The Practical Astronomer / Comprising illustrations of light and colours--practical descriptions of all kinds of telescopes--the use of the equatorial-transit--circular, and other astronomical instruments, a particular account of the Earl of Rosse's large telescopes, and other topics connected with astronomy

by Thomas Dick

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.

THE END.

L. SEELEY PRINTER, THAMES DITTON.