The Telescope

Telescopes are of two kinds, namely, refracting and reflecting telescopes. The refracting telescope consists of an object-glass which forms an image of the object, and an eye-glass by which the image is viewed. The reflecting telescope consists of a concave mirror which receives light from the distant object, and reflects it so that the rays converge to a focus and form an image, the image being viewed by an eye-glass. The terrestrial telescope consists of two telescopes like the preceding - which are called astronomical telescopes, and give an inverted image - the second inverting the inverted image of the first, and so giving an upright image. Eye-pieces generally have two lenses, and have names according to the position of the focus. Ramsden's eye-piece has two lenses, the focus being just beyond the field lens. It is called a positive eye-piece, and it can be used as a magnifying glass. Huyghens' eye-piece also has two lenses, the focus being between the two. It is called a negative eye-piece, and cannot be used as a magnifying glass. These compound eyepieces enable us to get rid of spherical and chromatic aberration. The achromatic object-glass is made by joining together two lenses, one of flint glass and the other of crown glass. The dispersion is made equal and opposite, but the bending powers are unequal. A lens is equivalent to a number of prisms placed base to base, the outer prisms having a greater angle to cause the rays to bend more, so that all the rays may come to one point, called the focus. The magnifying power of a telescope is found by dividing the focal length of the object-glass by the focal length of the eye-piece.

The Equatorial Telescope

The equatorial is an ordinary telescope, mounted in such a way that it can easily be directed to any part of the heavens. The polar axis is parallel to the earth's axis, that is to say, it is inclined at an angle equal to the latitude of the place, at Washington about 39°, at London about 51½°. The telescope can be moved round the polar axis in a plane which is parallel to the earth's equator, and this motion is said to be motion in right ascension. The telescope can also be moved up and down in a plane at right ancles to the earth's equator, and this motion is called motion in declination. Whatever part of the skies an object is in, the equatorial can be directed to it, and the object can be kept constantly in view, because there is a kind of clock which drives the instrument round at the same speed at which the earth is turning round.

The Transit Instrument

The transit instrument is a telescope mounted on a horizontal axis, so as to be capable of moving in the meridian only. It is used to determine the exact moment at which celestial bodies cross the meridian, that is, when they are in a true north or south position. It is also used for determining the declination of celestial objects, that is, how far in angular measures these bodies are from the celestial equator.

The Sidereal Clock

The sidereal clock is similar to an ordinary clock, but it is regulated to keep accurate time with the apparent diurnal movements of the stars, instead of with the mean sun. It shows the* same time as clocks and watches only once in a year, namely, at the Vernal Equinox, about the 21st of March. It gains about four minutes each day on the ordinary clock, and in a year it gains a whole day, so that there are 366 sidereal days and only 305 solar days in one year. The sidereal noon occurs when the first point of Aries passes the meridian, and the hours are reckoned from 0 to 24. The time by the sidereal clock at which a celestial body crosses the meridian is equal to the right ascension of that particular object. Conversely, if the exact right ascension of a star be known, the error of the clock can be determined by observing a transit of the star.

The Chronograph

The chronograph consists of a cylinder covered with paper, and made to rotate uniformly by clockwork. It is connected electrically with the sidereal clock, which, as it ticks, makes dots on the paper at equal distances by means of a recording pen, and these dots represent seconds. Fractions of a second are recorded by the observer touching a key, which causes a second pen to make a dot on the cylinder as it turns round. This dot would come between two second dots, and the distance is measured from these. In this manner the 1/100 or 1/1000 of a second can be estimated. The small fractions of a second obtained by the chronograph are necessary in fixing the right ascension and declination by the transit instrument.

The Micrometer

The micrometer is used for measuring small arcs. It consists of two wires, which can be brought together or separated at pleasure by means of a screw. An equatorial star appears to move through about 15° in one hour, 1° in four minutes, 15' in one minute, or 15" of arc in one second of time. The distance that the wire moves for one turn of the screw is found by allowing a star to pass from one wire to the other, and then allowing 15" of arc for every second of time taken in so doing. The diameter of the moon, the sun, or a planet can be estimated in angular measure by the micrometer, and then, knowing the distance of these objects, their size can be calculated from a knowledge of the relation that exists between the radius of a circle and its circumference.

The Theodolite

The theodolite is used for measuring horizontal and vertical angles, that is, altitude and azimuth. It consists of a small telescope, which can be moved up and down, and the inclination is shown by a graduated circle, called the altitude circle. The telescope can also be twisted around a vertical axis, and the angular distances of objects from the north point of the horizon measured, that is, azimuth.

The Sextant

The sextant is chiefly employed on board ship for observing the altitude of the sun, lunar distances, etc., in the determination of latitude and longitude. It consists of a telescope, through which the observer looks. Opposite to the telescope is a mirror, half silvered and half plain, so that he can see directly through the plain part to an object, and he can bring a second object to coincide with the first by means of a second mirror attached to the movable arm, which reflects its light on to the silvered part of the first mirror, and from thence through the telescope. The reading on the sextant then gives the angular distance between the two objects.