appear to follow sensibly in their succession the order of those of Newton's rings: it was required therefore only to compare the phænomena observed in the two cases, and to see whether they presented any hitherto unobserved differences; but it was impossible to recognise any. Thinking that perhaps a considerable augmentation of thickness in the plates might bring to view some appreciable differences, I repeated the experiment with pieces of rock crystal which were eight centimetres (3·149 inches) in thickness, and I saw nothing that could indicate that all the plates parallel to the axis did not comport themselves in the same manner with regard to light: whence it must be concluded that what we can learn respecting the structure of crystals by means of light, is not of the same order as that which sonorous vibrations may enable us to discover. It would appear from what precedes, that this latter process indicates more specially the elastic state and the force of cohesion of the integrant particles in the different directions of every plane, whilst the phænomena of light, depending more specially on the form of the particles and on the position they assume round their centre of gravity, are, to a certain point, independent of the mode of junction of the different plates of which the crystal is formed.
One of the modes of division of all the plates of this series remains constantly the same, fig. 3, bis; it is formed of two straight lines crossing each other rectangularly, and , one of these lines, is always the projection of the axis of the crystal on the plane of the plate. The other mode of division consists of two hyperbolic curves, which undergo various modifications depending on the inclination of the plates to the axis of the hexahedron, and which are in general analogous to those we have observed in the two first series of plates belonging to bodies possessing three rectangular axes of elasticity.
No. 1. represents the two modes of division of the plate perpendicular to the axis ; they are both composed of straight lines; or, if either is formed of two curves, their summits are so near each other that they appear to coalesce. Rock crystal being a crystal with one axis, in respect to light, it w£is natural to presume that the elasticity would be equal in every direction of the plane of the plate in question, and that, in consequence, this plate might assume only a single mode of division, having the property of placing itself in any direction; but this is not the case, even in plates cut with extreme care, and which by their optical properties appear sensibly perpendicular to the axis. Ne-
