TELESCOPE (from the Greek telheopoa, TnAfokoros," far-seeing"), an optical instrument consisting of a tube which contains a system of glass lenses having all their centres in one common axis, or a tube containing a metallic speculum in combination with such lenses : by either kind of instrument distant objects are caused to appear magnified, and more distinct than when viewed by the naked eye. Those which are constructed with glass lenses only are called diopteic, or refracting, and the others eatoptric, or reflecting telescopes. In the former kind the rays In the pencils of light which come from every part of the object viewed are, by the first lens on which they are incident, made to converge so as to form an image at the focus of the lens. In some cases the rays in each pencil are intercepted by a second lens, and, by its refractive power, are made to enter the eye in parallel directions : in other eases, the rays, after having crossed each other at the place where the image is formed, fall in a divergent state upon a second lens, and by it are refracted so as to emerge from it in parallel directions. Frequently, however, the parallelism of the rays is effected by two or more lenses in addition to that, called the object-glass, by which the image was formed. In reflecting telescopes an image is formed by the reflection of the rays in the pencils of light coming from the object, after having impinged upon the concave surface of the speculum : in some cases this image is viewed through one glass lens or more, but frequently the rays, before or after forming the image, are reflected from a second mirror, and are subsequently transmitted to the eye through lenses.
By these instruments objects even in the remotest depths of space are rendered accessible to human vision ; and terrestrial objects faintly visible in the.distance are brought, as it were, close to the eye. In the hands of astronomers they were the Means, almost immediately on being invented, of making more discoveries in the heavens than had been made during 5000 years previously; they form a valuable addi tion to the instruments employed by the mariner and the surveyor, and they will ever constitute the most agreeable companion of the traveller, by enabling him to distinguish, in every direction from him, objects which it might be difficult or impossible for him to approach.
In exhibiting the principles on which a telescope is constructed, it will be proper to commence with an explanation of the means by or to meet it in points beyond r and r, towards T : suppose the cur vature of this lens to be such that the red rays in the pencil P Q would, after refraction in both lenses, meet the axis in F (the ray taking which the image of an object is formed at the focus of a lens or of a reflecting mirror. With respect to a lens, if it be of the kind called (murex [ Lees], the rays in the pencils of light which proceed from every part of an object, as s F n, jig. l, in passing through the lens, supposing the latter to have a proper degree of curvature, are made to con. verge by the refracting power of the glass at points, as a, e, and I), and the assemblage of such points constitutes an image of the object : if a screen were placed at P perpendicularly to the axis el', the object would be represented on it, in an inverted position.
If the lens were of a concave form, the rays in the several pencils, after laming through it, would be made to diverge from one another, and consequently no imago could be formed : yet if the directions of the rays, after refraction, were produced backwards, they would unite between the lens and the object, in points corresponding to those which constitute the image formed by the convex lens.
If the rays in the pencils of light proceeding from different points, •, n,fig. 2, in an object are reflected from the surface of a concave mirror, supposing the latter to have a certain degree of curvature, those rays will unite in as many points, a, F, and li, and form an image of the object. If a screen were placed at F the object would be represented on it, in an inverted position. The rays in each pencil reflected from the surface of a convex mirror are made to diverge from each other ; and in that case no image is formed.