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