The only Lucite underwing fuel measuring sticks I ever saw were on the Lockheed L188 Electra. As far as I can recall the upper internal end was either a conical or chisel shaped prism. When above the fuel the viewing window at the lower end was clear or bright with the ambient light reflecting back down from the prism. As fuel covered the prism it went dark. To check the fuel level you pulled the stick down until the viewing window showed either a pinpoint of light for the conical or a thin straight line for the chisel shaped prism then read the figure on the side of the stick. For refuelling you set the stick to the required figure then shut off the fuel for that tank when the stick showed a pinpoint or straight line as before.