It is not to be understood, however, that the great Lick telescope still maintains its supremacy. The Yerkes telescope, which was exhibited at the World’s Fair Exposition in 1893, at Chicago, had an object glass of 3.28 feet in diameter and a focal distance of 65 feet, and it moved around a central axis in a vast cupola or dome 78 feet in diameter. The Grand Equatorial of Gruenewald, at the recent Berlin Exposition, was even still larger, since its object glass was 3 feet 7 inches, or nearly 2 inches larger than the Yerkes. [Illustration: FIG. 195.--GREAT TELESCOPE, PARIS EXPOSITION. 1900.] Even these great instruments have now been excelled in the Grande Lunette, of the Paris Exposition, in 1900. When it is remembered that an increase in the diameter of any circular body causes, for every additional inch, a vastly disproportionate increase in the cross-sectional area and weight, it will readily be seen how handicapped the instrument maker is in any increase in the power of such a telescope. An increased diameter of a few inches in the glass lens means an enormous increase in the cross section, its weight and the difficulties attending its successful casting free from imperfections, and the perfect grinding and polishing of the lens. An increased length of the tubular case of the telescope is liable to involve, from the great weight, a slight bending or springing out of axial alignment when supported near the middle for equatorial adjustment, and a few feet increase in the diameter of the massive and movable steel dome add greatly to the weight and incidental difficulties of constructing and delicately adjusting it. The great Lunette, see Fig. 195, changes entirely the method of manipulating the telescope, and also, in a measure, its principle of action, so as to avoid some of these difficulties. Its tube, instead of being pointed upwardly through the slot of a movable dome, and made adjustable with the dome, is laid down horizontally on a stationary base of supporting pillars, and an adjustable reflecting mirror and regulating mechanism, called a “siderostat,” is arranged at one end, to catch the view of the star, or moon, and reflect it into the great tube, and through its lenses on to the screen at the other end. The tube is 197 feet long, and the object glass or lens is a fraction over 4 feet in diameter. There are two of these, which together cost $120,000. The siderostat is supported on a large cast iron frame, and is provided with clockwork and devices for causing the mirror to follow the movement of the celestial object which is being viewed. The entire weight of the siderostat and base is 99,000 pounds, the movable part weighs 33,000 pounds, and the mirror and its cell weigh 14,740. The mirror itself is of glass, weighs 7,920 pounds, is 6.56 feet in diameter, and 10.63 inches thick. To facilitate the free and sensitive adjustment of this great mirror its base floats in a reservoir of mercury.