11 



From this it appears, that to adjust the horizon glass pro- 

 perly by two near objects, the face of both mirrors should be 

 covered, except the middle parts only, or means must be 

 used to view the images by those rays only, which are inci- 

 dent on the middle of both mirrors But if according to Mr. 

 Ludlam's direction, the object C be seen directly through 

 the middle of the glass ft, and if the image of no other part 

 of the glass a, but its middle part also, can be seen by re- 

 flection from the middle of the mirror b; then no rays inci- 

 dent on any other part g of the index glass could be seen 

 to coincide with the object C. Suppose this to be effected 

 as Mr. Ludlam directs, by covering the index glass with a 

 piece of card-paper, equal in size to itself, and lying close to 

 it, having a black line marked on the middle perpendicular to 

 the plane of the instrument : and the whole card to be made 

 visible in the horizon glass h, and the black line to appear in 



rectioB n m, visible iu the mirror not in the place/, but in m, by a ray dm, reflected in 

 w» n, making an angle with the former line of vision /A equal to the angle/ rf in ; and as 

 the eye shifts along the interval h n, carrying with it the axis of vision through the dif- 

 ferent points in that interval, tlie line of direction of the image, or its visible place will 

 also shift through the interval/ m in the mirror with an angular motion tinally equal to 

 the angle/ rf m. Hence the place of a very near object seen by reflection from a mirror 

 through a vane, also very close to the mirror as in the back observation for this adjust- 

 ment, may be very inaccurately determined, unless it be seen only in that place or spot 

 in the mirror from which spot the image had been reflected in a reversed position of the 

 quadrant in the adjustment. When the object d is so remote, that the angle/ d m be- 

 comes insensibly small, tben the apparent place of the image will be the same, in what- 

 ever part of / m in the mirror it is seen reflected from : but when the object is near, since 

 the axis of vision cannot be fixed by contracting the eye-hole to a point, the images must 

 be seen iu the same place iu the mirror. 



c 2 



