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.
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.
Evidently Hevelius had at this time no notion of the effect of the plano-convex form of lens as such in lessening aberration, but he mentions a curious form of telescope, actually due to De Rheita, in which the objective is double, apparently of two plano-convex lenses, the weaker ahead, and used with a concave eye lens. If properly proportioned such a doublet would have less than a quarter the spherical aberration of the equivalent double convex lens.
Hevelius also mentions the earlier form of re-inverting telescope above referred to, and speaks rather highly of its performance. To judge from his numerous drawings of the moon made in 1643 and 1644, his telescopes were much better than those of Scheiner and Fontana, but still woefully lacking in sharp definition.
Nevertheless the copper plates of the Selenographia, representing every phase of the moon, placed the lunar details with remarkable accuracy and formed for more than a century the best lunar atlas available. One acquires an abiding respect for the patience and skill of these old astronomers in seeing how much they did with means utterly inadequate.
One may get a fair idea of the size, appearance, and mounting of telescopes in this early day from Fig. 8, which shows a somewhat advanced construction credited by Hevelius to a suggestion in Descartes’ Dioptrica. Appearances indicate that the tube was somewhere about six feet long, approximately two inches in aperture, and that it had a draw tube for focussing. The offset head of the mount to allow observing near the zenith is worth an extra glance.