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