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.