Page:The American Cyclopædia (1879) Volume XV.djvu/652

 C22 TELESCOPE its rnsting from the dampness caused by the breath. Mr. Bell has received two patents in the United States for the telephone, one bearing date March 7, 1876, and the other Jan. 30, 1877; and his instrument is now in practical use in many cities in this country and in Europe. Among others who have made extensive experiments in the transmission of sounds to a distance are Prof. A. E. Dol- bear, of Tufts college, Massachusetts; G. M. Phelps, of New York ; and Thomas A. Edison, of New Jersey, the inventor of the phonograph (see PHONOGRAPH). The Dolbear telephone is essentially the same as the present form of the Bell instrument, and was devised by its inventor in the autumn of 1876 without any knowledge of the mechanism used by Bell. Prof. Dolbear claims that he was the first to utilize magneto-electric currents for the trans- mission of speech. The Phelps instrument is also the same in principle, but differs in ex- terior form. In the Edison telephone tho diaphragm is made to vibrate against a disk of carbon, which forms a part of an electric cir- cuit, tho current of which is generated by a two-cell battery. The resistance of this disk varies in accordance with the pressure on the diaphragm, and occasions a proportionate variation in the strength of the current, which is thus made to reproduce all the characteristics of vocai waves. TELESCOPE (Gr. r^Ae, far, and aitoirhv, to view), an instrument for aiding the eye in viewing distant objects. The general con- struction of the telescope is based upon the property possessed by a convex lens or con- cave mirror of converging to a focus the rays of light falling upon it from any object, and of forming at that focus an image of the ob- ject itself. This image may be rendered vis- ible, as in the camera obscura, by interposing at the focus a white screen, a plate of ground glass, or a cloud of light smoke within which the image will appear suspended. But if the rays be allowed to proceed without interrup- tion, and the eye be placed in the axis of the lens or mirror and at the proper distance from the focus, the image will be seen more dis- tinctly than before ; and if the focus be near- er to the eye than to the lens, the apparent dimensions of the image will be greater than the apparent dimensions of the object itself. This is the simplest, though not the common form of the telescope. Usually a second lens, of the object ; and thus is constituted the or- dinary telescope, which in its elementary con- struction consists of an "object glass" or "ob- ject mirror," of as large dimensions as prac- ticable, and an " eye lens," which enables the eye to receive the image under the greatest practicable angle. In fig. 1, M is the object glass and N the eye lens. The inverted image & a of a distant object A B is formed between the eye lens and its principal focus, and the eye lens then gives a magnified image of it, b' a'. The object glass is always necessarily convex, and the mirror concave, but the eye glass may be either ; if convex, it is placed at the proper distance beyond the focus, and, the rays having crossed, the image then appears inverted ; if concave, as in the common opera glass, it is placed within the focus, and objects appear in their natural position. The magni- fying power of the instrument is measured by dividing the focal distance of the object glass by that of the eye piece ; the illuminating FIG. 1. Astronomical Telescope. of shorter focus than the first, is introduced near the image, the effect of which is to in- crease still further the apparent magnitude FIG. 2. Terrestrial Telescope. power depends mainly on the size of the ob- ject glass. In the terrestrial telescope, com- monly called spy glass, the image is produced in its natural position. To effect this two ad- ditional lenses, O and P, fig. 2, called condens- ing glasses, are introduced between the real image and the eye lens. The object A B pro- duces an inverted and smaller image at b a. The lens O being at the distance of its princi- pal focal length from b a, the rays which fall on P will be parallel, and the image a' V in the principal focus of P will be erect, as will also be the magnified image a" b". It is believed by many authorities that the theory of both the telescope and the microscope was known to Roger Bacon, and the telescope is said to have been used by Digges before the 17th century ; but the first really definite accounts of the invention date from the latter part of the year 1608. Magnifying lenses had long been known, and even the compound micro- scope had been invented by the Jansens nearly 20 years before this date ; a discovery which has somewhat embarrassed the study of the question before us from confusion of the by no means explicit terms in which both instru- ments are described. But it is now generally conceded that the honor of making the first telescope belongs to one of two individuals, Hans Lippersheim, a spectacle maker in Mid- delburg, and Jacob Adriansz, called also Me- tius, a native of Alkmaar. Lippersheim, on Oct. 22, 1608, presented to his government three instruments with which "one could see things at a distance," applying at the same