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