Part 13 (1/2)
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-gla.s.s forty inches in diameter; and next to it the 36-inch Lick instrument on Mount Hamilton, California, built by Messrs. Alvan Clark of Cambridgeport, Ma.s.sachusetts. 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-gla.s.ses, 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 gla.s.s 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-gla.s.s 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-gla.s.s; 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.
a.s.suming 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 gla.s.ses, 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 ma.s.sive 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 Leon 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 ma.s.ses 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 ma.s.sive 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 ma.s.s.
The real romance of the construction of this huge telescope centres on the making of the lenses and mirror. First-cla.s.s 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 gla.s.s 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 polis.h.i.+ng. The presence of a single air-bubble in the molten gla.s.s, the slightest irregularity of surface in the polis.h.i.+ng may utterly destroy the value of a lens otherwise worth several thousands of pounds.
[Ill.u.s.tration: _Reproduced by the permission of Proprietors of ”Knowledge.”_
_General view, of the Great Paris Telescope, showing the eye-end. The tube is 180 feet long, and 59 inches in diameter. It weighs 21 tons._]
The object-gla.s.s of the great telescope was cast by M. Mantois, famous as the manufacturer of large lenses. The gla.s.s 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 gla.s.s, had to be constructed. The mirror, 6-1/2 feet in diameter and eleven inches thick, absorbed 3-3/4 tons of liquid gla.s.s; 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 polis.h.i.+ng, 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 gla.s.s 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 polis.h.i.+ng room, and the presence of dust. To insure success it was found necessary to warm all the polis.h.i.+ng machinery, and to keep it at a fixed temperature.
At the end of almost a year the polis.h.i.+ng 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 pus.h.i.+ng-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 gla.s.s 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.