TELESCOPE. The telescope is an optical instrument by which the image of ea distant sents the reflecting astronomical telescope as invented in 1669 by Sir Isaac Newton, and called the Newtonian Reflector. In this the light traverses the entire length. of the tube, at the lower end of which it strikes the con cave reflector A, which sends it back as a cone of rays to the diagonal reflector B; thence it travels to the focus F, where it is magnified by the eye-piece E, as in the refracting tele scope. Fig. 3 represents the most popular object is magnified so that it may be examined as if it were but a fraction of its actual dis tance from the observer. This instrument was invented by the Dutch optician, Lippershey, early in the 17th century. The first use of it for astronomical observations was made in Florence, by Galileo, who in 1609 invented the form of reflecting telescope, called the Casse grainian, invented by Cassegrain in 1672. The optical principles here are the same as in the Newtonian form except that the convergent cone of rays from the mirror A is intercepted by a convex reflector B and sent back through an opening in the centre of the mirror A to type known as the Galilean telescope. His first instrument was constructed of two spec tacle lenses set at the ends of a section of a leaden organ-pipe. Its power was three diam the focus F, where the image is magnified by the eye-piece as before. The Herschelian re flector was invented by Sir William Herschel (q.v.) in 1780. His great telescope, built in 1789, had a mirror 48 inches in diameter and a tube 40 feet in length.
In these forms of telescope the image of the object as seen through the eye-piece is necessarily inverted, which is, of course, un important in astronomical observations, but is a defect to be overcome in the terrestrial instru ment. The most common type of terrestrial in this telescope, the power resulting will be 672 diameters, and eye-pieces of longer or shorter focus will give correspondingly lower or higher powers. The practical limit of power
in telescopes of the highest degree of accuracy is usually considered to be about 100 diameters per inch of aperture. Thus the 36-inch Lick telescope may be practically used with an eye telescope is the Spy Glass (Fig. 4). The ob jective A has the same office as in the refract ing astronomical telescope, and forms an il lumined image of the object at the focus F. This image is then magnified by a compound eye-piece made up of several lenses B, C, D and E, which carry the light to the eye in such manner as to erect the image and show it in its natural position. Fig. 5 represents the piece which would give a power of 3,600 di ameters. Such high powers are, however, sel dom required and can be used only in the clearest atmosphere. By far the larger pro portion of astronomical observations are made with powers of less than 1,000 diameters. In telescope observations, the two elements °power) and ulight,)) while equally important, are always in opposition. Thus an object viewed with a Galilean telescope, which is the same in prin ciple as the ordinary opera glass. In this case the objective A condenses the light from the object observed, and would naturally make a small image at F, but the cone of rays, before reaching the focus, is intercepted by the double concave eye-piece C, and thence conveyed to the eye in erect position. Fig. 6 shows the Porro Prism instrument, the most modern and efficient form of terrestrial telescope. The objective and the lenses are in the same rela tion to each other as was first illustrated in the astronomical telescope, Fig. 1. Two double reflecting, 90-degree prisms are inserted within the cone of rays between the eye-piece and the objective (Fig. 6); their mission being to erect the image which, in the ordinary refracting telescope, is shown inverted. This system was the invention of the Italian engineer, Porro, who patented it in France about 1850.