However, the telescope had another, if less glamorous, use in the 18th century. This was its use in positional astronomy, in the ever more precise measurement of the relative positions of objects seen in the heavens. Measurement had been the purpose served by pre-telescopic astronomical instruments, the sighting bars of the Ptolemaic observers of Alexandria and the elegant quadrants of Tycho Brahe. For a time after the invention of the telescope the professional astronomer resisted the innovation, but by the end of the 17th century the new optical instrument was being adapted to the quadrant and other instruments for the precise measurement of the positions of heavenly bodies in relation to the time-honored astronomical coordinates. By the late 18th century telescopes were found serving three relatively distinct purposes: the increased magnification of the sky in general (in which use Herschel’s 48-inch reflector had made all others obsolete): the more precise measurement of planetary and stellar positions (and, conversely, of the Earth’s shape) by means of the quadrant, vertical circle, zenith sector, and similar instruments; and the simple edification of the educated but not learned classes, who wished not only to see what the astronomer saw, but to have an instrument also useful for looking occasionally at interesting objects on earth. Of these three purposes the second was the most unimpeachably scientific. It is remarkable that the earliest American-made telescopes of which we have knowledge were made for this purpose and not for the mere gratification of the curiosity of the educated layman. These are the telescopes of the remarkable Philadelphia mechanic, David Rittenhouse (1732-96). In an atmosphere not unlike the intellectual democracy that characterized the formation of the Royal Society a century earlier in London, Rittenhouse began as a clockmaker and ended as president of the American Philosophical Society, our counterpart of the Royal Society, in Philadelphia. He demonstrated not merely that an instrument-maker was capable of being a scientist, but also that the work of the instrument-maker, as it had developed by the late 18th century, was in itself scientific work. One of several observers assigned by the Society to the observation of the transit of Venus in 1769, he constructed instruments of the most advanced types, apparently employing European lenses, and used the instruments himself. Of these, a 1¾-inch refractor mounted as a transit instrument stands in the hall of the Philosophical Society. It is probably the oldest extant American-made telescope.