low in color contrast again the effect is maximized when the tourmaline c axis lies parallel to the alexandrite b axis the color of certain varieties of aquarmarine morganite beryl and ruby can be adjusted by combining them with dichroic plates of tourmaline or cordierite in doublet configurations as described above with the rotation angle being adjusted to produce the desired color the effects discussed above for a pleochroic doublet can also be produced using an optically active rotator element disposed between two pleochroic elements this variation relies on the known optical rotatory power or optical activity of u03b1 quartz and certain other compounds of the same crystal structure such as berlinite to rotate the polarization directions of light beams traveling in the quartz a beam of linearly polarized light traveling parallel to the c axis in a crystal of u03b1 quartz undergoes a progressive rotation of its plane of polarization depending on the distance traveled and the wavelength generally in a given path length light of short wavelength is rotated more than light of longer wavelength an alexandrite quartz alexandrite triplet illustrates a preferred embodiment of this aspect of the present invention it will be recalled that an alexandrite doublet comprising suitably oriented c axis plates transforms the e a beam in one element into the e b beam in the other element thus causing greater absorption of light in the u03bb u02dc 0 6 u03bcm spectral region than would take place without the invention in the present invention the rotational transformation is accomplished by the use of a quartz c axis plate interposed between two plates of similarly oriented alexandrite the thickness of the quartz plate is chosen to produce a.