{HEAD ~ The second reflecting surface} can be rotated around {TAIL ~ its longitudinal axis}, thereby the second reflecting surface can be inserted in or removed from an optical path of light traveling from the light source to the first reflecting surface 
rotated around
{ HEAD ~ The second reflecting surface } can be rotated around { TAIL ~ its longitudinal axis } , thereby the second reflecting surface can be inserted in or removed from an optical path of light traveling from the light source to the first reflecting surface
HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH 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 OTH OTH OTH OTH OTH OTH OTH