If the angles r L p, and p E r, were equal to each other, the telescope would not magnify, and they would be equal, if the lenses were of equal focal distance. Hence as the magnifying power of the telescope is produced by making the focal distance of the eye-glass less than that of the object-glass, it will easily be perceived, that the greater the difference of the focal lengths, the greater will be the magnifying power. It is found, however, that they may be so dispropor tionate, that the increased magnifying power is overbalanced by the indistinct: ness which ensues. In order, therefore, to obtain a great magnifying power with the preservation of just proportion, these telescopes have sometimes been made one hundred feet, or upwards, in length ; and, as they were mostly used for astronomical purposes, they were frequently used without a tube. The object lens was fixed on the top of a pole, in a frame capable of being moved by a cord or wire, in any required direction, and the fixed in a short tube, was held in the hand, or fitted to another frame, about the height of the observe:, so as to be capable of a simultaneous movement. A telescope of this descrip tion was called an denial telescope. Its use was evidently very incommodious ; but, such were the great pains taken by philosophers, in exploring the wonders which even the imperfect telescopes, at first constructed, promised to lay open, that with such an instrument, the five satellites of Saturn, and many other remarkable objects, were discovered.
The length of common" refracting telescopes must be increased in the propor tion of the square of the increase of their magnifying power • so that, in order to magnify twice as much as before, with the same degree of light and distinct ness, the telescope must be lengthened four times ; and to magnify three times as much, nine times. On this account, their unwieldy length, when great powers are desired, is unavoidable. The breadth of an object-glass adds nothing to the magnifying power; for, whatever the latter may be, the image will be equally formed at the distance of its focal length ; but the brilliancy of the image will be increased by the breadth, as a greater number of rays will then diverge from every point of the image.
The magnifying power, and the field of view, of this telescope may be in creased by using two plano-convex lenses, combined so as to act like one glass; and such a combination is now generally employed. If two plano-convex lenses be used, the curvature of both conjointly, will be less than the curvature of a single lens of equal magnifying power ; the combination therefore improves the eye-glass of a telescope, because the aberration of the rays passing through it, will be less than through a single lens of the same focus. Suppose I K to be a plano-convex lens, of which the focus is at F, so that an object placed at F would be seen magnified through it. If another lens L M, be placed between the first lens and its focus, the focus of the rays passing through both will be shortened, and will fall at about the distance f, so that, when thus combined, they will act like a single lens of much greater curvature. The telescope
called a night-glass, is simply a common astronomical telescope with tubes, and made of a short length, with a small magnifying power. It generally magni fies from 6 to 10 times. It is 'used by navigators at night, fur the purpose of discovering objects that are not very distinct, such as vessels, coasts, rocks, &c. From the smallness of its magnifying power it admits of large glasses being used, and consequently has a well-enlightened field of view.
The astronomical telescope, by the use of two additional eye-glasses, shows objects in their right position, and becomes a terrestrial or land telescope; and is sometimes called a perspective glass. This telescope is shown by the follow ing cut. The rays of each pencil coming from the image L M, of the object I K, emerge parallel from the lens A B, and having crossed at its focus 0, they continue in that direction to the lens E F ; in consequence of which they form an image S T, at the focus of the second lens ; and again diverging, they fall upon the third lens C D, in the same manner as they did upon the the lens A B; therefore after their emergence from this last lens, they fall parallel upon the eye at G. But as the last image S T, is not inverted as at L M, but in the same position as the object I K, the eye sees a true or upright picture, as if the rays had come directly from the object. The last lens, or the one nearest the eye, is now generally made of two plano-convex lenses, instead of a double convex one. By this means, all the best terrestrial telescopes contain four lenses in the tube next the eye.
The telescope of the celebrated Galileo consists of a convex object-glass, and a concave eye-glass, as represented by the following cut. The distance be tween the two lenses is less than the focal distance of the object-glass; • but the concave glass is situated so as to make the rays of each pencil fall parallel upon the eye, as is evident by conceiving the rays to go back again through the eye-glass towards 0 : E 0 being the focal length of the eye-glass. The field of view of the Galilean telescope does not depend, as in those with convex glasses, upon the size of the eye-glass, but upon the breadth of the pupil of the eye ; because the lateral pencils of rays diverge from the axis of the eye-glass at their emergence from it. On this account, the eye should be placed as near. to the eye-glass as possible, in order that it may receive the greatest lumber of pencils. No nearness of the eye, however, will wholly prevent the field of view from being more confined than with convex eye-glasses of equal curvature ; but this disadvantage is counterbalanced by the valuable property of superior dis tinctness.