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