Electrical-series operation of street lights was a very popular practice for decades. As you can see in the photo, two wires connected to the fixture; one from each of the dead-ended wires above. In some instances through the 1920s or so, specially colored glass insulators (such as cobalt blue and amber ones) were used to identify these lines. In subsequent years colored porcelain insulators served the same purpose; thus blue, yellow, green, etc. insulators.