Astronomical Instrument, South Yard, by Unknown, c. 1900, Smithsonian Archives - History Div, An image of a heliostat, a sophisticatedly mounted motor driven single flat mirror that take sunlight and feeds it into one of the APO buildings for analysis used by the Smithsonian Astrophysical Observatory is inside the wooden fence, which surrounds the Observatory located in the South Yard. The west side of the Arts and Industries Building is visible in the background..