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nothing, the image will be at reft while the mirror revolves round 

 BCiif therefore the ftem 5 C be fo turned by the adjufting fcrews, 



that 



fine of Ae angle CE!, CE being the coGne ; and leaft femiaxis the fine of the angle 

 CAI, made to fubtend an angle whofe cofine is alfo C E. 



Thefe ellipfes will be formed in any part of the mirror as well as the .middle of 

 it ; becaufe the fame change of poGtion of its furface during a revolution which is 

 prefcnted to the eye in whatever place it may view the image, will be the fame : alfo 

 the ellipfes will enlarge with the inelination or obliquity of the mirror to the axis 

 of motion or ftem of it, and their eccentricity will be inverfely as the altitude of the 

 objed, if the diftance of the eye and objea from the mirror does not vary -, for the 

 angles CEi and C A I, determining the ellipfe, enlarge with the angle B C l> or 

 HCh; but the lower the objed is, the nearer does the line of fight E S approach 

 the mirror, and the conjugate femiaxis of the ellipfe approaches to the vertex A of 

 the angle CA I, within which it is confined ; fo that this axis, which is the fine of 

 the angle CEI, diminiflies with the altitude of the objefl, while the angle C E /, 

 equal to twice BCb, remains unvaried. 



If the relative diftances of the obje£>; and the eye from the mirror be fuch, that 

 the direa interval between the image t and the line £ C is but little greater than 7C, 

 the viCble place or path of the image will degenerate into a circle : if the image is 

 at a lefs diftance from E C than this, its place will be again an ellipfe, whofe tranf- 

 verfe and conjugate axes will have changed place with thofe of the former one ; but 

 if the eye be at an infinite diftance with refpeft to the objeft, the place of the image 

 will be a right line at right angles to h n. Now from this enquiry refult the following 

 inftruftions : 



ift. That fuch an injudicious fituation of the eye and objeft might be pitched 

 upon for the adjuftment of the inftrument, that the image would appear to move in 

 a circle ; in which cafe it would be irapoffible to determine in what part the mirror 

 was moft elevated ; fo that the objed chofen for adjuftment (hould be much more 

 diftant than the eye, and alfo not too much elevated above the level of the mirror ; 



M 2 becaufe 



