A cw HeNe laser disposed for transmitting {TAIL ~ a first beam} in {HEAD ~ a first direction} 
A cw HeNe laser disposed for transmitting { TAIL ~ a first beam } in { HEAD ~ a first direction }
OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD