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