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