How We Spotted Planes Before Radar Was Invented
Conventional radar deployment was still almost 20 years away, but World War I's introduction of modern weaponry called for rapid resourcefulness. What appears to be a photo of steampunk Mousketeers is actually an image of Russian officers wearing visual and acoustic amplifying apparatuses in an effort to locate enemy aircraft during the war.
As described at Boing Boing:
"Drakegoodman scanned this 1917-ish photo of Soviet planespotters in exotic headgear; according to a commenter, the binox are focused at infinity "so that when you found the source of the sound by turning your head, you could see the aircraft creating that sound."