switch?
take the bulb that works and put it in the other position. if it doesn't light, chances are that the switch is the problem. it may be corroded. take the fixture apart and check it out. it might be time to replace the entire fixture. rob is right about how it works - 1 light, 2 lights, off. galley and head are good places to use a clear bulb and a red bulb if you plan to do any night cruising. with the red in the '1 light' position, you can grab something out of the ice box or hit the head without killing your night vision.