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CONVERSATION XIX.

Of the Refracting Telescope.

TUTOR. We now come to describe the structure of telescopes, of which there are two kinds ; viz. the refracting and the refecting telescope.

Charles. The former, or refracting telescope, depends, I suppose, upon lenses for the operation ; and the reflecting telescope acts chiefly by means of mirrors.

Tutor. These are the general principles upon which they are formed ; and we shall devote this morning to the explanation of the refracting telescope. Here is one completely fitted up.

James. It consists of two tubes, and two glasses.

Tutor. The tubes are intended to hold the glasses, and to confine the boundary of the view. I will therefore explain the principle by the following figure (Plate v. Fig. 34.) in which is represented the eye A B, the two lenses, m n, o p, and the object x y. The lens o p, which is nearest to the object, is called the object-glass, and that m n nearest to the eye is called the eyeglass.

Charles. Is the object-glass a double convex, and the eye-glass a double concave ?

Tutor. It happens so in this particular instance, but it is not necessary that the eye-glass should be concave ; the object-glass must, however, in all cases, be

convex.

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Charles. I see exactly, from the figure why the eye-glass is concave : for the con. vex lens converges the rays too quickly and the focus by that glass alone would be at e : and therefore the concave is put

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near the eye to make the rays diverge so much as to throw them to the retina before they come to a focus.

Tutor. But that is not the only reason : by coming to a focus at E, the image is very small, in comparison of what it is when the image is formed on the retina, by means of the

concave

lens. James, explain the reason of all the lines which you see in the figure ?

James. I think I can ;-there are two pencils of rays flowing from the extremities of the arrow, which is the object to be viewed. The rays of the pencil flowing from x, go on diverging till they reach the

convex lens op, when they will be so re- fracted by passing through the glass, as to

converge, and meet in the point x. Now the same may be said of the pencil of rays which come from y; and, of course, of all the pencils of rays flowing from the object between x and y. So that the image of the arrow would, by the convex lens, be form

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Tutor. And what would happen if there were no other glass?

James. The rays would cross each other and be divergent, so that when they got to the retina, there would be no distinct image formed, but every point as x or y, would be spread over a large space, and the image would be confused.

To prevent this, the concave lens m n is interposed ; the pencil of rays which would, by the convex glass, converge at x, will now be made to diverge, so as not to come to a focus till they arrive at the retina : and the pencil of rays which would, by the convex glass, have come to a point at y, will, by the interposition of the concave lens, be made to diverge so much as to throw the focus of the rays to b instead of y. By this means, the image of the object is magnified. Tutor. Can

you tell the reason why the tubes require to be drawn out more or less for different persons ?

Charles. The tubes are to be adjusted, in order to throw the focus of rays exactly

on the retina : and as some eyes are more convex than others, the length of the focus will vary in different persons, and, by sliding the tube up or down, this object is obtained.

Tutor. Refracting telescopes are used chiefly for viewing the terrestrial objects ; two things, therefore, are requisite in them; the first is, that it should show objects in an upright position, that is, in the same position as we see them without glasses ; and the second is, that they should afford a large field of view.

James. What do you mean, sir, by a field of view ?

Tutor. All that part of landscape which may be seen at once, without moving the eye or instrument. Now in looking on the figure again, you will perceive that the concave lens throws a number of the

rays beyond the pupil c of the eye, on to the iris on both sides, but those only are visible, or go to form an image, which pass through the pupil ; and therefore, by a

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