The magnifying power, and the field of view, of this telescope may be increased 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 distancef, 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 magnifies from 6 to 10 times.
It is used by navigators at night, for the purpose of discovering objects that are not very distinct, such as vessels, coasts, rocks, &e. 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-glas3es, shows objects in their right position, and become terrestrial or land telescope; and is sometimes called a perspective glass. This telescope is shown by the following 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 O, 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 between 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 O: E O being the focal length of the eye-gla3s. 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 number 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 distinctness.
The telescopes which we have hitherto described will only bear a small aperture, without exhibiting circular prismatic rings of colours, which are detrimental to their utility. Two causes contribute to this effect. 1. Spherical surfaces do not refract the rays of light accurately to a point; and 2. The rays of compounded light being differently refrangible, come to their respective force at different distances from the lens; the more refrangible rays converging sooner, of course, than the less refrangible. If the image of a paper painted entirely red, be cast, by means of a lens, upon a screen, it will be formed at a greater distance than the image of a blue paper cast by the same lens. The image of a white object is composed of an indefinite number of coloured images, the violet being the nearest, and the red the farthest from the lens; and the images of intermediate colours at intermediate distances. The whole image is therefore in some degree confused, though its extremities are most so; and this confusion being increased, not only by the magnifying power of the eye-glass, but also by the dispersive power which it has in common with the object-glass, the necessity for a certain proportion between the powers of the object and the eye-glass becomes indispensable
The late Mr. John Dolland, by making a compound lens of three different substances, of different refrangible powers, the rays of light which were too much dispersed by one convex lens, were brought nearer to a union with each. The telescopes made with an object-glass of this kind are now commonly used, and are distinguished by the name of achromatic telescopes, a term which signifies colourless. The object-glasses of Dolland's telescopes are composed of three distinct lenses, two of which are convex, and the other concave. The achromatic effect may be produced by the union of one convex and one concave lens, but not so perfectly as with three lenses.
The impossibility, however, of obtaining perfectly homogeneous glass, and the consequent failure of producing that complete correction of the aberration of the rays of light in the telescopes called achromatic, induced Dr. Blair to try the effects of fluid mediums; and his success was such as to induce him to give the term aplanatic, or free from error, to the object lenses he thus constructed. He made a compound lens, consisting of a plano-convex of crown glass, with its flat side towards the object, and a miniscus of the same material, with its convex side in the same direction, and its flatter concave next the eye; and the interval between the lenses he filled with a solution of antimony in a certain proportion of muriatic acid. The lens thus constructed did not exhibit the slightest vestige of any extraneous colour; but the invention, after a lapse of more than twenty years, has not come into general use, probably from the difficulty of preserving any fluid from growing turbid in the course of time.