Incidentally Hevelius, with perhaps pardonable pride, also explains the “Polemoscope,” a little invention of his own, made, he tells us, in 1637. It is nothing else than the first periscope, constructed as shown in Fig. 9, a tube c with two right angled branches, a fairly long one e for the objective f, a 45° mirror at g, another at a, and finally the concave ocular at b. It was of modest size, of tubes 1⅔ inch in diameter, the longer tube being 22 inches and the upper branch 8 inches, a size well suited for trench or parapet.
Fig. 8.—A Seventh Century Astronomer and his Telescope.
Even in these days of his youth Hevelius had learned much of practical optics as then known, had devised and was using very rational methods of observing sun-spots by projection in a darkened room, and gives perhaps the first useful hints at testing telescopes by such solar observations and on the planets. He was later to do much in the development and mounting of long telescopes and in observation, although, while progressive in other respects, he very curiously never seemed to grasp the importance of telescopic sights and consistently refused to use them.
Telescope construction was now to fall into more skillful hands. Shortly after 1650 Christian Huygens (1629-1695), and his accomplished brother Constantine awakened to a keen interest in astronomy and devised new and excellent methods of forming accurate tools and of grinding and polishing lenses.
Fig. 9.—The first Periscope.
By 1655 they had completed an instrument of 12 feet focus with which the study of Saturn was begun, Titan the chief satellite discovered, and the ring recognized. Pushing further, they constructed a telescope of 23 feet focal length and 2⅓ inches aperture, with which four years later Christian Huygens finally solved the mystery of Saturn’s ring.