Page:Collier's New Encyclopedia v. 09.djvu/336

LEFT TELEPHONE 284 TELESCOPE but its feeble currents are incapable of transmitting speech to a distance; hence most of the modifications in magnetic telephones have had the design of in- creasing the power, as by using both poles of the magnet, and in other ways. The microphone, on the other hand, uses the power of a battery in its circuit, but in some respects appears less delicately sensitive than the free membrane. In practice it is very general to employ some form of microphone as the transmitting or speaking instrument, and the Bell tele- phone, or one of its modifications, as the receiving or hearing instrument. There are now many forms of tele- phone in use, the principal varieties being the bi-telephone, in which there are two receivers, one for each ear; the capillary telephone, in which electro-capillarity is used to produce telephonic effects; the chemical telephone, in which chemical or electrolytic action is utilized; the elec- trostatic telephone, which utilizes elec- trostatic disturbances in the reproduc- tion of sound; the reaction telephone, in which two mutually reacting coils are used; and the thermo-electric telephone, in which a thermo-electric battery is used. The last-named telephone has never been used in practice. In 1892 a long-distance telephone was erected be- tween Chicago and the larger E. cities and such service has been extended everywhere. In 1902 a patent was ob- tained in the United States for a wire- less telephone service. De Forrest and others have since filed many patents. In 1915 the American Telegraph and Tele- phone Co. sent a message from Washing- ton to Paris without wires and subse- quently from Washington to Hawaii. Poi-table telephones were largely used in the World War. See Wireless Telegraphy and Tele- THONY. TELESCOPE, an optical instrument to assist the naked eye in examining distant objects. It does this in two ways — ^by magnifying the apparent angular dimen- sions of the object, and by collecting more light than the pupil of the eye could alone do to form the image on the retina. One point should be noted that is often misunderstood. If any object is large enough to be seen distinctly as a surface by the naked eye, it cannot be made to appear any brighter in a telescope; in fact, it will always be fainter on account of the loss of light by reflection, and ab- sorption in the object glass and eye piece. If, however, the object is faint and rather small to the naked eye, like a dis- tant clock face in the twilight, it can be read more easily in a telescope. If the person should approach the clock face till it subtended the same angular dimen- sions as its image in the telescope, he could read it more distinctly, and it would appear moi-e brightly illuminated than in the telescope at the first distance, provided he shielded his eyes from sur- rounding stray light, which is another way in which the telescope sometimes helps in seeing faint objects. If, how- ever, the sources of light examined are from points, like the apparent images of a star, which remain practically points under all magnifying powers, then these images will appear more and more bright the larger the aperture of the telescope, provided the whole of the emergent pencil of rays from the object glass enters the pupil of the eye. With large telescopes and low magnifying powers the emergent pencils are generally too large for this. Essential Parts. — The essential parts of a telescope are: (1) Either a concave mirror or a system of lenses, called the objective, for bringing to a point in the focal plane the cone of rays which pro- ceeds from each point of the luminous object and falls upon the objective (for objects at a very great — practically in- finite — distance these cones of rays that fall upon the objective may be regarded as cylinders). (2) An eye piece consist- ing of one or more lenses for examining and magnifying the image formed in the focal plane of the objective. (3) A tubo or framework of some kind to hold the objective and the eye piece in their proper relative positions. These are all the essential parts of the simple telescope itself, regarded as an optical instrument only. But for the purposes of convenient use many other accessories of mounting and conveniences of handling are re- quired. Moreover, a telescope fitted only as an optical instrument, for looking at objects simply, is of very little scientific value in these modern days of exact mea- surement and quantitative determination of everything examined, so that in many forms of astronomical, geodetic, and la- boratory instruments the optical tele- scope or microscope is only one of the important parts of the whole, the acces- sory apparatus attached to it, or to which it is attached, constituting in some in- struments the more important feature of the whole. Moreover, for purposes of greater convenience in handling and pointing or of comfort to the observer, many considerable modifications of the optical part itself are introduced, as in the "broken back" transit, where the cone of rays from the objective of the eye piece is reflected at right angles in its course and brought out at one end of the axis, where the observer works in one position without having to follow the rev-