21 



mirror, whose edge is grouiid accaratel}' at right angles to its 

 plane, and the edge also set up perpendicular to theplaneof the 

 instrument; (for which the purchaser must generally rely on 



reflected from C to B, thence refracterl again in B 11, and reflected by the mirror 7/' in 

 // E parallel to S A. Since the mirror //, and the axis of vision E H are fixed, the ray 

 a fj K also fixed, in all observations taken; and every object must be seen by rays nl- 

 timately coincident with B H. Suppose it be required to find the angulaV distance of 

 anotlier object s, from the object seen directly in the line E H; and that for this pur- 

 pose, and to make the image of s appear in E H, the index is moved to the position " 

 i g, through half the angular distance SAs of the objects, (the lines SA and EH 

 being supposed the same, and the interval AH to be accounted for) ; then the inwe 

 will be seen by the ray s a, inflected, as before traced, in the lines a c, c b, h H, and 

 HE', and the thickness of the index glass being moderate, there will be an interval 

 between the place of incidence on it of the rays S A and* a, so small as to be imper- 

 ceptible, and to occasion no error. But if the thickness of this mirror were great, as 

 AD ox ad, and the rays to be reflected from D and d; the image of S would be visible 

 by the ray J? j», proceeding in ;> D, D B, B H, and f/\E ; and the image of s by the 

 ray t e, e d, d b, b H, H E. So tlrat when the adjustment was made, by the ray Rj) 

 incident at p ; the object s wouhl afterward be seen, and the angle s A S measured, by 

 the r>y t e, incident at e, considerably distant from p. If the object s was very remote, 

 the rays saand fe would be as it were parallel, and their incidences and course the same ; 

 but if the object s were near, as at r, then the incidences would diflcr, and the error of 

 observation be equal to tlie angle era, so much the greater, as the object is nearer, or ■ 

 as the compIeme«it of the aDgu^dr distance observed is greater: and the same will be the 

 caacin the fore as well as in the back observation; which latter may be made as true as 

 the former, if the line of direction of the sight be accurately fixed, by a long eye-tube, 

 or telescope rightly placed, and if the other requisites above mentioned be observed. 



Thus though in observing remote objects, and for naulical uses, no iHconvenience will 

 arise from the thickness of tlieindex glass ; (which if it be duly proportioned, as here 

 stated, to that of the horizon glass, and its edge truly formed, is doubtless the best and 

 surest mechanical organ for adjusting the latter); and though no error can hence arise 

 in performing the above described adjustment ; wherein the position of tiie index glass 



