Provide two lighted candles, and place them upon a table before a
whitewashed or light papered wall: hold before one of the candles a
piece of coloured glass, taking care to remove to a greater distance
the candle before which the coloured glass is not placed, in order to
equalize the darkness of the two shadows. If you use a piece of green
glass, one of the shadows will be green, and the other a fine red; if
you use blue glass, one of the shadows will be blue, and the other a
pale yellow.