[ 88 ] 



In fig. 3, let the line C B reprefent the ftem of the level, kept 

 by the plummet in a vertical Situation : if the inftrument weie 

 truly adjufted, C B would be the axis of the mirror and at right 

 angles to its refleding furface ; but if it be inclined to this fur- 

 face, then no diameter drawn on it will beat right angles to CB 

 but one; all others v/ill be inclined to it in various angles of obli- 

 quity, the greateft being that at right angles to the diameter which 

 has no inclination ; and if the mirror revolve round the line CB, 

 this latter diameter will defcribe a circle, but the others interme- 

 diate between thefe two will defcribe oppofite conical furfaces, 

 having all of them different angles at their common vertex C. Let 

 HN be that diameter at right angles to CB the plumb-line, and 

 confequently in the horizon, and let it be in that plane in which 

 the eye, fixed at the place E, fees the image of a luminous point 

 S in the point C : from the law of refledion, the angle S C H, made 

 by the incident ray S C with the mirror, muft be equal to 

 ECN, made by the refleded ray CE with the fame; let now 

 the mirror revolve about the axis CB, 'till another diameter, fuppofe 

 that which is mofl inclined to CB, viz. b n, falls into the plane 

 of vifion or refleclion SHCE; the image of S cannot now be 

 feen at C, becaufe the ang. S C b \s not equal to the angle E Cn, 

 and it can be feen only in a part of the mirror where thefe angles 

 at the point of incidence will be equal, and that v/ill be (when 

 the object is very remote) at a point /, whofe angular diftane CE i 



from 



