Both, one-term as well as two-term Drude's rotatory dispersion formulae, express equally well the optical rotatory dispersion of the compounds in the visible range (6708 Å -4358 Å). The characteristic wave-lengths (\(\lambda_0\)), derived From these equations, do not agree with the observed ultraviolet absorption maxima. The complex rotatory dispersion equations in some cases are almost equivalent to the one-term simple equations, with the second term contributing practi­cally nothing to rotatory power ; in other cases both terms are significant and the two characteristic wave-lengths obtained in these cases are nearer, but not equal to the absorption bands. Thus the characteristic wave-lengths calculated From one and two-term rotatory dispersion equations are not the real absorption bands, but only the weighted average of the observed bands and those unobserved in the far ultraviolet. The method of building up a two-term equation by substituting the two observed absorption maxima in place of two characteristic wave-lengths not only suffers from the defect that some hypothetical bands can be fitted in equally well, but also that it ignores other unobserved ultraviolet bands beyond the working range of the instrument. The effect of chemical constitution on rotatory power and ultraviolet absorption bands is discussed.