{HEAD ~ the traversal beam} is converted by the beam splitter into {TAIL ~ a third vertical beam} spreading vertically, the third vertical beam forming the other part of the fractional light beam, 
is converted into
{ HEAD ~ the traversal beam } is converted by the beam splitter into { TAIL ~ a third vertical beam } spreading vertically , the third vertical beam forming the other part of the fractional light beam ,
HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL OTH OTH OTH OTH REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH