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