In the most precise model, the mercury surface is used to autocollimate the line of sight through {HEAD ~ the center} of {TAIL ~ cross hairs} to be horizontal for every pointing 
of
In the most precise model , the mercury surface is used to autocollimate the line of sight through { HEAD ~ the center } of { TAIL ~ cross hairs } to be horizontal for every pointing
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH