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