From David Turley:
On the BD-5 the fuel tank gages were made of acrylic rod attached to a
flat plate. Basically, there were five or six rods ranging from one to
six inches long, with one end bonded to the flat piece of acrylic which
was installed flush with the surface of the wing, and the ends of the
acrylic rods (1/2" diameter) ground and polished into a cone shape.
These ends hung down into the tank. This was a slick setup. When the
fuel covers the end of the fuel sensing "rod" the flat plastic plate
shows dark. When the rod is not touching fuel the plate shows a white
circle where the end of the acrylic rod is bonded. Must work through a
reflected light principal. Only works in daylight, needless to say.