opacity; on the other it savoured strongly of the black art, and, by its easy excursions through the human body, seemed likely to revolutionise medical and surgical methods. At first many strange ideas about the X-rays got afloat, attributing to them powers which would have surprised even their modest discoverer. It was also thought that the records were made in a camera after the ordinary manner of photography, but as a matter of fact Röntgen used neither lens nor camera, the operation being similar to that of casting a shadow on a wall by means of a lamp. In X-radiography a specially constructed electrically-lit glass tube takes the place of the lamp, and for the wall is substituted a sensitised plate. The object to be radiographed is merely inserted between them, its various parts offering varying resistance to the rays, so that the plate is affected unequally, and after exposure may be developed and printed from it the usual way. Photographs obtained by using X-rays are therefore properly called shadowgraphs or skiagraphs.

The discovery that has made Professor Röntgen famous is, like many great discoveries, based upon the labours of other men in the same field. Geissler, whose vacuum tubes are so well known for their striking colour effects, had already noticed that electric discharges sent through very much rarefied air or gases produced beautiful glows. Sir William Crookes, following the same line of research, and reducing with a Sprengel air-pump the internal pressure of the tubes to 1/100000 of an atmosphere, found that a luminous glow streamed from the cathode, or negative pole, in a straight line, heating and rendering phosphorescent anything that it met. Crookes regarded the glow as composed of “radiant matter,” and explained its existence as follows. The airy particles inside the tube, being few in number, are able to move about with far greater freedom than in the tightly packed atmosphere outside the tube. A particle, on reaching the cathode, is repelled violently by it in a straight line, to “bombard” another particle, the walls of the tube, or any object set up in its path, the sudden arrest of motion being converted into light and heat.

By means of special tubes he proved that the “radiant matter” could turn little vanes, and that the flow continued even when the terminals of the shocking-coil were outside the glass, thus meeting the contention of Puluj that the radiant matter was nothing more than small particles of platinum torn from the terminals. He also showed that, when intercepted, radiant matter cast a shadow, the intercepting object receiving the energy of the bombardment; but that when the obstruction was removed the hitherto sheltered part of the glass wall of the tube glowed with a brighter phosphorescence than the part which had become “tired” by prolonged bombardment. Experiments further revealed the fact that the shaft of “Cathode rays” could be deflected by a magnet from their course, and that they affected an ordinary photographic plate exposed to them.

In 1894 Lenard, a Hungarian, and pupil of the famous Hertz, fitted a Crookes’ tube with a “window” of aluminium in its side replacing a part of the glass, and saw that the course of the rays could be traced through the outside air. From this it was evident that something else than matter must be present in the shaft of energy sent from the negative terminal of the tube, as there was no direct communication between the interior and the exterior of the tube to account for the external phosphorescence. Whatever was the nature of the rays he succeeded in making them penetrate and impress themselves on a sensitised plate enclosed in a metal box.

Then in 1896 came Röntgen’s great discovery that the rays from a Crookes’ tube, after traversing the glass, could pierce opaque matter. He covered the tube with thick cardboard, but found that it would still cast the shadows of books, cards, wood, metals, the human hand, &c., on to a photographic plate even at the distance of some feet. The rays would also pass through the wood, metal, or bones in course of time; but certain bodies, notably metals, offered a much greater resistance than others, such as wood, leather, and paper. Professor Röntgen crowned his efforts by showing that a skeleton could be “shadow-graphed” while its owner was still alive.

Naturally everybody wished to know not only what the rays could do, but what they were. Röntgen, not being able to identify them with any known rays, took refuge in the algebraical symbol of the unknown quantity and dubbed them X-rays. He discovered this much, however, that they were invisible to the eye under ordinary conditions; that they travelled in straight lines only, passing through a prism, water, or other refracting bodies without turning aside from their path; and that a magnet exerted no power over them. This last fact was sufficient of itself to prevent their confusion with the radiant matter “cathode rays” of the tube. Röntgen thought, nevertheless, that they might be the cathode rays transmuted in some manner by their passage through the glass, so as to resemble in their motion sound-waves, i.e. moving straight forward and not swaying from side to side in a series of zig-zags. The existence of such ether waves had for some time before been suspected by Lord Kelvin.

Other authorities have other theories. We may mention the view that X represents the ultra-violet rays of the spectrum, caused by vibrations of such extreme rapidity as to be imperceptible to the human eye, just as sounds of extremely high pitch are inaudible to the ear. This theory is to a certain extent upheld by the behaviour of the photographic plate, which is least affected by the colours of the spectrum at the red end and most by those at the violet end. A photographer is able to use red or orange light in his dark room because his plates cannot “see” them, though he can; whereas the reverse would be the case with X-rays. This ultra-violet theory claims for X-rays a rate of ether vibration of trillions of waves per second.

An alternative theory is to relegate the rays to the gap in the scale of ether-waves between heatwaves and light-waves. But this does not explain any more satisfactorily than the other the peculiar phenomenon of non-refraction.

The apparatus employed in X-photography consists of a Crookes’ tube of a special type, a powerful shocking or induction coil, a fluorescent screen and photographic plates and appliances for developing, &c., besides a supply of high-pressure electricity derived from the main, a small dynamo or batteries.

A Crookes’ tube is four to five inches in diameter, globular in its middle portion, but tapering away towards each end. Through one extremity is led a platinum wire, terminating in a saucer-shaped platinum plate an inch or so across. At the focus of this, the negative terminal, is fixed a platinum plate at an angle to the path of the rays so as to deflect them through the side of the tube. The positive terminal penetrates the glass at one side. The tube contains, as we have seen, a very tiny residue of air. If this were entirely exhausted the action of the tube would cease; so that some tubes are so arranged that when rarefaction becomes too high the passage of an electrical current through small bars of chemicals, whose ends project through the sides of the tube, liberates gas from the bars in sufficient quantity to render the tube active again.

When the Ruhmkorff induction coil is joined to the electric circuit a series of violent discharges of great rapidity occur between the tube terminals, resembling in their power the discharge of a Leyden jar, though for want of a dense atmosphere the brilliant spark has been replaced by a glow and brush-light in the tube. The coil is of large dimensions, capable of passing a spark across an air-gap of ten to twelve inches. It will perhaps increase the reader’s respect for X-rays to learn that a coil of proper size contains upwards of thirteen miles of wire; though indeed this quantity is nothing in comparison with the 150 miles wound on the huge inductorium formerly exhibited at the London Polytechnic.

If we were invited to an X-ray demonstration we should find the operator and his apparatus in a darkened room. He turns on the current and the darkness is broken by a velvety glow surrounding the negative terminal, which gradually extends until the whole tube becomes clothed in a green phosphorescence. A sharply-defined line athwart the tube separates the shadowed part behind the receiving plate at the negative focus—now intensely hot—from that on which the reflected rays fall directly.

One of us is now invited to extend a hand close to the tube. The operator then holds on the near side of the hand his fluorescent screen, which is nothing more than a framework supporting a paper smeared on one side with platino-cyanide of barium, a chemical that, in common with several others, was discovered by Salvioni of Perugia to be sensitive to the rays and able to make them visible to the human eye. The value of the screen to the X-radiographer is that of the ground-glass plate to the ordinary photographer, as it allows him to see exactly what things are before the sensitised plate is brought into position, and in fact largely obviates the necessity for making a permanent record.

The screen shows clearly and in full detail all the bones of the hand—so clearly that one is almost irresistibly drawn to peep behind to see if a real hand is there. One of us now extends an arm and the screen shows us the ulna and the radius working round each other, now both visible, now one obscuring the other. On presenting the body to the course of the rays a remarkable shadow is cast on to the screen. The spinal column and the ribs; the action of the heart and lungs are seen quite distinctly. A deep breath causes the movement of a dark mass—the liver. There is no privacy in presence of the rays. The enlarged heart, the diseased lung, the ulcerated liver betrays itself at once. In a second of time the phosphorescent screen reveals what might baulk medical examination for months.

If a photographic slide containing a dry-plate be substituted for the focusing-screen, the rays soon penetrate any covering in which the plate may be wrapped to protect it from ordinary light rays. The process of taking a shadowgraph may therefore be conducted in broad daylight, which is under certain conditions a great advantage, though the sensitiveness of plates exposed to Röntgen rays entails special care being taken of them when they are not in use. In the early days of X-radiography an exposure of some minutes was necessary to secure a negative, but now, thanks to the improvements in the tubes, a few seconds is often sufficient.

The discovery of the X-rays is a great discovery, because it has done much to promote the noblest possible cause, the alleviation of human suffering. Not everybody will appreciate a more rapid mode of telegraphy, or a new method of spinning yarn, but the dullest intellect will give due credit to a scientific process that helps to save life and limb. Who among us is not liable to break an arm or leg, or suffer from internal injuries invisible to the eye? Who among us therefore should not be thankful on reflecting that, in event of such a mishap, the X-rays will be at hand to show just what the trouble is, how to deal with it, and how far the healing advances day by day? The X-ray apparatus is now as necessary for the proper equipment of a hospital as a camera for that of a photographic studio.

It is especially welcome in the hospitals which accompany an army into the field. Since May 1896 many a wounded soldier has had reason to bless the patient work that led to the discovery at Würzburg. The Greek war, the war in Cuba, the Tirah campaign, the Egyptian campaign, and the war in South Africa, have given a quick succession of fine opportunities for putting the new photography to the test. There is now small excuse for the useless and agonising probings that once added to the dangers and horrors of the military hospital. Even if the X-ray equipment, by reason of its weight, cannot conveniently be kept at the front of a rapidly moving army, it can be set up in the “advanced” or “base” hospitals, whither the wounded are sent after a first rough dressing of their injuries. The medical staff there subject their patients to the searching rays, are able to record the exact position of a bullet or shell-fragment, and the damage it has done; and by promptly removing the intruder to greatly lessen its power to harm.

The Röntgen ray has added to the surgeon’s armoury a powerful weapon. Its possibilities are not yet fully known, but there can be no doubt that it marks a new epoch in surgical work. And for this reason Professor Röntgen deserves to rank with Harvey, the discoverer of the blood’s circulation; with Jenner, the father of vaccination; and with Sir James Young Simpson, the first doctor to use chloroform as an anæsthetic.

Photography in the Dark.

Strange as it seems to take photographs with invisible rays, it is still stranger to be able to affect sensitised plates without apparently the presence of any kind of rays.

Professor W. J. Russell, Vice-President of the Royal Society of London, has discovered that many substances have the power of impressing their outlines automatically on a sensitive film, if the substance be placed in a dark cupboard in contact with, or very close to a dry-plate.

After some hours, or it may be days, development of the plate will reveal a distinct impression of the body in question. Dr. Russell experimented with wood, metal, leaves, drawings, printed matter, lace. Zinc proved to be an unusually active agent. A plate of the metal, highly polished and then ruled with patterns, had at the end of a few days imparted a record of every scratch and mark to the plate. And not only will zinc impress itself, but it affects substances which are not themselves active, throwing shadowgraphs on to the plate. This was demonstrated with samples of lace, laid between a plate and a small sheet of bright zinc; also with a skeleton leaf. It is curious that while the interposition of thin films of celluloid, gutta-percha, vegetable parchment, and gold-beater’s skin—all inactive—between the zinc and the plate has no obstructive effect, a plate of thin glass counteracts the action of the zinc. Besides zinc, nickel, aluminium, pewter, lead, and tin among the metals influence a sensitised plate. Another totally different substance, printer’s ink, has a similar power; or at least some printer’s ink, for Professor Russell found that different samples varied greatly in their effects. What is especially curious, the printed matter on both sides of a piece of newspaper appeared on the plate, and that the effect proceeded from the ink and not from any rays passing from beyond it is proved by the fact that the type came out dark in the development, whereas if it had been a case of shadowgraphy, the ink by intercepting rays would have produced white letters. Professor Russell has also shown that modern writing ink is incapable of producing an impression unaided, but that on the other hand paper written on a hundred years ago or a printed book centuries old will, with the help of zinc, yield a picture in which even faded and uncertain characters appear quite distinctly. This opens the way to a practical use of the discovery, in the deciphering of old and partly obliterated manuscripts.

A very interesting experiment may be made with that useful possession—a five-pound note. Place the note printed side next to the plate, and the printing appears dark; but insert the note between a zinc sheet and the plate, its back being this time towards the sensitised surface, and the printing appears white; and the zinc, after contact with the printed side, will itself yield a picture of the inscription as though it had absorbed some virtue from the note!

As explanation of this paradoxical dark photography—or whatever it is—two theories may be advanced. The one—favoured by Professor Russell—is that all “active” substances give off vapours able to act on a photographic plate. In support of this may be urged the fact that the interposition of glass prevents the making of dark pictures. But on the other hand it must be remembered that celluloid and sheet-gelatine, also air-tight substances, are able to store up light and to give it out again. It is well known among photographers that to allow sunlight to fall on the inside of a camera is apt to have a “fogging” effect on a plate that is exposed in the camera afterwards, though the greatest care be taken to keep all external light from the plate. But here the glass again presents a difficulty, for if this were a case of reflected light, glass would evidently be less obstructive than opaque vegetable parchment or gutta-percha.

SOLAR MOTORS.

One day George Stephenson and a friend stood watching a train drawn by one of his locomotives.

“What moves that train?” asked Stephenson.

“The engine,” replied his friend.

“And what moves the engine?”

“The steam.”

“And what produces the steam?”

“Coal.”

“And what produces coal?”

This last query nonplussed his friend, and Stephenson himself replied, “The sun.”

The “bottled sunshine” that drove the locomotive was stored up millions of years ago in the dense forests then covering the face of the globe. Every day vegetation was built by the sunbeams, and in the course of ages this growth was crushed into fossil form by the pressure of high-piled rock and débris. To-day we cast “black diamonds” into our grates and furnaces, to call out the warmth and power that is a legacy from a period long prior to the advent of fire-loving man, often forgetful of its real source.

We see the influence of the sun more directly in the motions of wind and water. Had not the sun’s action deposited snow and rain on the uplands of the world, there would be no roaring waterfall, no rushing torrent, no smooth-flowing stream. But for the sun heating the atmosphere unequally, there would not be that rushing of cool air to replace hot which we know as wind.

We press Sol into our service when we burn fuel; our wind-mills and water-mills make him our slave. Of late years many prophets have arisen to warn us that we must not be too lavish of our coal; that the time is not so far distant, reckoning by centuries, when the coal-seams of the world will be worked out and leave our descendants destitute of what plays so important a part in modern life. Now, though waste is unpardonable, and the care for posterity praiseworthy, there really seems to be no good reason why we should alarm ourselves about the welfare of the people of the far future. Even if coal fails, the winds and the rivers will be there, and the huge unharnessed energy of the tides, and the sun himself is ready to answer appeals for help, if rightly shaped. He does not demand the prayers of Persian fire-worshippers, but rather the scientific gathering of his good gifts.

Place your hand on a roof lying square to the summer sun, and you will find it too hot for the touch. Concentrate a beam of sunshine through a small burning-glass. How fierce is the small glowing focal spot that makes us draw our hands suddenly away! Suppose now a large glass many feet across bending several square yards of sun rays to a point, and at that point a boiler. The boiler would develop steam, and the steam might be led into cylinders and forced to drudge for us.

Do many of us realise the enormous energy of a hot summer’s day? The heat falling in the tropics on a single square foot of the earth’s surface has been estimated as the equivalent of one-third of a horse-power. The force of Niagara itself would on this basis be matched by the sunshine streaming on to a square mile or so. A steamship might be propelled by the heat that scorches its decks.

For many centuries inventors have tried to utilise this huge waste power. We all know how, according to the story, Archimedes burnt up the Roman ships besieging his native town, Syracuse, by concentrating on them the sun heat cast from hundreds of mirrors. This story is less probable than interesting as a proof that the ancients were aware of the sun’s power. The first genuine solar machine was the work of Ericsson, the builder of the Monitor. He focused sun heat on a boiler, which gave the equivalent of one horse-power for every hundred square feet of mirrors employed. This was not what engineers would call a “high efficiency,” a great deal of heat being wasted, but it led the way to further improvements.

In America, especially in the dry, arid regions, where fuel is scarce and the sun shines pitilessly day after day, all the year round, sun-catchers of various types have been erected and worked successfully. Dr. William Calver, of Washington, has built in the barren wastes of Arizona huge frames of mirrors, travelling on circular rails, so that they may be brought to face the sun at all hours between sunrise and sunset. Dr. Calver employs no less than 1600 mirrors. As each of these mirrors develops 10-15 degrees of heat it is obvious, after an appeal to simple arithmetic, that the united efforts of these reflectors should produce the tremendous temperature 16,000-24,000 degrees, which, expressed comparatively, means the paltry 90 degrees in the shade beneath which we grow restive multiplied hundreds of times. Hitherto the greatest known heat had been that of the arc of the electric lamp, in which the incandescent particles between pole and pole attain 6000 degrees Fahrenheit.

The combined effect of the burning mirrors is irresistible. They can, we are told, in a few moments reduce Russian iron to the consistency of warmed wax, though it mocks the heat of many blast-furnaces. They will bake bricks twenty times as rapidly as any kiln, and the bricks produced are not the friable blocks which a mason chips easily with his trowel, but bodies so hard as to scratch case-hardened steel.

There are at work in California sun-motors of another design. The reader must imagine a huge conical lamp-shade turned over on to its smaller end, its inner surface lined with nearly 1800 mirrors 2 feet long and 3 inches broad, the whole supported on a light iron framework, and he will have a good idea of the apparatus used on the Pasadena ostrich farm. The machine is arranged in meridian, that is, at right angles to the path of the sun, which it follows all day long by the agency of clockwork. In the focus of the mirrors is a boiler, 13 feet 6 inches long, coated with black, heat-absorbing substances. This boiler holds over 100 gallons of water, and being fed automatically will raise steam untended all the day through. The steam is led by pipes to an engine working a pump, capable of delivering 1400 gallons per minute.

The cheapness of the apparatus in proportion to its utility is so marked that, in regions where sunshine is almost perpetual, the solar motor will in time become as common as are windmills and factory chimneys elsewhere. If the heat falling on a few square yards of mirror lifts nearly 100,000 gallons of water an hour, there is indeed hope for the Sahara, the Persian Desert, Arabia, Mongolia, Mexico, Australia. That is to say, if the water under the earth be in these parts as plentiful as the sunshine above it. The effect of water on the most unpromising soil is marvellous. Already in Algeria the French have reclaimed thousands of square miles by scientific irrigation. In Australia huge artesian wells have made habitable for man and beast millions of acres that were before desert.

It is only a just retribution that the sun should be harnessed and compelled to draw water for tracts to which he has so long denied it. The sun-motor is only just entering on its useful career, and at present we can but dream of the great effects it may have on future civilisation. Yet its principle is so simple, so scientific, and so obvious, that it is easy to imagine it at no far distant date a dangerous rival to King Coal himself. To quarry coal from the bowels of the earth and transform it into heat, is to traverse two sides of a triangle, the third being to use the sunshine of the passing hour.

LIQUID AIR.

Among common phenomena few are more interesting than the changes undergone by the substance called water. Its usual form is a liquid. Under the influence of frost it becomes hard as iron, brittle as glass. At the touch of fire it passes into unsubstantial vapour.

This transformation illustrates the great principle that the form of every substance in the universe is a question of heat. A metal transported from the earth to the sun would first melt and then vaporise; while what we here know only as vapours would in the moon turn into liquids.

We notice that, as regards bulk, the most striking change is from liquid to gaseous form. In steam the atoms and molecules of water are endowed with enormous repulsive vigour. Each atom suddenly shows a huge distaste for the company of its neighbours, drives them off, and endeavours to occupy the largest possible amount of private space.

Now, though we are accustomed to see water-atoms thus stirred into an activity which gives us the giant steam as servant, it has probably fallen to the lot of but few of us to encounter certain gaseous substances so utterly deprived of their self-assertiveness as to collapse into a liquid mass, in which shape they are quite strangers to us. What gaseous body do we know better than the air we breathe? and what should we less expect to be reducible to the consistency of water? Yet science has lately brought prominently into notice that strange child of pressure and cold, Liquid Air; of which great things are prophesied, and about which many strange facts may be told.

Very likely our readers have sometimes noticed a porter uncoupling the air-tube between two railway carriages. He first turns off the tap at each end of the tube, and then by a twist disconnects a joint in the centre. At the moment of disconnection what appears to be a small cloud of steam issues from the joint. This is, however, the result of cold, not heat, the tube being full of highly-compressed air, which by its sudden expansion develops cold sufficient to freeze any particles of moisture in the surrounding air.

Keep this in mind, and also what happens when you inflate your cycle-tyre. The air-pump grows hotter and hotter as inflation proceeds: until at last, if of metal, it becomes uncomfortably warm. The heat is caused by the forcing together of air-molecules, and inasmuch as all force produces heat, your strength is transformed into warmth.

In these two operations, compression and expansion, we have the key to the creation of liquid air—the great power, as some say, of to-morrow.