It's an 1822 print depicting the famed "looking-glass curtain" at London's Royal Coburg Theatre (later known as "The Old Vic"). The curtain, made of 63 plates of mirrored glass, enabled an audience to admire a reflection of itself before and after performances. It certainly enhanced the "to see and be seen" culture of theater in the time period, while also giving audiences a direct sense of control over what occurred onstage. Jim Davis and Victor Emeljanow report that "the weight of the curtain made it unmanageable and...poor ventilation coated it with a misty opaque film, thereby defeating its whole purpose," (Reflecting the Audience: London Theatregoing, 1840–1888, Hatfield: University of Hertfordshire Press, 2001: 35) but still the novelty of the curtain created a sensation. When it debuted in 1821, it was apparently more popular than the plays onstage. This particular print (London: G. Humphrey, St. James's Street, 1822), by the way, depicts Ramo Samee, a juggler, magician, and sword swallower, juggling on stage, in front of the mirror, something that must have created a number of double reflections--an audience looking at itself looking at itself--while also looking at a performance.
Spectatorship is never simple.