12 OS AN IMPROVED REFLECTING CIRCLE. 



oppofite one another, might however be advantageous, in 

 order to correct the errors of eccentricity ; but, in my opi- 

 nion, a greater number ought not, in any cafe, to be ufed. 

 It may be ufed Before I conclude this Paper, I (hall remark, that my im- 

 X^tdefcone'iT P rovemcnt ma >' ^ c partially applied to a circle, where the 

 attached. telefcope and the horizon glafs arc attached, or fixed, to the 



main frame of the inftrument. The flying nonius, acting 

 then with the fingle centre index, will only give the fame 

 re fult as Borda's circle ; but this conilruclion feems to me 

 greatly preferable to all the other plans executed till now ; 

 the whole apparatus being more folitl and fimple, and its ufe 

 not liable to the errors which arife from the motion of the 

 horizon index. 

 It aftbrds a me- With this conftruclion, we may likewife employ a method 

 ^Je^ionTbe? ** afcertaining the ■ place where the parallelifm of the glades 

 ginning from a was obferved to take place, and of fettjng the index afterwards 

 *ft" in. the'fame fituatmn, as often as is neceflary for the repetition 



of the observations. A piece may be ufed, fo contrived as to 

 be attached to, or detached from, one fide of the index, by 

 means of a ferew ; and provided befides with other fcrews^ 

 to fatten it to any part of the limb. This rectification piece, 

 being previoufly attached to, and carried with, the index, 

 muft be fixed in the fituation it occupied when the contacl of 

 the images was obferved. The index will then be detached 

 from it, in order to obferve the diflance, and afterwards mufr 

 be brought back to the fame pofition as before, contiguous to 

 the rectification piece. The like alternate procefs may be 

 repeated ; and the flying nonius, going with the index in the 

 motions forwards, and Handing ftill in the motions backwards/ 

 will give the multiple of the obferved angle, without per- 

 forming the obfervation for the parallelifm more than once in 

 the beginning. In Mayer's circle, as well as in Borda's, 

 there is . a great objection to any attempt for that purpofe ; 

 beeaufe, as the horizon glafs moves round with the index, its 

 perpendicular pofition is deranged by the inequalities of the 

 plane of the limb; but, in my construction for multiplying 

 with the horizon glafs fixed, that inconvenience is removed ; 

 and the method of afcertaining the identical pofition of the 

 glafles may be employed in practice, with advantage, it being 

 done when the index is at the fame point of the frame. By 

 fuggefting this idea, I do not, however, mean to* reprefent 



it 



