TELESCOPE, one of the most useful optical instruments, consisting of a long tube furnished with several glasses, for the purpose of viewng distant objects.—This term is mostly applied to the larger kinds of telescopes; the smaller ones being called perspective-glasses, spy-glasses, opera-glasses; and a particular species, which is much brighter than any other, is known under the name of night-glasses.
Telescopes are of extensive utility, both in naval and military pursuits, but especially for contemplating the celestial bodies: hence the merit and honour of their original invention have been claimed by many ingenious men. According to the most accurate accounts, however, the authentic contriver was Zacharias Jansen, a Dutchman, who produced his first instrument in the year 1590. Since that period, various improvements and additions have been made by Galileo, Kepler, Huygens, Mersennus, Gregory, Short, Ramsden, Dollond, Adams, and numerous other philosophers and artists. The most important are, those accomplished by Dr. Herschel, whose grand instrument, when furnished with proper glasses, magnifies upwards of six thousand times.
It would be incompatible with our plan, to enter into the rationale, or an account of the principles on which telescopes are in general constructed. We shall, therefore, only remark, that a complete description of the machinery of Dr. H.'s noble instrument is inserted in the Philosophical Transactions of the Royal Society, Part ii. for 1795;—and we shall proceed to state the principal patents which have been granted to different individuals.
The first, within our knowledge deserving notice, is that obtained in April, 1791, by Mr. Robert Blair, for his method of improving refracting telescopes, and other dioptrical instruments; which is fully detailed in the 7th vol. of the Repertory of the Arts, &c.
In January, 1799, a privilege was granted to Mr. Cater Rand, for an improved military and naval telescope, serving to ascertain distances, &c. at sight, by means of a new micrometrical adjustment. His instruments are made refracting, achroamatical, or reflecting; and the micrometer is furnished with four parallel wires, fitted to brass, silver, or other metallic, converging and diverging, plates, together with other machinery; the whole of which is so arranged, that the intermediate distance between two unknown places may, together with the height, or size of such distance, be easily ascertained.—A full specification of this ingenious contrivance is inserted in the 12th vol. of the work above quoted, where it is illustrated by an engraving.
The last patent we shall mention, is that procured by Mr. Dudley Adams, in May, 1800; for his invention of a mode of rendering telescopes, and other optical glasses, more portable. This useful object is effected by employing certain springs, which are so contrived as to occupy the least possible space, while they render the instrument to which they may be applied, strong and steady, on being drawn out: and, in order that the several glasses may acquire their true positions or distances, when such joints or tubes are drawn out, he has invented certain flaunches, for fixing and retaining them in every situation, in which an observer might find it necessary to place them. For a minute account, the curious reader is referred to the 15th vol. of the Repertory of Arts, &c. where the patentee's improvements are exemplified by a plate.