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