Tel′e-scope.
An instrument for magnifying distant objects, so as to make them appear comparatively near.
Gerbert of
Auvergne, who taught astronomy in his school at
Rheims, observed the stars through a tube, A. D. 1000.
He derived it from his tutors at
Cordova, and they, no doubt, from the
Alexandrian savans.
In both places, celestial observations were made through long tubes with object and ocular diopters at the respective ends.
No lenses as yet.
Who was the first discoverer of the telescope cannot now be determined.
Spectacles were known in the thirteenth century.
Roger Bacon employs some expressions indicative of a knowledge of the effect of the combination of lenses.
Dr. Dee mentions (1570) that “perspective glasses” will enable a commander to ascertain the strength of an enemy's forces, referring apparently to an optical instrument then in use.
Baptista Porta said: “If you properly combine a concave and a convex lens, you will see distant and near objects larger and clearer.”
Digges states that by an arrangement of mirrors and transparent glasses the image of a small object at a distance may be so augmented as to be brought apparently near to the observer.
The matter is restated in a second edition of his works, published in 1591.
Toward the middle of the seventeenth century, Borelli, a Dutch mathematician, interested himself in determining the question of inventorship, and decided in favor of
Jansen and Lippersheim, spectacle-makers of
Middelburg, Holland, about 1590.
Galilco, hearing of the principle of the new wonder, constructed one in 1509, magnifying four times, a second magnifying seven times, and then one magnifying thirty-two times.
It is only fair to give another account, for which we are indebted to
Descartes, who lived at about the time when the invention was made public, and had a right to know whereof he affirmed.
He says it was made by
James Metius, and was due to a fortunate accident.
James was a glass-cutter, and had a brother who was a professor of mathematics and a maker of mirrors and burning-glasses.
James, it appears, was amusing himself by trying the effect of looking through two glasses, held in line and at a distance, by the respective hands.
Fortunately he tried the experiment with a concave and convex glass, which gave the wonderful effect now so familiar.
They were fitted in a wooden tube, and made the first telescope ever used in the world, says
Descartes.
The inventor was a suspicious character, and tried to keep the invention secret even on his deathbed.
But his brother and some few others had seen it, and were able to follow the track, which they opened to the world, and which was followed by Galilco.
Humboldt says:—
The accidental discovery of the space-penetrating power of the telescope was first made in Holland, probably as early as the close of the year 1608.
According to the latest documentary investigations, this great invention may be claimed by Hans Lippershey (or Laprey), a native of Wesel, and spectacle-maker at Middelburg; Jacob Adriansz, also called Metius, who is said to have made burning-glasses of ice; and Zacharias Jansen.
Lippershey, on the 2d of October, 1608, offered to the States-General “three instruments with which one can see to a distance.”
On the 17th of the same month, Metius, in his offer to the States-General, states that “through meditation and industry he had constructed such instruments for two years.”
Zacharias Jansen, who, like Lippershey, was a spectacle-maker at Middelburg, together with his father, Hans Jansen, invented the compound microscope having a concave lens for its eyeglass, toward the end of the sixteenth century (probably about 1590), but discovered the telescope only in 1610.
When the news of the recent Dutch invention reached Venice, Galileo was accidentally present; he at once divined what were the essential conditions of the construction, and immediately completed a telescope at Padua for his own use. He directed it first to the mountains of the moon, and showed the method of measuring their hights; attributing, like Leonardo da Vinci and Mostlin, the ashy-colored light of the moon to the light of the sun reflected back upon her from the earth.
He examined with small magnifying powers the group of the Pleiades, the cluster of stars in Cancer, the Milky Way, and the group of stars in the head of Orion.
Then followed in quick succession the great discoveries of the four satellites of Jupiter, the two ‘handles’ of Saturn, — or his surrounding ring imperfectly seen, so that its true character was not at first recognized, — the solar spots, and the crescent form of Venus.
As early as November, 1610, Galileo wrote to Kepler that “Saturn consists of three heavenly bodies in contact with each other.”
In this observation there was the germ of the discovery of Saturn's ring.
Hevelius described, in 1656, the variations in the form of Saturn, the unequal opening of the “handles,” and their occasional entire disappearance.
But the merit of having explained scientifically all the phenomena of the ring of Saturn taken as one belongs to Huyghens (1655). Dominic Cassini first saw the black stripes in the ring (1684), and recognized its division into at least two concentric rings.
The spots on the sun were first observed through telescopes by John Fabricius of East Friesland, and by Galileo either at Padua or Venice.
Fabricius published his discovery in June, 1611, while Galileo did not make his generally known until May, 1612.
Galileo noticed that the same spots sometimes returned, and was persuaded that they belonged to the sun itself.
The difference in their dimensions when near the center of the sun's disk and on approaching his margin attracted his attention; though it does not appear, from his second letter to Welser, in August, 1612, that he had observed the inequality of the ashycolored border at the two sides of the black nucleus, when approaching the limb of the sun. Fabricius, like Galileo, recognized the fact that the spots belonged to the sun itself, and also noticed that spots which he had observed disappeared and returned again, and these phenomena taught him the rotation of the sun, which had been conjectured by Kepler before the discovery of the spots.
The cycle of admirable discoveries, which scarcely occupied two years, was completed by the observation of the phases of Venus.
As early as 1610, Galileo noticed the sickle or crescent form of the planet, and, according to a practice much in vogue in those days, concealed the important discovery in an anagram.
Huyghers, with an object-glass polished by himself, first, discovered one of Saturn's satellites (the sixth) in March, 1655; and from a superstitious notion, entertained by some astronomers of the period, that the number of satellites could not exceed that of the primary planets, did not seek to discover any more of them.
Four other of Saturn's moons were discovered by Dominic Cassini; the seventh, or outermost, which has great alternations of brightness, in 1671, the fifth in 1672, and the third and fourth in 1684, with an object-glass of Campani's having a focal length of 100-136 feet. The two innermost, or the first and second, were discovered in 1788 and 1789 by Herschel, with his colossal reflector.
The second satellite offers the remarkable phenomenon of performing its revolution around the principal planet in less than one of our days.
The telescopes which Galilco constructed himself, and others which he used for observing Jupiter's satellites, the phases of Venus, and the solar spots, magnified four, seven, and thirty-two times in linear dimensions, — never more.
The Arenarius of Archimedes says very distinctly that “Aristarchus had confuted the astronomers who imagined the earth to be immovable in the center of the universe; that this center was occupied by the sun, which was immovable, like the other stars, while the earth revolved around it.”
Turning the invention to immediate account, Galileo discovered the spots on the sun. On January 7, 1610, he discovered three of the moons of
Jupiter, and the fourth shortly after.
His discovery of the phases of
Venus furnished another proof of the truth of the heliocentric theory.
The papal persecution which followed Bruno, who was burnt in 1600, and Galileo, who was denounced in 1616, are familiar to readers.
Galileo was visited by
Milton while in prison, became blind, then deaf, and died a prisoner of the Inquisition, which followed him after death, denied his right to make a will, to be buried in consecrated ground, or to have a monument.
The nineteenth century has attended to the latter duty.
The eye-glasses of the Galilean telescope were double-concave.
Kepler first pointed out the possibility of making telescopes with two convex lenses.
Scheiner, in 1650, reduced it to practice.
De Rheita made one with three lenses; he also made a binocular telescope.
The focal length of some of these telescopes was immense.
Campani, in 1672, made one for Louis XIV., the focal length of whose object-lens was 136 feet. Auzont had one of 600 feet. Huyghens had one of 123 feet, which is still preserved by the Royal Society of
London.
These were used without tubes.
Huyghens adopted the plan of placing his object-glass in a short tube, having a balland-socket joint, at the top of a tall pole.
To this tube was connected a string, so that the observer could bring its axis in line with that of another short tube containing the eye glass.
The great difficulty of managing these cumbrous instruments led to the invention of the reflecting-telescope.
The difficulty of the problem is thus stated by
Whewell, in his “History of the
Inductive Sciences” :—
“If we endeavor to augment the optical power of this instrument, we run, according to the path we take, into various inconveniences, — distortion, confusion, want of light, or colored
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images.
Distortion and confusion are produced if we increase the magnifying power, retaining the length and the aperture of the object-glass.
If we diminish the aperture, we suffer from loss of light.
What remains, then, is to increase the focal length.”
Comes Mr. Reeve with a twelve-foot glasse.
Up to the top of the house, and then we endeavoured to see the moon, and Saturn, and Jupiter, but the heavens proved cloudy. — Pepys's Diary, 1668.
The May-pole which stood close to the site of the church of St. Mary-le-strand was begged in 1717 by Sir Isaac Newton, and removed to Wanstead, where it was used in raising the largest telescope then known. — pennant's London.
Telescopes are of two kinds, reflecting (or catoptric) and refracting (or dioptric). In the former, an image of the object to be viewed is produced by a concave reflector; in the latter, by a converging lens.
Reflecting telescopes are of four kinds.
The Gregorian telescope (
A,
Fig. 6272) was invented by
James Gregory of
Aberdeen in 1663.
It has an annular metallic speculum and a smaller concave speculum placed in the axis of the tube, at a distance from the larger speculum greater than its focal length.
The eye-piece is placed in a smaller tube at the extremity of the longer tube.
The Cassegrainian telescope (
B,
Fig. 6272), was invented by Cassegrain in 1672.
It is similar to the Gregorian, except that the smaller speculum is convex instead of concave, and that it is placed in the tube at a distance from the larger speculum less than its focal length.
A telescope of this kind, having a speculum of 4 feet diameter and 30 feet 6 inches focal length, was sent from
England to
Melbourne in 1868.
The speculum weighed 3,500 pounds, and was composed of 32 parts copper and 1477 tin.
Fig 6271 is a representation of the Melbourne telescope, which is really a very fine instrument.
The tube is of open work, in order to avoid the air-currents, which with such powerful and delicate instruments are sometimes very annoying in large closed tubes.
The mirror in this telescope is placed at the bottom of the tube, and a hole is pierced in the center, in which is placed the
ocular, in this respect like the Gregorian and Cassegrainian.
The hole does not make any practical difference in the working of the mirror.
The clock-work for driving the instrument is seen attached to it. The astronomer directs his
ocular to a little plane mirror at the upper part of the tube, where the star image is reflected from the parabola.
The Newtonian telescope (
C,
Fig. 6272) was invented by
Sir Isaac Newton in 1669.
A large concave reflector is placed at one end of the tube.
At a distance from the larger mirror less than its focal length is placed, at an angle of 45° to the optic axis of the telescope, a plane reflector, by which the rays proceeding from the object are turned to the side of the tube and viewed by an eye-piece whose axis is at right angles to the axis of the large tube.
Foucault used a prism, which was an improvement, especially in making solar observations.
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Reflecting-telescopes. |
Fig 6273 represents the reflecting-telescope made by
Sir Isaac Newton's own hands, and presented to the Royal Society, in whose possession it remains.
Fig. 6274 is the new reflecting-telescope in the national observatory of
Paris.
It has a parabolic glass mirror, weighing 1,300 pounds, at the bottom of the tube, from which the rays are reflected to a plane mirror in the head of the telescope, the image there formed being examined through a system of magnifyingglasses known as the
ocular. The telescope is suspended on a mov able axis, and has all the usual adjustments in altitude and azimuth with the automatic clock-work motion.
The platform on which the observer stands travels on the rails shown on the floor, and is omitted from the engraving.
The weight of the movable portion of the telescope is 9 tons; its cost, $40,000.
In the Gregorian, Cassegrainian, and Newtonian instruments the central rays are lost.
In the telescope (
D,
Fig. 6272) of
Sir William Herschel, invented in the latter part of the last century, the large speculum is inclined to the axis of the tube, and the image of the object observed is brought to the interior edge of the tube, where it is examined by the eye-piece instead of through the medium of the second reflector.
By this instrument
Sir William Herschel made his numerous and important discoveries.
Herschel's 7, 10, and 20 foot reflectors were made about 1779 He discovered
Uranus, March 21, 1781.
In 1779, he completed his 40-foot telescope, which was taken down in 1822.
The largest instrument of this class ever made was erected by Lord Rosse on his estate at Parsonstown,
Ireland, in 1842.
The diameter of the speculum is 6 feet, having a reflecting surface of 4,071 square inches, — more than double that of
Herschel's 40-foot telescope, with which he discovered the moons of Saturn and
Uranus, and which had a polished surface of but 1,811 square inches.
A foundry was constructed for the special purpose of casting the speculum, which is composed of copper and tin combined in very nearly their atomic proportions, or 126.4 copper to 58.9 parts tin.
The metal was molten in cast-iron crucibles 2 feet in diame-
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ter and 2 1/2 feet deep; these were allowed to heat ten hours previous to the introduction of the metal, which required ten more hours to acquire sufficient fluidity for pouring.
The bottom of the mold was formed of layers of hoop-iron turned to the required shape, and its sides of sand.
When sufficiently cooled, the cast was removed to the annealing-oven, where it was allowed to remain for sixteen weeks previous to undergoing the grinding process.
This was effected under water, by means of a cast-iron grinder having grooves cut length wise, transversely, and circularly in its face, emery being used to abrade the surface of the speculum and wear it to a truly circular form; this operation required six weeks.
The tube of this huge telescope, including the speculum-box, is 56 feet long, and is made of 1-inch deal boards hooped with iron.
On the inside, at distances of 8 feet, are also iron rings 3 inches deep and 1 inch broad, for strengthening the sides.
The diameter of the tube is 7 feet, and it is fixed to masonry in the ground by a universal joint, to allow it free movement in any required direction.
At 12 feet distance on each side, walls are built 72 feet long and 56 feet high at their highest parts, the walls being 24 feet distant from each other, and lying exactly in the direction of the meridian.
These allow the telescope a motion of but about 15 degrees on either side of the meridian, in an east and west direction: but in the opposite direction, that of the meridian, it may be lowered until nearly parallel with the horizon, when directed south, and so as to point to the north pole of the heavens in the other direction.
Elevation or depression is effected by means of a chain and windless, and the telescope being counterpoised in every direction, two men can perform these operations with great facility, though the total weight is about 15 tons, the speculum alone weighing 3 tons.
The total cost of the instrument was not less than £ 12,000.
Specula for reflecting-telescopes are now made of silvered glass.
Its use was first suggested by
M. Foucault.
Dr. Henry Draper of New York has been very successful in constructing specula of this kind; his process is described in the fourteenth volume of the “Smithsonian contributions to knowledge.”
M. Secretan of
Paris has constructed one for the observatory of
Marseilles exceeding 2 1/2 feet in diameter.
The advantages possessed by glass are that it weighs but from one half to one third as much as speculum metal, that it can be made much thinner, it is more easily wrought, and the loss of light by reflection from the silvered surface is comparatively trifling, while that from a metallic mirror amounts to from one third to one half; besides, should the silvering become tarnished, it can be removed by solution, and replaced without the necessity of regrinding the speculum.
The following remarks on the
apertures and
powers of telescopes, by
Mr. Tomlinson, will be interesting:—
The largest achromatic telescopes, such as those at Dorpat and Kensington, have each a clear opening of 13 inches, while that of Lord Rosse's reflector is 6 feet. Taking the diameter of the pupil of the eye at 1/8 inch, the former instruments admit 10,816 times and the latter 331,776 times the quantity of light which is received from any object by the unassisted eye. But as every speculum absorbs about half the light that it receives, the latter number must be reduced to 165,888.
These numbers, then, show how much the area of any object may be magnified by these telescopes without rendering it less bright than it appears to be to the naked eye; and their square roots, 104 and 407, show their magnifying powers in such a case.
We may also, from the size of the aperture of any other telescope, estimate what is called its absolute or penetrating power, which is independent of its length or internal arrangements, and depends solely on the size of the object-glass.
The number obtained by the above rule shows how many times farther any object may be seen with the telescope than without it, supposing its brightness to remain the same at all distances as it does in vacuo.  |
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Great reflecting-telescope of Paris. |
The magnifying power is totally independent of this, and can be made as great as we please, however small the telescope; but it is obvious that as long as the quantity of light admitted is the same, the more the image is enlarged the fainter it will be, its brightness being always proportional to the quotient of the absolute power divided by the magnifying power.
If the latter exceed the former, as it does in astronomical telescopes, the object will be less bright than to the naked eye; but if the absolute exceed the magnifying power, the object will be seen brighter with the telescope than without it.
The telescope at the
Washington Observatory, having double the aperture of either of the refractors referred to above, of course admits four times the quantity of light.
In this respect it is, however, surpassed in a nearly fourfold ratio by the great reflector of Lord Rosse.
A refractingtelescope in its simplest form consists merely of a double convex lens (the objectglass), which forms an image of the object to be viewed, and a second and smaller double convex lens, called the
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eye-piece, used as a simple microscope to examine the object formed by the first.
For perfection of result, the object-glass is made double or triple, to neutralize certain optical inconveniences, called spherical and chromatic aberration, and the eye-glass is generally composed of two lenses suitably combined.
The telescope of Galileo (
A,
Fig. 6276) has a double convex object-glass and a double concave eye-glass.
The common operaglass is made on this principle.
The eye-pieces of telescopes are the Ramsden, or
positive, and the Huyghen, or
negative.
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Eart of Rosse's 6-foot reflector. |
The positive (
B,
Fig. 6276) has two plano-convex lenses, with the convex sides toward each other.
The inner one is the field-glass, and the outer the eye-glass.
Their focal lengths are equal.
It is suited for micrometers and other instruments having wires in the focus of the object-glass.
The negative eye-piece (
C,
Fig. 6276) has two plano-concave lenses, the convex sides of both being turned toward the object-glass.
The ratio of the focal lengths is usually 3 to 1, the latter representing the eye-glass. — G. Chambers. See lens; achromatic lens.
The equatorial telescope is so mounted that its vertical axis points toward the pole, having an inclination corresponding to the latitude, so that a single motion, one of rotation around that inclined axis, will cause the line of sight, the optical axis, to trace upon the sphere a circle corresponding to that in which any heavenly body appears to move.
The circles increase or diminish as the telescope is moved upon its horizontal pivot, changing the angle between the line of sight and the inclined axis, just as the circles apparently described by the heavenly bodies increase or diminish according to their polar distance.
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Refracting-telescopes. |
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Telescope mounted on a pillar and claw-stand. |
The equatorial of the
Cincinnati Observatory was purchased in
Munich by
Professor O. M. Mitchell, the originator and director of the observatory.
The object-glass has a diameter of nearly 12 inches. The death of this talented and Christian gentleman in the service of his country has been a great loss to science and to the social circle, where he was highly esteemed for his modesty and merit.
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Telescope with motion in altitude and azimuth. |
Large refractors are now universally mounted equatorially, and are made of dimensions which but a few years ago were deemed impracticable.
The improvement in this respect is in a large degree due to the exertions of
Mr. Alvan Clark, of
Cambridgeport.
Mass. who, like most other great improvers of the telescope, is a self-taught artist.
It would seem that opticians, like poets, are born, not made.
Mr. Clark's first objectglass-es.
strange to say, were made for
England, but in 1862 he completed one having a clear aperture of 18 1/2 inches for the
Chicago.
Astronomical Society.
This was followed by the great telescope of 26 inches aperture for the
United States Naval Observatory
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at
Washington, completed in November. 1873.
Mr. Clark is confident of being able to produce an object-glass of 5 feet 6 inches clear aperture and 75 feet focal length.
The largest that had previously been made were those at the observatories of
Cambridge, Mass., and Pulkowa,
Russia, each having a clear aperture of 15 inches. These were large in comparison with the instruments that had previously been used, those at Dorpat and
Kensington having but 13 inches aperture.
In the
United States, at that time, among the largest were those at.
| Cincinnati | 12 inches aperture; 17 feet focal length. |
| West Point | 9 3/4 inches aperture; 14 feet focal length. |
| Washington | 9 1/8 inches aperture; 14 ft. 4 1/2 in. focal length |
| Alleghany City | 13 inches aperture. |
The disks for
Clark's lenses are made by Chance & Co., of
Birmingham, England.
The crucibles are of clay, and are built up gradually in rings of about 2 inches in hight, the process requiring a whole year for its completion.
Optical glass of the best quality is then selected and crushed, and the fragments separated according to their specific gravity by a hydraulic separator, in the manner employed for treating ores.
Those of uniform quality and size are selected and melted by the most intense heat of a Siemen's gas-furnace; the mass is then cooled very slowly, and the central portion sawn out. This may be reheated until sufficiently fluid, and molded to approximately the desired shape.
The disks are then tested, to ascertain if the glass is homogeneous and free from flaws.
This is effected by throwing the light from a lamp through a lens on one side of the disk, and placing the eye in the focus of the lens at the other side.
Any imperfections thus appear greatly magnified, and if not removable by grinding, cause the rejection of the piece, at least for a lens of the size for which it was intended.
Polarized light also affords a very delicate test as to whether or not it has been equally annealed.
The disk is ground upon concave plates of cast-iron of the proper curvature by pushing it back and forth, at the same time giving a slow rotary movement.
Emery, with water, is used as an abradant, finer sizes being successively used.
The polishing is effected by coating the tool with a thin layer of pitch, which is pressed into the proper shape; this is covered with rouge and water, and the disk manipulated as in the grinding process.
The pieces forming the lens are, when fin ished, put together and set on edge, facing a luminoous point placed at a distance equal to twelve or fifteen times the focal distance of the lens; the appearance of this point through the lens is examined with an eye-piece of high power, or by the eye placed in the focus; the optician thus judges what parts have an excess or a deficiency of curvature.
The polishing process is then repeated upon such portions as are too prominent; the lens is re-examined, and this process repeated until no departure from the proper curve at any point can be detected.
This in the case of large lenses is a long and tedious operation, requiring many trials and repetitions of the process.
The object-glass has but two pieces, — a plano-concave lens of flint and a double convex lens of crown glass.
The
Washington equatorial (shown in Plate LXIX.) has a clear aperture of 26 inches and a focal length of 31 feet 6 inches, its total length being 32 feet 6 inches. The rough glass for the object-lens was received by
Messrs. Clark in December, 1871, and was ground, polished, and finished in November, 1872.
Another year was required to finish the tube and complete the other parts of the instrument.
The tube is of thin steel, in three pieces, and is mounted upon a pillar of brick supported by an arched foundation of bluestone, and capped by a block of sandstone weighing about two tons.
The dome inclosing the instrument is 41 feet in diameter and 25 feet high.
It rests upon a tower of equal diameter and 21 feet in hight.
For lightness, and in order that the temperature may be maintained at that of the air outside, it is made of pine, covered with galvanized iron.
It rests upon thirty-two iron rollers, running upon a circular track, and is rotated by a reaction-wheel driven by water from a main of the
Washington aqueduct, and operating also the clock-work mechanism and the conical pendulum of the instrument.
The circles are each read by two microscopes reaching from the eye end of the telescope along its sides to the rightascen-sion circle, which is divided to seconds, the declination circle being divided to tenths of a minute of circle.
The instrument has the usual number of eye-pieces and ring micrometer, a filar micrometer, an Airey double-image micrometer, a mica-scale micrometer, and a spectroscope.
The disks for the object-glass cost $7,000; the whole instrument, about $48,000; and the building for its accommodation, about $14,000.
Next in size to this is a telescope constructed by
Messrs. T. Cooke and
Sons of
York, England, for
Mr. R. S. Newall, the contractor for the first Atlantic cable.
This has an object-glass of 25 inches aperture and 29 feet focal length, and is represented as being a superior instrument.
Another English telescope, constructed by
Rev. Mr. Craig in connection with
Mr. Cravatt, F. R. S., has a 24-inch object-glass, with a focal length of 76 feet, the length of the tube being 85 feet. This was mounted on Wandsworth Common.
The great focal length of this appears remarkable, the tendency of late having been to reduce this as far as possible in proportion to the aperture of the lens.
A telescope is now in course of construction in
Dublin for the Austro-Hungarian government.
Its object-glass will have an aperture of 27 inches, and its total length will be about 32 feet.
The ratio of focal lengths, mode of construction, powers, proportions, adjustments, and mounting are fully explained in many treatises on the subject.
See
Pearson,
Loomis, Heather,
Simms, etc., etc.
See under the following heads:—
| Astronomical telescope. | Newtonian telescope. |
| Binocular telescope. | Object-glass. |
| Cassegrainian telescope. | Opera-glass. |
| Comet-seeker. | Perspective-glass. |
| Equatorial telescope. | Reflecting-telescope. |
| Eye-glass. | Refracting-telescope. |
| Field-glass. | Submarine telescope. |
| Finder. | Teinoscope. |
| Galilean telescope. | Telemeter. |
| Gregorian telescope. | Terrestrial telescope. |
| Herschelian telescope. | Transit-instrument. |
One of the most important applications of the telescope is to instruments for making observations upon the heavenly bodies, both to determine their positions and for ascertaining the latitude and longitude.
By means of a very small telescope of low power attached to his sextant, the navigator is enabled to observe with far greater accuracy the contact of the sun's disk with the horizon, or the contact of the limbs of the sun and moon, than he could with the plain sight tube.
In fact, the lunar method of finding the longitude would be of little value without the telescope, and the nicety and precision now attained in observations on land, both in astronomical observations and in triangulations for surveys, would be utterly impracticable.
Sir John Herschel says: “The honor of this capital improvement” (the application of the telescope to the measurement of astronomical angles) “has been successfully vindicated by Derham to our young, talented, and unfortunate countryman,
Gascoigne, from his correspondence with
Crabtree and Horrockes, in his, Derham's, possession.”
The passages cited by Derham from these letters leave no doubt that, so early as 1640,
Gascoigne had applied telescopes to his quadrants and sextants,
with threads in the common focus of the glasses; and had even carried the invention so far as to illuminate the field of views by artificial light, which he found “
very helpful when the moon appeareth not, or it is not otherwise light enough.”
These inventions were freely communicated by him to
Crabtree, and through him to his friend Horrockes, the pride and boast of British astronomy, both of whom expressed their unbounded admiration of this and many other of his delicate and admirable improvements in the art of observation.
Gascoigne, however, perished at the age of twenty-three at the battle of
Marston Moor; and the premature and sudden death of Horrockes, at a yet earlier age, will account for the temporary oblivion of the invention.
It was revived or reinvented in 1667 by
Picard and Auzont (Lalande, Astron.), after which its use became universal.
Morin, even earlier than
Gascoigne (in 1635), had proposed to substitute the telescope for plain sights; but it is the thread or wire stretched in the focus, with which the image of a star can be brought to exact coincidence, which gives the telescope its advantage in practice; and the idea of this does not seem to have occurred to
Morin.
