{HEAD ~ the third light receiving portion} consists of {TAIL ~ two third optical receivers}, one for receiving the third vertical beam, the other for receiving the fourth vertical beam 
consists of
{ HEAD ~ the third light receiving portion } consists of { TAIL ~ two third optical receivers } , one for receiving the third vertical beam , the other for receiving the fourth vertical beam
HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL 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