Part 13 (2/2)
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 a.s.sembled 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 a.s.sistant 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 circ.u.mstances, 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-gla.s.s before our eyes the moon's image crept up from one corner until it had overspread the gla.s.s 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 Rontgen of Wurzburg 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 Rontgen 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 gla.s.s tube takes the place of the lamp, and for the wall is subst.i.tuted 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 Rontgen 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 phosph.o.r.escent 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 gla.s.s, 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 gla.s.s wall of the tube glowed with a brighter phosph.o.r.escence 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 gla.s.s, 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 phosph.o.r.escence. 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 Rontgen's great discovery that the rays from a Crookes' tube, after traversing the _gla.s.s_, 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 pa.s.s 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 Rontgen 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. Rontgen, not being able to identify them with any known rays, took refuge in the algebraical symbol of the unknown quant.i.ty 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, pa.s.sing 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. Rontgen thought, nevertheless, that they might be the cathode rays trans.m.u.ted in some manner by their pa.s.sage through the gla.s.s, 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.
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