Provide a circular piece of coloured glass, and pierce its centre by
means of a common awl, well moistened with oil of turpentine: encircle
the glass with the fingers and thumb, hold it in the sunshine or the
strong light of a lamp, and the following beautiful effects will be
produced. If the glass be red, the luminous spot in the centre will be
reflected green; if the glass be green, the spot will be red; if blue,
orange; and if yellow, indigo.