The exemplary embodiment is a reservoir for use with a cigarette lighter has a chamber for containing gas and liquid fuel and a lens made of a transparent material and having an internal surface adjacent the chamber. The internal surface allows optical transmission of visible light into and from the liquid fuel when in contact therewith, but denies the transmission of light into the gas when in contact therewith. The internal surface is contoured such that the transmissions are facilitated by optical refraction, and the denial is facilitated by optical reflection. The transmissions facilitate comparisons of the relative amounts of the liquid fuel and the gas in the chamber. The chamber has within a non-transparent object viewable through the lens during the transmissions and non-viewable through the lens during the denial. The object is a planar panel disposed substantially parallel to and displaced from the internal surface. Indicia proximate the lens may be provided for facilitating the comparison of the relative amounts of the liquid fuel and the gas in the chamber. The reservoir may be removable from the lighter for replacement and the lens may be viewable through a window in the lighter's housing.