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CONVERSATION XX.

Of Reflecting Telescopes.

TUTOR. This is a telescope of a different kind, and is called a reflecting teles scope.

Charles. What advantages does the reBlecting telescope possess over that which you described yesterday?

Tutor. The great inconvenience attending refracting telescopes is their length, and on that account they are not very much used when high powers are required. A reflector of six feet long will magnify as much as a refractor of a hundred feet.

James. Are these, like the refracting telescopes, made in different ways ?

Tutor. They were invented by Sir I. Newton, but have been greatly improved since his time. The following figure (Plate vi. Fig. 36.) will lead to a description of one of those most in use. You know that there is a great similarity between convex lenses and concave mirrors.

Charles. They both form an inverted focal image of any remote object, by the convergence of the pencils of rays.

Tutor. In instruments, the exhibitions of which are the effects of reflection, the concave mirror is substituted for the convex lens. TT (Fig. 36.) represents the large tube, and t t the small tube of the telescope, at one end of which is D F, a concave mirror, with a hole in the middle at P, the principal focus of which is at i K; opposite to the hole p is a small mirror L, concave towards the great one ; it is fixed on a strong

and may, by means of a long screw on the outside of the tube, be made to move

wire M,

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backwards or forwards. A B is a remote object; from which rays will flow to the great mirror D F.

James. And I see you have taken only two rays of a pencil from the top, and two from the bottom.

Tutor. And in order to trace the progress of the reflections and refractions, the upper ones are represented by full lines, the lower ones by dotted lines. Now the rays at c and e falling upon the mirror at d and F, are reflected, and form an inverted image

at m.

Charles. Is there any thing there to receive the image?

Tutor. No: and therefore they go on towards the reflector L, the rays from different parts of the object crossing one another a little before they reach L.

James. Does not the hole at P tend to distort the image ?

Tutor. Not at all; the only defect is, that there is less light. From the mirror L the rays are reflected nearly parallel througłz P, there they have to pass the plano convex lens R, which causes them to converge at e h, and the image is now painted in the small tube near the eye.

Charles. What is the other plano convex lens s for?

Tutor. Having by means of the lens R, and the two concave mirrors, brought the image of the object so nigh as at ab, we only want to magnify the image.

James. This, I see, is done by the lens s.

Tutor. It is, and will appear as large as cd, that is, the image is seen under the angle cf d.

Charles. How do you estimate the mag. nifying power of the reflecting telescope ?

Tutor. The rule is this : “Multiply the focal distance of the large mirror by the distance of the small mirror from the image m: then multiply the focal distance of the small mirror by the focal distance of the eye-glass; and divide these two products

by one another, and the quotient is the magnifying power.

James. It is not likely that we should know all these in any instrument we pos

sess.

Tutor. The following then is a method of finding the same thing by experiment. “ Observe at what distance you can read any book with the naked eye, and then remove the book to the farthest distance at which you can distinctly read by means of the telescope, and divide the latter by the former."

Charles. Has not Dr. Herschel a very large reflecting telescope ?

Tutor. He has made many, but the tube of the grand telescope is nearly 40 feet long, and 4 feet ten inches in diameter. The concave surface of the great mirror is 48 inches, of polished surface, in diameter, and it magnifies 6000 times. This noble instrument cost the Doctor four years' severe labour : it was finished August 28, 1789, on which VOL. 111.

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