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way of what may be termed universal wireless eters. A second instrument had the power of telephony for social, commercial and other pur- eight diameters and a later one 30. poses it is confidently expected by competent The many and widely varying forms of telephone engineers and inventors that its ulti- telescopes may all be grouped simply as tubes mate consummation is not beyond the possibil- (Milton, seeing Galileo's in Florence, called it ity of practical realization. In the meantime the «Optik Tube”), in which are placed the wireless telephony if only available,
several combinations of lenses or reflectors; mercially, for comparatively short distances, each combination, however, producing the one obviously could be installed to great advantage result, namely, first; gathering the light from the in the officer's room of every ship that floats object and concentrating it at the focus in a ocean, lake, river or harbor, because of the brilliantly illuminated but small image; and, fact that telephony requires no specially trained second, magnifying this image with a microoperator.
scope called an eye-piece. One of the common Bibliography.- Bucher, Vacuum Tubes in types is the refracting astronomical telescope Wireless Communication; Fleming, Radio (Fig. 1), in which A is the object-glass or Telegraphy and Radio Telephony); Goldsmith, objective, and F the focus, where the small Radio Telephony'; Maver, Wireless Teleg- image is formed. The two lenses, C and D, raphy and Telephony
form the microscopic eye-piece, which magWilliam Maver, JR., nifies this image. The first reflecting telescope Author American Telegraphy and Encyclo- was the Gregorian, invented in 1663 by James pedia of the Telegraph.
Gregory. It has not survived. Fig. 2 repreA
Fig. 1. — Astronomical Telescope.
TELEPHOT, or TELEPHOTЕ, any one sents the reflecting astronomical telescope as of several theoretical instruments designed to invented in 1669 by Sir Isaac Newton, and reproduce scenes at a distance by photography
called the Newtonian Reflector. In this the and the aid of electricity: Telephotography
light traverses the entire length of the tube, has been an interesting field for inventors, but
at the lower end of which it strikes the conno commercially successful machine has been cave reflector A, which sends it back as a cone developed.
of rays to the diagonal reflector B; thence it
travels to the focus F, where it is magnified TELESCOPE. The telescope is an optical by the eye-piece E, as in the refracting teleinstrument by which the image of •a distant scope. Fig. 3 represents the most popular
Fig. 2. — Newtonian (eye-piece on side of tube). object is magnified so that it may be examined form of reflecting telescope, called the Casseas if it were but a fraction of its actual dis- grainian, invented by Cassegrain in 1672. The tance from the observer. This instrument was optical principles here are the same as in the invented by the Dutch optician, Lippershey, Newtonian form except that the convergent early in the 17th century. The first use of it cone of rays from the mirror A is intercepted for astronomical observations was made in by a convex reflector B and sent back through Florence, by Galileo, who in 1609 invented the an opening in the centre of the mirror A to
Fig. 3. Cassegrainian (secondary mirror convex). type known as the Galilean telescope. His the focus F, where the image is magnified by first instrument was constructed of two spec- the eye-piece as before. The Herschelian retacle lenses set at the ends of a section of a Alector was invented by Sir William Herschel leaden organ-pipe. Its power was three diam- (q.v.) in 1780. His great telescope, built in
FIG:4. - Terrestrial Telescope (spy glass). telescope is the Spy Glass (Fig. 4). The ob- piece which would give a power of 3,600 dijective A has the same office as in the refract- ameters. Such high powers are, however, seling astronomical telescope, and forms an il- dom required and can be used only in the lumined image of the object at the focus F. clearest atmosphere. By far the larger proThis image is then magnified by compound portion of astronomical observations are made eye-piece made up of several lenses B, C, D with powers of less than 1,000 diameters. In and E, which carry the light to the eye in telescope observations, the two elements "power such manner as to erect the image and show and "light,” while equally important, are always it in its natural position. Fig. 5 represents the in opposition. Thus an object viewed with a
Fig. 5. - Galilean Telescope (opera glass). Galilean telescope, which is the same in prin- power of 100 diameters is four times as brilciple as the ordinary opera glass. In this case liantly illuminated as would be the case if an the objective Acondenses the light from the eye-piece giving 200 diameters were used. It object observed, and would naturally make a follows, therefore, that the observer will use small image at F, but the cone of rays, before the power best adapted to his purpose, both reaching the focus, is intercepted by the double as to magnification and light. concave eye-piece C, and thence conveyed to The telescope giving the minimum power is the eye in erect position. Fig. 6 shows the the opera glass, usually magnifying two and Porro Prism instrument, the most modern and one-half or three diameters, which is sufficient efficient form of terrestrial telescope. The for indoor use, while for outdoor use the Galiobjective and the lenses are in the same rela- lean binocular has a power of four or five tion to each other as was first illustrated in the diameters, and the prism binocular of six, astronomical telescope, Fig. 1. Two double- eight, 10 or even 12 diameters. The eightreflecting, 90-degree prisms are inserted within power is, however, considered as high as can the cone of rays between the eye-piece and the be held in the hands with sufficient steadiness objective (Fig. 6); their mission being to erect to give the best results. The power used in the image which, in the ordinary refracting terrestrial telescopes steadily mounted on a telescope, is shown inverted. This system was tripod usually ranges from 15 to 100 diameters, the invention of the Italian engineer, Porro, depending on the condition of the atmosphere. who patented it in France about 1850.
The most important element in a refracting The magnifying power of telescopes is usu- telescope is the objective, and, in a reflecting ally expressed in diameters and is indicated by telescope, the mirror or speculum. The objecthe ratio of the focal length of the objective tive of the early refracting telescope was a to that of the combination of lenses forming the double-convex lens, which could not give a diseye-piece. For example, the Lick telescope has tinct image because it separated each ray of a focal length of 56 feet or 672 inches. If, light into its various prismatic colors, and each therefore, an eye-piece of one inch focus is used color, having a refracting power different from