The progress in astronomical science throughout the nineteenth century has naturally to a great extent depended upon the advances made both in the optics of the telescope and the way in which they are mounted, either with circles to record exact times and positions, or made to move so as to keep a star or other celestial objects in the field of view while under observation. The perfection of definition and the magnitude of the lenses employed in the modern instrument have been responsible for many important discoveries. Ever since the telescope was invented—Galileo’s lens was smaller than those used in spectacles—men’s minds have been concentrated on producing instruments of larger and larger size to fathom the cosmos to its innermost depths. At the beginning of the century we were, as we have seen already, in possession of reflectors of large dimensions; Herschel’s four-foot mirror, the instrument he was using in 1801, which had a focal length of forty feet, was capable of being employed with high magnifying powers; and it was the judicious use of these, on occasions when the finest of weather prevailed, that enabled him to enrich so extensively our knowledge of the stellar and planetary systems. For the ordinary work of astronomy, however, especially when circles are used, refractors are the more suitable instruments. This form suffers less from the vicissitudes of weather and temperature, and is, therefore, more suited where exact measurements are required. Towards the end of the eighteenth century a Swiss artisan, Pierre Guinard, after many years of patient labor, succeeded in producing pure disks of flint glass as large as six inches in diameter. The modern refracting telescope thus became possible. In 1804 there was started at Munich the famous optical and mechanical institute, which soon made its presence felt in the astronomical world. Reforms in instrument making were soon taken in hand, and under the leadership of the great German astronomer, Bessel, great strides were made in instruments of precision. Fraunhofer, who had been silently working away at the theory of lenses, and making various experiments in the manufacture of glass, was joined, in 1805, by Guinard. In 1809 Troughton invented a new method of graduating circles, according to Airy the greatest improvement ever achieved in the art of instrument making. In 1824 Fraunhofer successfully completed and perfected an object-glass of 9.9 inches in diameter for the Dorpat Observatory. This objective might literally have been called a “giant,” for nothing approaching it in size had been previously made.