ror there, inclined so as to reflect the light to the side of the tube, when 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.

533. 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.

534. 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.

A

B

Here the concave speculum is seen to be inclined a little to the lower side of the tube, so 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.

535. 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.

533. Cassegranian? Difference?

584. Herschelian? Where eye-piece? How observer sit? 535. First Herschelian telescope? What called? Next?

SIR WILLIAM HERSCHEL'S FORTY FEET REFLECTOR.

536. The speculum of this instrument is 4 feet in diameter, 3 inches thick, and weighed, before being ground, 2,118 pounds. 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 to use this telescope-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 observatious and discoveries which have immortalized his name, and which have so abundantly enriched and advanced the science of astronomy.

536. Herschel's forty feet reflector? Size of speculum? Length and weight? How mounted? Observer where?

Weight? Tube?
Usefulness?

537. 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 th 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 4,071 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 a universal hinge. It can be lowered to the horizon, and raised to the zenith, and lowered northward till it takes in the Pole star. Its motion from east to west is limited to 15 degrees. This magnificent instrument is situated at Burr Castle, Ireland. It was constructed by the Earl of Rosse, at an expense of $60,000.

537. Lord Rosse's telescope? Weight of speculum? Diameter? Thickness? Cooling? Tube? Entire weight? How mounted? What motion? Where located? Cost?

538. A Transit Instrument is a telescope used for observing the transit of celestial objects across the meridian, for the purpose of determining differences of right ascension, or obtaining correct time. They are usually from six to ten feet long, and are mounted upon a horizontal axis, between two abutments of mason work; so that the instrument, when horizontal, will point exactly to the south. It will then take objects in the heavens, when they are exactly on the meridian.

Let A D in the cut represent the telescope, and E and W the east and west abutments, between which it is placed. On the left is seen, attached to the mason work, a graduated circle; and on the eastern end of the axis of the telescope is seen an arm, n, extending to the circle, as an index. Now, suppose the index n to be at o, in the upper part of the circle, when the telescope is horizontal; then if the meridian altitude of the object to be taken is 100, the index must be moved 100 from o, as the degrees on the circle and the altitude of the object will correspond.

538. What is a transit instrument? Size? How mounted? (Describe parts as shown in the cut. How set the instrument for the meridian altitude of a star?)

539. An Astronomical Clock is a clock adapted to keep exact sidereal time (136).

Taking the vernal equinox in the heavens as the zero point, and reckoning 24 hours eastward to the same point again, the time-reckoning 150 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.

540. A Mural Circle is a large graduated circle, with a telescope crossing its center, used for the measurement of the altitudes and zenith distances of the heavenly bodies, at the instant of their crossing the meridian. They are usually fixed upon a horizontal axis, that turns in a socket firmly fixed in a north and south wall. The degrees, minutes, and seconds on the circle are read by means of microscopes, and indicate the altitude of the object.

In the cut, A is a reading microscope, and BCDE the wall to which the circle is at tached. The telescope would denote an altitude of about 400, which would leave 500 as the zenith distance.

539. An astronomical clock? How set? How indicate right ascension o objects?

540. Describe a mural circle? Its uses? How mounted? (How ascertain altitude and zenith distance by it?)