The great reflectors designed and constructed by Ritchey, formerly of Chicago and now of Pasadena, deserve especial mention. While connected with the Yerkes Observatory he constructed a two-foot reflector for that institution, with which he had exceptional success in photography of the stars and nebulæ. Later he built a 5-foot reflector, now at the Carnegie Observatory on Mount Wilson, California, with which the spiral nebulæ and many other celestial objects have been especially well photographed. Ritchey's later years have been spent on the construction of an even greater mirror, no less than 100 inches in diameter, which was completed in 1919, and has already yielded photographic results dealt with farther on, and far surpassing anything previously obtained. Theoretically this huge mirror, if its surface were perfectly reflective so that it would transmit all the rays falling upon it, would gather 160,000 times as much light as the unaided eye alone. Whether a 72-inch refractor, should it ever be constructed, would surpass the 100-inch reflector as an all-round engine for astronomical research, is a question that can only be fully answered by building it and trying the two instruments alongside. Probably three-quarters of all the really great astronomical work in the past has been done by refractors. They are always ready and convenient for use, and the optical surfaces rarely require cleaning and readjustment. With increase of size, however, the secondary spectrum becomes very bothersome in the great lenses; and the larger they are, the more light is lost by absorption on account of the increasing thickness of the lenses. With the reflector on the other hand, while there is clearly a greater range of size, the reflective surface retains its high polish only a brief period, so that mere tarnish effectively reduces the aperture; and the great mirror is more or less ineffective in consequence of flexure uncompensated by the lever system that supports the back of the mirror. Both types of telescope still have their enthusiastic devotees; and the next great reflector would doubtless be a gratifying success, if mounted in some elevated region of the world, like the Andes of northern Chile, where the air is exceptionally steady and the sky very clear a large part of the year. The highest magnifying powers suitable for work with such a telescope could then be employed, and new discoveries added as well as important work done in extension of lines already begun on the universe of stars.