The thicker the plate of quartz, the larger the arc through which the analyzing plate must be moved round, to cause a change from one tint to another.

This fact is seen more clearly by using homogeneous light, which indeed is most conveniently employed in many of these experiments.

If we use that from red glass, when the quartz is 004 inches thick, the red spot in the centre of the crystal will be brightest when the planes of reflection and polarization are at right angles; and the tint will cease to be visible, when the analyzing plate is moved through an arc of 17 49 With a crystal double that thick- ness, the analyzing plate must revolve through an arc of 35 Two right, or two left handed crystals produce an effect which is the same as that of a crystal, the thickness of which is equal to the sum of their thicknesses.

But, if one is right, and the other left handed, the effect will be that of a crystal which is equal to the difference of their thicknesses : and it will be right or left handed, according as the right, or left handed used is the thicker.

25 Circular polarization is not confined to quartz; it is found also in fluids.