2. The old Mural Circle is now being rapidly superseded by the Meridian Circle in the best observatories.

706. A Comet Seeker is a refracting telescope with a large aperture and short focal distance. As comets cannot be found by their right ascension and declination, but often have to be searched up, by sweeping around the heavens with a telescope, before they became visible to the naked eye, it is important to have telescopes that will cover considerable space-that is, of wide aperture and short focal distance.

A COMET SEEKER.

I

Such a telescope was

made by Mr. Fitz for Miss Mitchel, of Newport, R. I.

Miss Mitchel is an amateur astronomer, and has the honor of having discovered a number of new comets.

707. An Astronomical Clock is a clock adapted to keep exact sidereal time. Taking the vernal equinox in the heavens as the zero point, and reckoning 24 hours eastward to the same point again, the time-reckoning 15° to an hour-when an object crosses the meridian, will always represent the right ascension of the object. Hence right ascension is usually given in hours, minutes, and seconds; or in time by the astronomical clock, set by the vernal equinox.

Professor Mitchel, we believe, made some valuable improvements in astronomical clocks. A very fine instrument of this kind is located in the Dudley Observatory, at Albany, N. Y.

REFLECTING TELESCOPES.

708. The Reflecting Telescope is one in which the light is converged to a focus by means of a concave metallic reflector or speculum. Like the Refractors, they may be constructed with very little mounting; though for convenience in use, it is necessary to place the reflector in a tube.

The student should fully understand the difference between the two kinds of telebcopes, viz.: refractors and reflectors. In one respect they are alike, as they both converge the rays of light to a focus; but they do it by widely different processes, as the following pages will show.

706. What is a comet scekor? Why necessary? 707. What is an astronomical clock? 708. Describe a reflecting telescope. Simplest form?

SIMPLEST FORM OF A REFLECTING TELESCOPE.

B

A

A

In this cut, the light A is seen passing from the object on the right, and falling upon the concave surface of the reflector at B, from which it is reflected back to a focus, and enters the eye of the observer at C. This telescope has no eye-piece.

708. The focal distance of a concave reflector is equal to half the radius of the sphere formed by the concave surface produced. Hence to grind a reflector for a focus of 20 feet, it will be necessary to have the curve that of a circle whose radius is 40 feet.

B

FOCAL DISTANCE OF A CONCAVE REFLECTOR.

Here the curve of the speculum B is that of a circle, whose center is C; while the focus of the speculum is at D, which is only half the distance from B to C.

709. Reflecting telescopes are of several kinds-viz., the Gregorian, the Newtonian, the Cassegranian, the Herschelian, &c. The Gregorian Reflector has an aperture in the center of the speculum, and a small concave mirror in the focus of the specuun, which reflects the light back through the aperture to the eye-piece. In this way the observer is enabled to face the object, and to direct the telescope toward it, as if it were a refractor.

708. Foca, distance? 709. How many kinds of reflectors? Describe the Gregorian. Why called Gregorian?

Here the light A falls upon the speculum at B, and is reflected back to the small mirror C, by which it is thrown out through the aperture in the speculum, to the eye of the observer at D. The object is supposed to be off on the right, in the direction towards which the instrument is pointed. It is called a Gregorian telescope, after Mr. James Gregory, who first suggested the construction of reflecting telescopes.

710. The Newtonian Reflector is so called after Sir Isaac Newton, its inventor. Instead of a concave mirror in the focus of the speculum, he placed a plane mirror there, inclined so as to reflect the light to the side of the tube, where it was received by the observer.

NEWTONIAN REFLECTOR.

The light from the speculum is here shown falling upon the inclined mirror in the center, and reflected out to the eye of the observer.

711. The Cassegranian Reflector is so called from M. Cassegrain, its inventor. It resembles the Gregorian, except that the speculum placed in the focus of the reflector is convex instead of

concave.

The Herschelian Reflector differs from all others, in having no small reflector whatever; the light being reflected back to a focus at the top of the telescope, and near the edge of the tube, where the eye-piece is placed, and where the observer sits looking into the mirror with his back to the object.

HERSCHELIAN TELESCOPE.

Here the concave speculum is seen to be inclined a little to the lower side of the tube, that the parallel rays A are reflected back to the observer at B, at the side of the instrument, where the eye-piece is placed.

710, Newtonian reflectors? eye-piece? How observer sit?

T11. Cassegranian? Difference? Herschelian? Where

14*

712. The first telescope constructed upon this plan was that by Sir William Herschel, in 1782. This was called his 20 feet reflector, and was the instrument with which he made many of his observations upon the double stars. In 1789, he completed his forty feet reflector, until recently the largest telescope ever constructed.

713. The speculum of this instrument is 4 feet in diameter, 3 inches thick, and weighed, before being ground, 2,118 pounds.

712. First Herschelian telescope? What called? Next? 713. Herschel's forty feet reflector? Size of Speculum? Weight? Tube? Length and Weight? How mounted?

The tube is made of sheet iron riveted together, and painted within and without.

The length of the tube is 39 feet 4 inches, and its weight 8,260 pounds. It is elevated or lowered by tackles, attached to strong frame-work; and the observer, who sits in a chair at the upper end of the tube, and looks down into the reflector at the bottom, is raised and lowered with the instrument. Three persons are necessary o use this tele scope-one to observe, another to work the tube, and a third to note down the observations. A speaking tube runs from the observer to the house where the assistants are at work. By this telescope, the sixth and seventh satellites of Saturn were discovered; and it was the chief instrument used by its distinguished owner, in making the observations and discoveries which have immortalized his name, and which have so abundantly enriched and advanced the science of astronomy.

714. This is the largest reflecting telescope ever constructed. The speculum, composed of copper and tin, weighed three tons as it came from the mould, and lost about 4th of an inch in grinding. It is 5 inches thick, and 6 feet in diameter. It was cast on the 13th of April, 1842, and was cooled gradually in an oven for 16 weeks, to prevent its cracking, by a sudden or unequal reduction of the temperature. This speculum has a reflecting surface of 4071 square inches. The tube is made of deal wood, one inch thick, and hooped with iron. Its diameter is seven feet, and its length 56.

The entire weight of this telescope is twelve tons. It is mounted between two north and south walls, 24 feet apart, 72 feet long, and 48 feet high. The lower end rests upon an universal hinge. It can be lowered to the horizon, and raised to the zenith, and lowered northward till it takes in the Pole Star.

Observer where?
Diameter ?

Usefulness?

Thickness? Cooling?

714. Lord Rosse's telescope? Weight of speculuin? Tube? Entire weight? How mounted? What