[ S6 ] 



fcrewlng up the oppoiite fcrew ; for this will make the end of 



the ftem approach towards or make a lefs angle with the 



mirror on the fide where that angle was too great, -i. e. where 



the mirror was too much elevated : alfo if the image, during the 



revolution of the mirror, appears fujl to dejccnd towards the 



obferver, and then to afcend, that part of the mirror in which 



the image is loweft is too much elevated above the horizon, 



and it will be depreiled by withdrawing the fcrew next to the 



loweft place of the image, (which is now the highell part of the 



mirror) and advancing the oppofite fcrew as before ; fo that the 



adjtxfler muft be cautious not to miftake the order of the afcent 



and defcent of the image, nor the direB'ton of its motion ; for as 



it has (in every complete revolution) both an afcent and defcent 



towards and from the higheft and loweft points, if the obferver 



miftakes the one' for the other he may move the fcrews erro- 



neoully, fo that he muft previoufly afcertain the higheft and 



loweft place of the image. 



That it is by the elevation of the mirror on the fide in 

 which the image appears it is made to afcend, may be ocularly 

 demonftrated by giving that diameter of the mirror, which is 

 in the fame plane of refledlion with the eye, the objedt, and 

 image, an angular motion in that plane upon the point of fuf- 

 penfion ; but if one would choofe to know the optical reafon of 



this 



