the longitudinal beam is converted by the beam splitter into {TAIL ~ a third horizontal beam} spreading horizontally, the third horizontal beam forming a part of {HEAD ~ the fractional light beam}, 
the longitudinal beam is converted by the beam splitter into { TAIL ~ a third horizontal beam } spreading horizontally , the third horizontal beam forming a part of { HEAD ~ the fractional light beam } ,
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH