When the second reflecting surface is removed from {TAIL ~ the optical path}, {HEAD ~ the second reflecting surface} is located in a shadow region of a third reflecting surface 
When the second reflecting surface is removed from { TAIL ~ the optical path } , { HEAD ~ the second reflecting surface } is located in a shadow region of a third reflecting surface
OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH