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A group of liquid crystals, mainly derivates of cholesterol, shows remarkable optical properties, including strong rotatory power and selective reflexion of circularly polarized light in a narrow region of wave-lengths. In this paper it is shown how these properties can be explained by the assumption that the molecules are arranged in a special way, so that the electrical axes rotate screw-like. It is inessential whether this occurs in small steps or continuously. When the axes make one revolution over a thickness p, then light in a region around A=pn will be reflected (n = refractive index). The second important parameter is the value of the double refraction ~= (n~-nl)/n. From p and ~ all optical properties can be calculated. No accurate data for testing the theory are available but qualitatively the agreement is complete.
Hl. de Vries (1951) studied this question.