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this efFed and of the preceding diredions, (and the knowledge 



of the theory of any inflrmnent enables the reader to appreciate 



its merits, and alfo preclu^les injudicious attempts at alteration 



or improvement ; on which accounts I think the theory fhould 



aecompanv the defcription of every machine) j he may underftand 



it from confidering the following circumftances ; and indeed fince 



differences in the diftances of the near objeds,.by which alone 



this mirror, when not level, could fometirnes be adjufted *, will 



produce a variety of motions and places of the reflcdcd image, 



while the mirror is turned round the ptumb-line, even when the 



altitude of the objeds does not vary, and vice verfa, differences 



in the altitudes when the diftances are unvaried ; which might 



puzzle and difgufl the perfon adjufting it, and create doubts of 



its accuracy, it is the more neceffary to give feme explanation of 



its optical properties when it is propofed for ufe. 



In 



* Though the want of adjuftment of this level may be difcovercd in every obfer- 

 vation of the celeftial bodies, in which cafe their great diltaiice renders the place 

 of the eye, and confequently of the quadrant ufed, immaterial ; yet it can fcarcely 

 be adjufted by an obfervation of the fun, &c. unlefs they are very near the meridian, 

 when there might not be time enough to accomplifh it, and alfo unlefs the altitude 

 of the fun, &c. be not too great : therefore the fun will notanfwer well for adjufting 

 it in low latitudes, nor the moon except when at full : it will on thefe accounts be 

 moft generally convenient to adjufl; it by a near objeft, in which cafe the eye muft 

 be fixed, and then there will be a greater variety in the motions of the reflefted 

 image on the mirror, according to the relative diftances of the eye and objeft from 

 it ; and there ftiould be an attention to more circumftances in the procefs of adjuft- 

 ment : wherefore I thought it proper to give forae account of thefe particulars. 



