A great many modifications of the original idea have been made. For instance, instead of counterpoising the end of the tube containing the mirror by extending the tube to a distance beyond the altitude or horizontal axis, I introduced a system of counterpoise levers which allows the telescope to work in a space little more than its own focal length across. This construction permits both ends of the tube to be supported, the lower one on a wire rope, and gives the greatest freedom from tremor, the parts coming quickly to rest after a movement. In the use of the telescope for photography, as we shall see, the system of bringing the mass of the instrument to complete rest before exposing the sensitive plate, and only driving that plate itself by a clock, is always adopted. The obvious disadvantage connected with the alt-azimuth mounting--the difficulty of finding some objects--has not been a source of embarrassment. In fact the instability of the optical axis in reflecting instruments, if the mirror is unconstrainedly supported, as it should be, renders them unsuitable for determinations of position. A little patience will enable an observer to find all necessary tests, or curious objects. The mounting is divided into: a. The Tube; and b. The supporting frame. a. _The Tube._ The telescope tube is a sixteen sided prism of walnut wood, 18 inches in diameter, and 12 feet long. The staves are 3/8 of an inch thick, and are hooped together with four bands of brass, capable of being tightened by screws. Inside the tube are placed two rings of iron, half an inch thick, reducing the internal diameter to about 16 inches. At opposite sides of the upper end of the tube are screwed the perforated trunnions _a_, Fig. 28 (of which only one is shown), upon which it swings. Surrounding the other end is a wire rope _b b′ b″_, the ends of which go over the pulleys _c_ (_c′_ not shown) on friction rollers, and terminate in disks of lead _d d′_. These counterpoises are fastened on the ends of levers _e e′_, which turn below on a fixed axle _f_. By this arrangement as the tube assumes a horizontal position and becomes, so to speak, heavier, the counterpoises do the same, while when the tube becomes perpendicular, and most of its weight falls upon the trunnions, the counterpoises are carried mostly by their axle. A continual condition of equilibrium is thus reached, the tube being easily raised or depressed to any altitude desired. It is necessary, however, to constrain the wire rope _b b′ b″_, to move in the arc of the circle described by the end of the tube and ends of the levers and hence the twelve rollers or guide pulleys _g g′ g″_. Over some of the same pulleys a thin wire rope _h h′_ runs, but while its ends are fastened to the lower part of the tube at _b_, the central parts go twice around a roller connected with the winch _i_, near the eye-piece, thus enabling the observer to move the telescope in altitude, without taking the eye from the eye-piece.