Soak a cotton wick in a strong solution of salt and water, dry it, place it in a spirit lamp, and when lit it will give a bright yellow light for a long time. If you look through a piece of blue glass at the flame, it will lose all its yellow light and you will only perceive feeble violet rays. If before the blue glass you place a pale yellow glass, the lamp will be absolutely invisible, though a candle may be distinctly seen through the same glasses.