The following figure shews a section of the machinery connected with this telescope. It exhibits a view of the inside of the eastern wall, with all the machinery as seen in section. A is the mason-work on the ground, B the universal joint, which allows the tube to turn in all directions; C the speculum in its tube; D the box; E the eye-piece; F the moveable pulley; G the fixed one; H the chain from the side of the tube; I the chain from the beam; K the counterpoise; L the lever; M the chain connecting it with the tube; Z the chain which passes from the tube to the windlass over a pulley on a truss-beam which runs from W to the same situation on the opposite wall—the pulley is not seen. X is a railroad on which the speculum is drawn either to or from its box; part is cut away to show the counterpoise. The dotted line a represents the course of the weight R as the tube rises or falls; it is a segment of a circle of which the chain I is the radius. The tube is moved from wall to wall by the ratchet and wheel at R; the wheel is turned by the handle O, and the ratchet is fixed to the circle on the wall. The ladders in front, as shown in the preceding sketch, enable the observer to follow the tube in its ascent to where the galleries on the side wall commence; these side galleries are three in number, and each can be moved from wall to wall by the observer, after the tube, the motion of which he also accomplishes by means of the handle O.
I shall conclude the description of this wonderful instrument in the words of Sir James South.
‘What will be the power of this telescope when it has its Le Mairean form’ [that is, when it is fitted up with the front view] ‘it is not easy to divine;—what nebulæ will it resolve into stars; in what nebulæ will it not find stars;—how many satellites of Saturn will it show us;—how many will it indicate as appertaining to Uranus;—how many nebulæ never yet seen by mortal eye, will it present to us;—what spots will it show us on the various planets; will it tell us what causes the variable brightness of many of the fixed stars;—will it give us any information as to the constitution of the planetary nebulæ;—will it exhibit to us any satellites encircling them; will it tell us why the satellites of Jupiter, which generally pass over Jupiter’s face as disks nearly of white light, sometimes traverse it as black patches;—will it add to our knowledge of the physical construction of nebulous stars;—of that mysterious class of bodies which surround some stars, called, for want of a better name, ‘photospheres;’—will it show the annular nebulæ of Lyra, merely as a brilliant luminous ring, or will it exhibit it as thousands of stars arranged in all the symmetry of an ellipse; will it enable us to comprehend the hitherto incomprehensible nature and origin of the light of the great nebulæ of Orion;—will it give us, in easily appreciable quantity, the parallax of some of the fixed stars, or will it make sensible to us the parallax of the nebulae themselves;—finally, having presented to us original portraits of the moon and of the sidereal heavens, such as man has never dared even to anticipate—will it, by Daguerreotype aid, administer to us copies founded upon truth, and enable astronomers of future ages to compare the moon and heavens as they then may be, with the moon and heavens as they were? Some of these questions will be answered affirmatively, others negatively, and that, too, very shortly; for the noble maker of the noblest instrument ever formed by man, “has cast his bread upon the waters, and will, with God’s blessing, find it before many days.”’
HINTS TO AMATEURS IN ASTRONOMY RESPECTING THE CONSTRUCTION OF TELESCOPES.
As there are many among the lower ranks of the community who have a desire to be possessed of a telescope, which will show them some of the prominent features of celestial scenery, but who are unable to purchase a finished instrument at the prices usually charged by Opticians, the following hints may perhaps be acceptable to those who are possessed of a mechanical genius.
The lenses of an Achromatic telescope may be purchased separately from glass-grinders or Opticians, and tubes of a cheap material may be prepared by the individual himself for receiving the glasses. The following are the prices at which achromatic object-glasses for astronomical telescopes are generally sold. Focal length 30 inches, diameter 2¼ inches, from 2 to 3½ guineas. Focal length 42 inches, diameter 2¾ inches, from 5 to 8 guineas. Focal length 42 inches, diameter 3¼ inches, from 12 to 20 guineas. Focal length 42 inches, diameter 3¾ inches, from 25 to 30 guineas. Eye-pieces, from 10s. 6d. to 18 shillings. The smallest of these lenses, namely that of 2¼ inches diameter, if truly achromatic, may be made to bear a power of from 80 to 100 times, in clear weather, for celestial objects, which will show Jupiter’s moons and belts, Saturn’s ring and other celestial phenomena. The tubes may be made either of tin plates, papier maché, or wood. Wood, however, is rather a clumsy article, and it is sometimes liable to warp, yet excellent tubes have sometimes been made of it. Perhaps the cheapest and most convenient of all tubes when properly made, are those formed of paper. In forming these a wooden roller of the proper diameter should be procured, and paper of a proper size, along with book-binder’s paste. About three or four layers only of the paper should be pasted at one time, and, when sufficiently dry, it should be smoothed by rubbing it with a smooth stick or ruler; after which another series of layers should be pasted on, and allowed to dry as before, and so on till the tube has acquired a sufficient degree of strength and firmness. In this way, I have, by means of a few old Newspapers, and similar materials, formed tubes as strong as if they had been made of wood. If several tubes be intended to slide into each other, the smallest tube should be made first, and it will serve as a roller for forming the tube into which it is to slide.
An achromatic object glass of a shorter focal distance, and a smaller diameter than any of those stated above, may be fitted up as a useful astronomical telescope, when a better instrument cannot be procured. In the Pawn-broker’s shops in London, and other places, an old achromatic telescope, with an object-glass 20 inches focal distance and about 1½ inch diameter, may be purchased at a price varying from 15 to 20 shillings. By applying an astronomical eye-piece to such a lens, if a good one, it may bear a power, for celestial objects, of 50 or 60 times. If two plano-convex glasses, ¾ inch focal distance, be placed with their convex sides near to each other, they will form an eye-piece which will produce a power on such an object-glass, of above 50 times, which will show Jupiter’s belts and satellites, Saturn’s ring, the solar spots, and the mountains and cavities of the moon. I have an object-glass of this description which belonged to an old telescope, which cost me only 12 shillings, and with which I formerly made some useful astronomical observations. It was afterwards used as the telescope of a small Equatorial instrument, and, with it, I was enabled to perceive stars of the first and second magnitude, and the planets Venus, Jupiter, and Mars, in the day-time.
But, should such a glass be still beyond the reach of the astronomical amateur, let him not altogether despair. He may purchase a single lens, 3 feet focal distance, for about a couple of shillings, and by applying an eye-glass of 1 inch focus, which may be procured for a shilling, he will obtain a power of 36 times, which is a higher power than Galileo was able to apply to his best telescope; and consequently, with such an instrument, he will be enabled to perceive all the celestial objects which that celebrated astronomer first described, and which excited so much wonder, at that period, in the learned world. But, whatever kind of telescope may be used, it is essentially requisite that it be placed on a firm stand in all celestial observations: and any common mechanic can easily form such a stand at a trifling expence.
There is a certain optical illusion to which most persons are subject, in the first use of telescopes, especially when applied to the celestial bodies, on which it may not be improper to make a remark. The illusion to which I allude is this—that they are apt to imagine, the telescope does not magnify nearly so much as it really does. They are apt to complain of the small appearance which Jupiter and Saturn, for example, present when magnified 160 or 200 times. With such powers they are apt to imagine, that these bodies do not appear so large as the moon to the naked eye. Yet it can be proved that Jupiter, when nearest the earth, viewed with such a power, appears about 5 times the diameter of the full moon, and 25 times larger in surface. This appears from the following calculation. Jupiter, when in opposition, or nearest the Earth, presents a diameter of 47´´: the mean apparent diameter of the moon is about 31´. Multiply the diameter of Jupiter by the magnifying power, 200, the product is 9400´´ or 156´ or 2° 36´, which, divided by 31´, the moon’s diameter, produces a quotient of 5, showing that this planet with such a power appears five times larger in diameter than the full moon to the naked eye, and consequently 25 times larger in surface. Were a power of only 50 times applied to Jupiter, when nearest the earth, that planet would appear somewhat larger than the full moon. For 47´´ multiplied by 50 gives 2350´´ or 39´, which is 8´ more than the diameter of the moon. Yet with such a power most persons would imagine that the planet does not appear one third of the size of the full moon.
The principal mode by which a person may be experimentally convinced of the fallacy to which I allude is the following:—At a time when Jupiter happens to be within a few degrees of the moon, let the planet be viewed through the telescope with the one eye, and the magnified image of the planet be brought into contact with the moon as seen with the other eye—the one eye looking at the moon, and the other viewing the magnified image of Jupiter through the telescope when brought into apparent contact with the moon—then it will be perceived, that with a magnifying power of 50 the image of Jupiter will completely cover the moon as seen by the naked eye;—and with a power of 200—when the moon is made to appear in the centre of the magnified image of the planet—it will be seen that Jupiter forms a large and broad circle around the moon, appearing at least 5 times greater than the diameter of the moon. This experiment may be varied as follows: Suppose a person to view the moon through a small telescope or opera-glass, magnifying three times, he will be apt to imagine, at first sight, that she is not in the least magnified, but rather somewhat diminished. But let him bring the image as seen in the telescope in contact with the moon as seen with the naked eye, and he will plainly perceive the magnifying power, by the size of the image. It may be difficult in the first instance to look, at the same time, at the magnified image and the real object, but a few trials will render it easy.