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