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The Christian Philosopher; or, Science and Religion

Note X, p. 117.—On Telescopes; with a brief notice of a New Reflecting Telescope, constructed by the Author

Note X, p. 117.—On Telescopes; with a brief notice of a New Reflecting Telescope, constructed by the Author

It is doubtful to what particular individual we owe the invention of the telescope. Some have supposed that Roger Bacon and Baptista Porta invented this instrument. Borelli ascribes the invention to Zacharias Jansen, a native of Middleberg. Perhaps the account given in the article to which this note refers, and which is stated by a variety of authors, may be as probable as any other. It is certain the telescope was not in general use until the beginning of the 17th century, and that no discoveries in the heavens were made with it until the year 1609

There are two kinds of telescopes, Refracting and Reflecting. In refracting telescopes, the rays of light pass through convex or concave glasses or lenses. The object-glass is always convex, and forms an image or picture of the object in an inverted position in its focus, which image is viewed by the eye-glass; and the magnifying power is in the proportion of the focal distance of the object-glass to that of the eye-glass. The focal distance of a convex glass may be ascertained by holding it in the rays of the sun, opposite to a piece of white paper, and measuring the distance between the glass and the white spot, or burning point, formed on the paper. An astronomical telescope for viewing celestial objects may be constructed with only two glasses. If an object-glass, 30 inches focal distance, be fixed in the end of a tube, and an eye-glass of one inch focus be placed at the other end, at the distance of 31 inches from the object-glass, a telescope will be formed which will magnify in the proportion of one to thirty, or 30 times; that is, objects seen through such a telescope will appear thirty times larger in diameter, or thirty times nearer than to the naked eye. By such an instrument, the inequalities on the Moon's surface, and some of the satellites of Jupiter, may be perceived; but when directed to land objects, they will appear inverted or turned upside down. In order to reverse the appearance of the object, two other eye-glasses are required;—or, if a concave eye-glass of a similar focus be placed at 29 inches from the object-glass, the object will appear in its natural position, and the magnifying power will be the same; but the field of view will be much smaller. Astronomical telescopes of this construction were formerly made of 120, and even of 200 feet in length, and were used without a tube; the object-glass being placed on the top of a long pole; but these are now entirely superseded by achromatic telescopes. In the achromatic telescope, the object-glass is compounded of two, and sometimes of three lenses, placed close to each other, one of which is a double concave of white flint glass, and the other a double convex of crown glass. By this means an image is formed without being blended with the prismatic colors; and it will, therefore, bear a large aperture, and a much greater magnifying power, than a common refractor. A good achromatic telescope four feet long will magnify objects as much as a common refractor 100 feet long.

In Reflecting telescopes the images of objects are formed by speculums or mirrors, instead of page 154 lenses. They are of two kinds, the Gregorian and the Newtonian. The Gregorian Reflector consists of a tube in which a concave mirror, having a hole in its center, is placed. The rays of light from distant objects falling upon this mirror, form an image before it, in its center or focus. This image is intercepted by a smaller mirror, which reflects it back, through the hole in the large mirror to an eye-glass, through which the observer views the object. In the Newtonian Reflector, a plane mirror, placed at an angle of 45 degrees, is substituted in place of the small mirror, in the Gregorian construction, and the observer looks upon the object through the side of the tube. Sir David Brewster has suggested an interesting improvement in the construction of this instrument, which is described in the Edin. Encyc., Art. Optics, p. 644.

New Reflector.—Several years ago, the Author commenced a series of experiments on Reflecting Telescopes; and has lately constructed several on a new plan and principle. In this construction there is no small speculum, either plane, convex, or concave; there is no tube, except a short one of two or three inches in length, for holding the speculum. The observer sits with his back to the object, and views the image formed by the speculum through an eye-piece, which requires to be nicely directed and adjusted. Three or four instruments of this construction have been fitted up, with specula of 16, 28, 35, and 49 inches focal distance. One of them, having a speculum of eight inches focus, and two inches diameter, with a terrestrial eye-piece, magnifying about 25 times, forms an excellent parlor telescope for viewing land-objects, and exhibits them in a brilliant and novel aspect. When compared with a Gregorian of the same size and magnifying power, the quantity of light upon the object appears nearly doubled, and the image is equally distinct. It represents objects in their natural colors, without that dingy and yellowish tinge which appears when looking through a Gregorian. Another of these instruments, having a speculum of 28 inches focal distance, and an eye-piece producing a magnifying power of about 100 times, serves as an excellent astronomical telescope. By this instrument the belts and satellites of Jupiter, the ring of Saturn, and the mountains and cavities of the Moon, may be contemplated with great ease and distinctness. By placing the pedestal on the floor of the apartment, when the object is at a high elevation, we can view celestial phenomena with the same ease as if we were sitting at a writing-desk reading a book. With a magnifying power of about 40 times applied to this telescope, terrestrial objects appear extremely bright and well defined. A speculum of 49 inches focal distance, and 61/2 inches diameter, has lately been fitted up on the same principle. With magnifying powers of from 100 to 130 times, it exhibits distinct and interesting views of the Moon's surface and of the ring of Saturn, and with a power of 56 times it affords a beautiful view of land objects. The specula used in some of these instruments are far from being good; being of a yellowish color, and scarcely half polished, and having large holes in the center; as they were originally intended for Gregorian reflectors; yet the brightness of vision approaches nearly to that of Achromatic Telescopes. The experiments which have been made on this subject demonstrate, that a tube is not necessary for a reflecting Telescope, when viewing either celestial or terrestrial objects; and, therefore, this construction of the instrument may be denominated, The Aerial Reflector. The simplicity of the construction, and the excellence of the performance of these instruments, have been much admired by several scientific gentlemen to whom they have been exhibited.*

In the system of Optics, lately published in the Edinburgh Encyclopedia (one of the most luminous and comprehensive treatises which has yet appeared on this subject), the writer in his introduction to the account of Sir D. Brewster's improvement on the Newtonian Telescope, remarks:—“If we could dispense with the use of the small specula in telescopes of modern length, by inclining the great speculum, and using an oblique, and consequently a distorted reflection, as proposed first by La Maire, we should consider the Newtonian Telescope as perfect, and, on a large scale, or when the instrument exceeds 20 feet, it has undoubtedly this character, as nothing can be more simple than to magnify, by a single eye-glass, the image formed by a single speculum.—As the front view is quite impracticable, and, indeed, has never been attempted in instruments of a small size, it becomes of great practical consequence to remove as much as possible the evils which arise from the use of a small speculum,” etc. The instruments noticed above have effectuated the desirable object alluded to by this respectable writer; and the principle of the construction is neither that of Sir W. Herschel's front view, nor does it coincide with that proposed by La Maire, which seems to have been a mere hint, which was never put into execution.

* The reader will find a more particular account of these instruments, accompanied with engravings, in the Edinburgh Philosophical Journal for July, 1826; the “London Encyclopedia,”Art. Telescope, and in the “London Mechanics’ Magazine” for August, 1826. The Author has given a more particular description of these and various other astronomical instruments, in a work recently published.