ADJUSTING HADLEY's QUADRANT. 159 



placing itfelf with great exadnefs oppofite the centre of 

 the eye-hole. 



The fame objedion and remedy are applicable to the 

 flits in the fight- vanes, of the common furveying inftru- 

 ments. 



7. Mr. Maflvyline, and others, recommend the fun 

 itfelf as the beft objedt by which to adjuft the fore-hori- 

 zon-glafs. — There is however confiderable difficulty at- 

 tending this mode of adjuftment. — The fun is too bright 

 to be viewed directly, without a piece of coloured glafs 

 interpofed between it and the eye ; and even if the eye- 

 piece be furnlfhed with fuch an appendage (which in the 

 common odant is feldom the cafe) ftill the two images 

 will be of very different fliades ; and either the one feen 

 by dire£l vifion will be too bright, or that feen by re- 

 fledlion will be too faint for an accurate obfervation of 

 their coincidence or contadl. — This difficulty may how- 

 ever be obviated in the following manner. 



Every octant is furniflied with at leaft two coloured 

 glaffes, of different fliades — take the darkeft of thefe out 

 of its frame, and with a thread faften it on behind the 

 horizon-glafs, and turn down the other, between this 

 and the index-fpeculum : The two images of the fun will 

 thus be generally of the fame fhade, or nearly fo, and 

 the adjuftment may then be made with the utmofi eafe 

 and fafety to the eye. Or, which is perhaps better. — 

 Place your eye behind the fore-horizon-glafs, and, look- 

 ing through this towards the centre of the index fpe- 

 culum, hold the inftrument in fuch a poiition as that 

 the line of vifion may be diredtcd to any point in the 

 heavens, &c. at the angular diftance of 90 degrees from 

 the fun ; and then, the index being placed at o, two 

 images of the fun will appear, both by a fingle reflexion, 

 one from the index-fpeculum, and the other from tlie 

 back part of the fore-horizon-glafs ; and by bringing 

 thefe images into coincidence or contacl, as when you 

 X 2 look 



