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which M. Arago first observed, and of which M. Biot determined the experimental laws. The most simple arrangement to exhibit these colours, is to place a thin lamina of some crystallized substance, in the direction of a white ray, previously polarized by reflexion, and to analyze the transmitted light by means of a double-refracting prism. The light is thus separated into two portions, of which the colours are complementary to each other, and identical with those of the rings between two glasses. One of these portions appears to have preserved its primitive polarization, whilst the other exhibits a new polarization, of which the direction depends on that given to the axes of the crystal by turning the lamina round in its own plane.
Following gradually in this manner the direction of the polarization given to a molecule of light, transmitted through different thicknesses of a crystalline medium, it will be found to undergo periodical alternations, which, if light be a matter emitted, indicate an oscillatory motion of the axes of the molecules accompanying their progressive motion. M. Biot has designated this fact by the name of moveable polarization, which is merely the expression of results observed.
If the system of undulations be adopted, the colours of the two images may be attributed to the interference of the two pencils into which the incident polarized light separates, in passing through the lamina. This is what Dr. Young does, and it is remarkable that calculations founded on this principle gave him the nature of the tints, and the periods after which they recur, precisely as M. Biot had determined them by experiment. As to the alternations of polarization, they become, in the undulation system, a compound result produced by the mutual influence of the interfering rays, and it is easy to deduce from observation the conditions to which the mixture of the waves must be subjected to produce the new direction of apparent polarization. M. Fresnel has done this, and the indications of his formulæ have been found conformable in all respects to the laws deduced by M. Biot from observation.
These interferences of the rays may be produced without the assistance of crystalline laminæ; we may equally employ thick plates, provided the rays pass through them at very small incli-
