16 



will make no sensible difference in the angle of incidence and 

 reflection of the rays, nor consequently alter the place of the 

 images, as would be the case if the objects were near. 



If the quadrant, instead of being turned half round from the 

 position A F Q to ap q^ were to be so inverted, that tlie in- 

 dex and horizon glasses A and B should be placed on the lines 

 E c, C e, the adjustment could not be made, unless the ob- 

 jects were so remote, that the interval between the glasses 

 would make an insensible angle at either of the objects, and 

 that any little motion on either side of a line joining the 

 objects, which might accidentally be given in reversing the 

 quadrant, would cause likewise only an imperceptible divari- 

 cation of the images. For if the quadrant were to be turned 

 upside down, and so that the centres of the mirror would fall 

 on the lines C c, E c as before, the centre A on D, and B 

 on F ; then the angle of incidence of a ray falling from C 

 on Dy would be different from that of a ray from C on A ; 

 it would therefore not be reflected to F ; so that it would be 

 necessary to turn the instrument in its plane, in order to 

 make the image of C be visible in the horizon glass ; by which 

 the glasses in the 2d. position would not be parallel to them- 

 selves as they were in the 1st. nor is there any certain posi- 

 tion in which they could be placed, as this will depend on 

 the distance of the objects. So that the horizon glass cannot 

 be adjusted by reversing the face of the octant, unless the 

 objects by which this is to be done, are so far removed, that 

 the distance between the glasses subtends at them an imper- 



