der 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 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 eyeglass 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 De c, that is, the image m Ed will be magnified 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 constructed 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 surface 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 object-glass, 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 object-glass 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 hundred feet, must have an eye-glass whose focus must be about six inches from it. How much will each of these glasses magnify?

:

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

restrial 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 necessary 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.

CONVERSATION XX.

Of Reflecting Telescopes.

Tutor. This is a telescope of a different kind, and is called a reflecting telescope.

Charles. What advantages does the reflecting telescope possess over that which yesterday?

you described

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 mir

rors.

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

Tutor. In instruments, the exhibitions of