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telescope made in this way, the middle part of the object is only seen, or, in other words, the prospect is by it very much diminished.

Charles. How is that remedied?

Tutor. By substituting a double convex eye-glass g h (Plate v. Fig. 35.) instead of the concave one. Here the focus of the double convex lens is at E, and the glass g h must be so much more convex than o p, as that its focus may be also at E: for then the rays flowing from the object x y, and passing through the object glass op, will form the inverted image m E d. Now by interposing the double convex g h, the image is thrown on the retina, and it is seen under the large angle Dec, that is, the image med will be mag. nified to the size C E D.

James. Is not the image of the object in the telescope inverted ?

Tutor. Yes it is : for you see the image on the retina stands in the same position as the object; but we always see things by

having the images inverted : and, therefore, whatever is seen by telescopes con

structed as this is, will appear inverted to ! the spectator, which is a very unpleasant

circumstance with regard to terrestrial objects it is on that account chiefly used for celestial observations.

Charles. Is there any rule for calculating the magnifying power of this telescope ?

Tutor. It magnifies in proportion as the focal distance of the object-glass is greater than the focal distance of the eye-glass. Thus, if the focal distance of the object glass is ten inches, and that of the eye-glass only a single inch, the telescope magnifies the diameter of an object ten times : and the whole sunface of the object will be magnified a hundred times.

Charles. Will a small object, as a silver penny for instance, appear a hundred times larger through this telescope than it would by the naked eye?

Tutor. Telescopes, in general, represent terrestrial objects to be nearer and not larger: thus looking at the silver penny a hundred yards distant, it will not appear to be larger, but at the distance only of a single yard.

James. Is there no advantage gained, if the focal distance of the eye-glass, and of the object-glass, be equal ?

Tutor. None ; and therefore in telescopes of this kind we have only to increase the focal distance of the objectglass, and to diminish the focal distance of the eye-glass, to augment the magnifying power to almost any degree.

Charles. Can you carry this principle to any extent?

Tutor. Not altogether so : an objectglass of ten feet focal distance, will require an eye-glass whose focal distance is rather more than two inches and a half : and an object-glass with a focal distance of a huna dred feet, must have an eye-glass whose focus must be about six inches from it. How much will each of these glasses magnify ?

Charles. Ten feet divided by two inches and a half, give for a quotient forty-eight :

hundred feet divided by six inches, give two hundred : so that the former magnifies 48 times, and the latter 200 times.

Tutor. Refracting telescopes for viewing terrestrial objects, in order to show them in their natural posture, are usually constructed with one object-glass, and three eye-glasses, the focal distances of these last being equal.

James. Do you make use of the same method in calculating the magnifying power of a telescope constructed in this way, as you did in the last ?

Tutor. Yes; the three glasses next the eye having their focal distances equal, the magnifying power is found by dividing the focal distance of the object-glass by the focal distance of one of the eye-glasses. We have now said as much on the subject as is nesessary to our plan.

Charles. What is the construction of operaglasses, that are so much used at the theatre? Tutor. The opera glass is nothing more than a short refracting telescope.

The night telescope is only about two feet long; it represents objects inverted, much enlightened, but not greatly magnified. It is used to discover objects, not very distant, but which cannot otherwise be seen for want of sufficient light.

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