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