Highly evacuated glass tube filled with low-pressure argon and containing multiply distilled sodium for demonstrating the resonance fluorescence of sodium vapor. The entire tube emits yellow light at the wavelength of the sodium 'D' line when heated using the Heating Chamber and illuminated with diffuse sodium spectral light. If the tube is instead illuminated with full-spectrum white light (e.g. from an incandescent bulb), the transmitted light exhibits a dark absorption line at the position of the sodium D line.