{HEAD ~ The beams} which are focused onto the optical disc D1 or D2 are reflected by {TAIL ~ the optical disc}, and then are incident on and reflected by the beam splitter 22 to be received on the photo detector 30 
are reflected by
{ HEAD ~ The beams } which are focused onto the optical disc D1 or D2 are reflected by { TAIL ~ the optical disc } , and then are incident on and reflected by the beam splitter 22 to be received on the photo detector 30
HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH