28 ON THE CONSTRUCTION AND USE OF 



an equatorial. But aftrr rcticctinfT on the fact that it was intended for photography, 

 and that absohite freedom from tremor was essential, a condition not attained in 

 'the equatorial when driven by a clock, and in addition that in the case of the moon 

 rotation upon a polar axis does not suffice to counteract the motion in declination, 

 I was led to adopt the other form. 



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 diffi- 

 culty of finding some objects — has not been a source of embarrasment. In fact 

 the instability of the optical axis in reflecting instruments, if the mirror is uncon- 

 strainedly 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. TJie Tube. 



The telescope tube is a sixteen sided prism of walnut wood, 18 inches in diameter, 

 and 12 feet long. The staves are | 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 per- 

 forated trunnions a, Fig. 28 (of which only one is shown), upon which it swings. 

 Surrounding the other end is a wire rope b V &", the ends of which go over the 

 pulleys c (c' not sho^'sai) 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 /. 



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 trvmnions, 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 h h' l>" , 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' (j". Over some of the same pu^lleys a thin wire rope 7i h' runs, but while 

 its ends are fastened to the lower part of the tube at h, the central parts go twice 

 around a roller connected with the Avinch i, near the eye-piece, thus enabling the 

 observer to move the telescope in altitude, without taking the eye from the eye-uiece. 



