This shortly took the form of a genuine micrometer consisting of a pair of parallel blades in the focus, moved in opposite directions by a screw of duplex pitch, with a scale for whole revolutions, and a head divided into 100 parts for partial revolutions. With this he observed much from 1638 to 1643, measured the diameters of sun, moon and planets with a good degree of precision, and laid the foundations of modern micrometry. He was equipped by 1639 with what was then called a large telescope. His untimely death, leaving behind an unpublished treatise on optics, was a grave loss to science, the more since the manuscript could not be found, and, swept away by the storms of war, his brilliant work dropped out of sight for above a score of years. Meanwhile De Rheita (1597-1660), a Capuchin monk, and an industrious and capable investigator, had been busy with the telescope, and in 1645 published at Antwerp a somewhat bizarre treatise, dedicated to Jesus Christ, and containing not a little practical information. De Rheita had early constructed binoculars, probably quite independently, had lately been diligently experimenting with Descartes’ hyperbolic lens, it is needless to say without much success, and was meditating work on a colossal scale—a glass to magnify 4,000 times. But his real contribution to optics was the terrestrial ocular. This as he made it is shown in Fig. 6 where _a b_ is the image formed by the objective in front of the eye lens r, s and t two equal lenses separated by their focal lengths and _a′ b′_ the resultant reinverted image. This form remained in common use until improved by Dolland more than a century later. [Illustration: FIG. 7.—Johannes Hevelius.] A somewhat earlier form ascribed to Father Scheiner had merged the two lenses forming the inverting system of Fig. 6, into a single lens used at its conjugate foci. Closely following De Rheita came Johannes Hevelius (1611-1687) of Danzig, one of the really important observers of the seventeenth century. His great treatise _Selenographia_ published in 1647 gives us the first systematic study of the moon, and a brief but illuminating account of the instruments of the time and their practical construction. At this time the Galilean and Keplerian forms of telescope were in concurrent use and Hevelius gives directions for designing and making both of them. Apparently the current instruments were not generally above five or six feet long and from Hevelius’ data would give not above 30 diameters in the Galilean form. There is mention, however, of tubes up to 12 feet in length, and of the advantage in clearness and power of the longer focus plano-convex lens. Paper tubes, evidently common, are condemned, also those of sheet iron on account of their weight, and wood was to be preferred for the longer tubes.