The black cannister you see in the foreground is the ballast or transformer that powered this fixture. Shown is GE's first mercury vapor luminaire style introduced in 1949. A separate photocontrol, time switch, etc., operated these then-popular fixtures. Like some other 120V and 240V street lights that ran in strings for their dusk-to-dawn control, the dedicated wires that operated them sometimes were identified by blue, green or yellow porcelain insulators as delineators.