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