In 1812, as Arago pursued further qualitative experiments and other commitments, Jean-Baptiste Biot reworked the same ground using a gypsum lamina in place of the mica, and found empirical formulae for the intensities of the ordinary and extraordinary images. The formulae contained two coefficients, supposedly representing colors of rays "affected" and "unaffected" by the plate—the "affected" rays being of the same color mix as those reflected by amorphous thin plates of proportional, but lesser, thickness.Darrigol, 2012, pp. 194–195 (ordinary intensity); Frankel, 1976, p. 148 (both intensities).
In 1812 as Arago pursued further qualitative experiments and other commitments Jean-Baptiste Biot reworked the same ground using a gypsum lamina in place of the mica and found empirical formulae for the intensities of the ordinary and extraordinary images The formulae contained two coefficients supposedly representing colors of rays "affected" and "unaffected" by the plate—the "affected" rays being of the same color mix as those reflected by amorphous thin plates of proportional but lesser thicknessDarrigol 2012 pp 194–195 (ordinary intensity); Frankel 1976 p 148 (both intensities)