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