[ n6 ] 



furfaces will be leflened in the proportion of the 

 fine of the inclination to the radius -, fo that at laft, 

 when the common fection becomes parallel to the 

 plane of the quadrant, the error intirely vanifhes. 

 For this reafon, Mr. Hadleyvery properly directed the 

 thickeft and thinned edges of the index-glafs to be 

 placed parallel to the plane of the quadrant. But, as it 

 may well be queftioned whether this care is always 

 taken by the inftrument-maker, and it cannot be fup- 

 pofed that the glaffes can be ground perfect parallel 

 planes, it would certainly be an advantage acquired to 

 the instrument, could the error ariling from a want of 

 parallelifm of the planes be removed in whatever 

 pofition the common fection of the planes mould be 

 placed with refpect to the plane of the quadrant. 

 This will be effected for celeftial obfervations, if the 

 upper part of the index-glafs be left unfilvered on the 

 back, and made rough and blacked, the lower part 

 of the glafs being filvered as ufual, which muft be 

 covered whenever any celeftial obfervations are made. 

 Then, if the telefcope be fufficiently raifed above 

 the plane of the quadrant, it is evident that the ob- 

 fervations will be made by the rays reflected from 

 the fore-furface of the upper part of the index-glafs, 

 and confequently, if the quadrant be adjufted by 

 making ufe of the fame part of the index-glafs, the 

 obfervations will be true whether the two furfaces 

 of the index-glafs are parallel planes or not. The 

 Sun or Moon may be thus obferved by reflection 

 from the unfilvered parts of the index-glafs and ho- 

 rizon-glafs, fo that a paler darkening glafs will fuf- 

 iice, and they will appear much diftincter than from 

 an index-glafs wholly filvered with a deeper darken- 

 ing 



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