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