Provide a piece of plain and polished smalt-blue glass, such as
sugar-basins and finger glasses are made of. It should be of unequal
thickness. Look through this glass at a strong light, as that from the
crack of a window-shutter, in a darkened room, and, at the thinnest
part, the colour will be purely blue. As the thickness increases, a
purple tinge will come on, which will become more and more ruddy; and,
if the glass be very thick, the colour will pass to a deep red.