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