Crompton 24v "Gunsight" lamps

A nice pair of old lamps, found recently on Ebay for a very reasonable price. These interesting little lamps were, I believe, used onboard British fighter planes in the 2nd World War, including the iconic Spitfire. I'm not sure how they were used, as I can't seem to find any description of that online! Essentially however these two lamps both serve the same function, whatever it may be, and the different coating colours do not seem to set apart their usage.