[5] In Fig 13, _A_ is the support of the tube and focussing screw, _B_ the main mirror, an inch in diameter, _CD_ the oblique mirror, _E_ the principal focus, _F_ the eye lens, and _G_ the member from which the oblique mirror is carried. [Illustration: FIG. 13.—Newton’s Model of his Reflector.] His error in judgment doubtless came from lack of practical astronomical experience, for he assumed that the whole real trouble with existing telescopes was chromatic aberration, which in fact worried the observer little more than the faults due to other causes, since the very low luminosity toward the ends of the spectrum enormously lessens the indistinctness due to dispersion. As a matter of fact the long focus objective of small aperture did very creditable work, and its errors would not compare unfavorably with those of a spherical concave mirror of the wide aperture planned by Newton. Had he actually made one of his telescopes of fair dimensions and power the definition would infallibly have been wrecked by the aberrations due to spherical figure.[6] [6] In fact a “four foot telescope of Mr. Newton’s invention” brought before the Royal Society two weeks after his original paper, proved only fair in quality, was returned somewhat improved at the next meeting, and then was referred to Mr. Hooke to be perfected as far as might be, after which nothing more was heard of it. [Illustration: FIG. 14.—De Bercé’s sketch of Cassegrain’s Telescope.] It is quite likely that appreciation of this, and the grave doubts of both Newton and Huygens as to obtaining a proper parabolic curve checked further developments. About the beginning of the year 1672 M. Cassegrain communicated to M. de Bercé a design for a reflecting telescope, which eventually found its way into the _Philosophical Transactions_ of May in that year, after previous publication in the _Journal des Sçavans_. Figure 14 shows de Bercé’s rough original sketch. It differed from Gregory’s construction in that the latter’s elliptical concave mirror placed outside the main focus, was replaced by a convex mirror placed inside focus. The image was therefore inverted. The inventor is referred to in histories of science as “Cassegrain, a Frenchman.” He was in fact Sieur Guillaume Cassegrain, sculptor in the service of Louis Quatorze, modeller and founder of many statues. In 1666 he was paid 1200 livres for executing a bust of the King modelled by Bertin, and later made many replicas from the antique for the decoration of His Majesty’s gardens at Versailles. He disappeared from the royal records in 1684 and probably died within a year or two of that date.