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