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