28 ON THE CONST RUCTION AND USE OF 



an equatorial. But after reflecting on the fact that it was intended for pliotograpliy, 

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

 the equatorial wjien driven by a clock, aiul in addition that in the case of tlie 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 nia(l(>. For instance, 

 instead of counterpoising the end of the tul)e ((mtaining the mirror by extending 

 t]H> tube to a distance beyond the altitude or horizontal axis, I introduced a system 

 of coiuiterpoise levers Avhich 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 tlie greatest freedom from tremor, 

 the parts coming (|iii<kiy to rest after a mov(>ment. In the use of the telescope for 

 photography, as Ave shall see, the system of bringing the mass of the instrument to 

 complete rest b(>fore ex])()sing the sensiti\i' ])lnt<'. and only driAing that ])late itself 

 by a clock, is always adopted. 



The obvious disadvantage connected wifli tlic ;dt-azimutli monnting — the diffi- 

 culty of finding some objects — has not been a source of eml)arrasiii( ut. hi fact 

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

 strainedly su])])orted, as it shonld \h', renders them unsuitable for determinations of 

 position. A little ])atience will enable an obserscr to find all necessary tests, or 

 curious objects. 



The mounting is dixided into: a. The Tiilic ; and li. Tlic sup])orting frame. 



a. T/i' '/'iihr. 



The teles(o])(' tn])e is a sixteen sided ])rism of \\ alinit wood, IS inches in diameter, 

 aiul \'2 feet long. The staves are gof an inch tliiik, and are liooped together with 

 four bands of brass, capable of being tightened by screws. Inside the tube are 

 ])laced two rings of iron, half an inch thick, reducing the internal diameter to about 

 IG inches. At opposite sides of the ui)])er end of the tube are screwed the per- 

 forated trunnions a. Fig. '28 (of which only one is shown), u])on Avhich it swings. 

 Surrounding the other (nid is a Avire rope // />' />" . the ends of which go OA»'r the 

 pulleys c ((;' not shoAvn) on friction rollers, and terminate in disks of leatl (/ <!'. 

 These counterpoises are fasten(>d (m the ends of levers r i'\ Avhich turn beloAv on a 

 fixed axle/. 



By this arrangement as the tube assumes a horizontal ])osition and becomes, so 

 to speak, heavier, the counterpoises do the same, Avhile Avhen the tid)e becomes 

 perpendicular, and most of its Aveight falls upon the trunnions, the counterpoises are 

 carried mostly by their axle. A continual condition of e(iuilil)rium is thus reached, 

 the tube being easily raised or depressed to any altitude desired. Tt is necessary, 

 however, to constrain the Avirc rope hh' h" , to move in the arc of the circle described 

 by the end of the tube and ends of the levers and hence the tAvelve rollers or guide 

 pulleys (/ (/' (/". Over some of the same pulleys a thin Avirc rope 7/ 7/ rinis, but Avhile 

 its ends are fastened to the loAver part of the tube at h, the central i)arts go tAvice 

 around a roller connected Avith the Avinch /, near the eye-piece, thus enabling the 

 obserA^er to move the telescope in altitude, Avithout taking the eye from the eye-piece. 



