When the light source is illuminated, however, the viewer sees {HEAD ~ a complete image} on {TAIL ~ the exterior surface}  
on
When the light source is illuminated , however , the viewer sees { HEAD ~ a complete image } on { TAIL ~ the exterior surface }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH