gems, corundum, color.4, pleochroism

Absorption in colored birefringent media differs essentially from the ordinary phenomenon to which the inherent colors of bodies relate. Here, as in many other parts of modern optics, the first observations, those constituting the discovery, are due to M. Arago. He showed that in colored birefringent media, and notably in yellowish baryte sulfate, polarized light developed varying tints depending on the direction in which it passed through the crystal. M. Biot and M. Brewster, but especially M. Biot, then studied the phenomenon, though without formulating the definitive law. In a note formerly communicated to the Academy, I had stated that colored birefringent crystals, called negative or repulsive, such as Iceland spar, tourmaline, and sapphire, absorb the ordinary ray more strongly, whereas positive or attractive crystals, such as quartz, zircon, and common apophyllite, extinguish the extraordinary ray to a greater extent, and consequently allow the ordinary ray to pass as partially polarized light. However, there are some exceptions, such as beryl, though not emerald. Finally, in the memoir on the optical properties of minerals, dichroism had not led to the statement of the law of the phenomenon to whose demonstration the experiments of the present memoir refer.