'15 



the horizon glass, will point out the axis of visfon and direc- 

 tion of the telescope. If the position of the latter be wrong, 

 the observations will be erroneous, unless Mr. Hadley's cor- 

 rection be applied. * 



If the object which bj the eye at JE is seen in m, i. e. the 

 object c removed to m, were to be brought to appear to the 

 eye at e to coincide with C, by giving the mirror h an angular 

 motion sufficient for thisj such motion would be too great ; 

 for then the incident ray ma, and the reflected ray 6 €\ would 

 not be parallel, nor consequently the glasses perpendicular: 

 only half this motioH must be given, and then the stand 

 changed, or the object w moved to c, till the object and 

 image are made to unite; (it being the same in effect, whe*- 

 ther the stand be moved toward the object, or line joining the 

 objects, or the object toward the stand) ; and then the qua^ 

 drant must be turned half round to its first position, and the 

 images brought half way together by turning the horizon glass 

 and united as before: this- to be repeated atevery semirevolu- 

 tion, so often as necessary, till the adjustment of the horizon 

 glass is perfected'. 



When the objects C &cc are very distant, a small removal of 

 the quadrant to the right or left of a line joining the objects^ 



* Whether the eye, which- is itself a telescope, and with a large aperture, ever re- 

 quires a correction of this sort, when it looks through a sight vane,, is not questioned ; 

 nor whether it views any thing obliquely ; i. e. whether its axis be always the axis of 

 its vision ; but enough is said here to shew the errors that may arise in some cases, 

 from looking through an eytj hole or vane of too great magnitude; and these errors 

 would not be corrected by using, a telescope, unless Mr. Hadley's correction, (in hia- 

 ith corollary,) were applied. 



