{HEAD ~ A lens} may be provided to focus {TAIL ~ both beams} on a small hole in the mirror 
provided to focus
{ HEAD ~ A lens } may be provided to focus { TAIL ~ both beams } on a small hole in the mirror
HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH