achromatic (Art. 315). The eye-glass is made quite convex, so as to magnify the image formed by the object-glass.

Fig. 214 represents an astronomical telescope mounted for use. It rests upon a cast-iron stand, with three feet, called a tripod. The tripod supports a vertical axis, capable of turning around in its supports; the telescope is attached to the top of this axis by a hinge joint. These arrangements enable the observer to direct the telescope to any point of the heavens. The telescope may be raised or depressed by means of a rack, worked by toothed wheels, set in motion by a crank, as shown at the bottom of the figure.

A smaller telescope with a larger field of view is attached to it, to aid the observer in fixing the instrument on any object. This telescope is called the seeker.

The Terrestrial Telescope.

320. The TERRESTRIAL TELESCOPE differs from the astronomical telescope, in having two additional lenses, which together constitute what is called an erecting piece. The object of the erecting piece is to invert the image formed by the object-lens, so that objects may appear erect when viewed through the telescope.

B

m

Fig. 215.

Fig. 215 shows the course of the rays in a terrestrial telescope. AB is the object, o is the object-lens, m and n, two convex lenses, constitute the erecting piece, and is the eye-piece.

The erecting piece is so placed that the distance of the image, I, shall be at a distance from m, equal to its principal focal distance.

What is said of the object-glass and of the eye-piece? (320.) In what respect does the Terrestrial differ from the Astronomical Telescope? What is the object of the erecting piece?

A pencil of rays from A, falling upon the object-lens, is converged to a focus at the lower end of the image, I; the pencil proceeding from I, is converted into a beam by the lens, m, directed obliquely upwards, which beam is converged to a focus at i. In this manner an erect image, i, is formed, which is then viewed by the eye-piece, O. The eye-piece refracts the pencils coming from the image i, so as to make them appear to come from ab.

The angle, under which ab is seen, is the visual angle, and being greater than the angle under which AB would be seen without the telescope, the object is magnified.

The number of times which the visual angle of the image contains the visual angle of the object, is the magnifying power of the telescope.

The terrestrial telescope is used at sea and on land for viewing objects at a distance. It may, for convenience, be mounted in the same way as the astronomical telescope shown in Fig. 214.

321.

Reflecting Telescopes.

A REFLECTING TELESCOPE is one in which the image of a distant object is formed by means of a reflector or speculum, which image is then viewed by an eye-piece. The eye-piece is either a single lens or a combination of lenses.

One of the first telescopes of this description was constructed by NEWTON, and this is the only one of the kind which we shall describe in detail,

Newtonian Telescope.

322. Fig. 216 shows a NEWTONIAN TELESCOPE, as con. structed by M. FROMENT, of Paris, with improvements introduced by that distinguished physicist..

Describe the course of the rays in the terrestrial telescope. What is the magnifying power? What is the use of the terrestrial telescope? (321.) What is a reflecting telescope? (322,) Describe the Newtonian Telescope,

Fig. 217 shows the same telescope in section, and indi cates the course of the rays of light.

M is a concave mirror placed at the bottom of a long tube. This reflector tends to form a small image of an object at the other end of the tube. But before the rays reach the image, they are intercepted by a prism of glass, mn, so arranged that the rays enter its first face without

deviation, and strike its second face so as to be totally reflected, which causes the image to be formed at ab. The image thus formed is viewed by an eye-piece through the

B

m

a

M

side of the telescope. The eye-piece in this telescope is made of two plano-convex lenses, as shown in the figure, the combined effect of which is to cause the image to appear in the position BA, giving a great power to the telescope.

Fig. 216 shows the manner of viewing the image. It also shows a small seeker attached to the tube of the main instrument, which is used in directing the telescope to any required object.

Herschel's Telescope.

323. SIR WILLIAM HERSCHEL, of London, modified the Newtonian telescope by inclining the mirror, M, so as to throw the image to one side of the tube, where it could be viewed by a magnifying eye-piece, the observer's back being turned towards the object.

The large telescope made by this eminent astronomer was forty feet in length, and the speculum had a diameter of about five feet. It was with this gigantic instrument that he made some of his most brilliant discoveries.

Lord Ross's Telescope.

324. LORD Ross, of Ireland, has recently constructed a reflect

Explain Fig. 216. (323.) What modification did HERSCHEL make in the Newtonian telescope? Describe HERSCHEL'S telescope. (324.) Describe LORD Ross's telescope.

ing telescope still larger than HERSCHEL'S. The tube is 56 feet in length, and the diameter of the reflector is more than 6 feet. The speculum weighs over 4 tons, and the entire instrument more than 18 tons. This telescope is supported by two walls of masonry 48 fect high, 72 feet long, and 24 feet distant from each other. strument is said to have cost the owner $60,000.

Microscopes.

The in

325. A MICROSCOPE is a modification of the telescope, for viewing near objects.

Microscopes, like telescopes, may be composed of a combination of lenses alone, or they may be composed of a combination of reflectors and lenses. Reflecting microscopes are but little uscd. We shall only describe the refracting microscope, of which there are two kinds, the simple and the compound.

The Simple Microscope.

The SIMPLE MICROSCOPE Consists of a double con

326. vex lens of short focal distance. It is usually set in a frame of metal or of horn, and held in the hand.

Fig. 218 shows the manner of using it. It is held at a distance from the object to be viewed, a little less than its principal focal dis

tance. In this

case, each pencil of light falling upon it will be deviated so

(325.) What is a Microscope? How may a microscope be constructed? (326) What is a Simple Microscope? Explain Fig 218.