The improvement 11 wherein said detecting means includes {HEAD ~ means} for detecting {TAIL ~ the longitudinal position} of said beam spot on said second scan path 
The improvement 11 wherein said detecting means includes { HEAD ~ means } for detecting { TAIL ~ the longitudinal position } of said beam spot on said second scan path
OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH