258 FELIX savart's researches on the 



appear to follow sensibly in their succession the order of those of New- 

 ton's rings : it was required therefore only to compare the phaenomena 

 observed in the two cases, and to see whether they presented any hi- 

 therto 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 phaenomena 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. 



Second Series. Plates cut round the Edge a b, Jig. 1, and according 

 to the different Azimuths of the Plane m n Xop Y,Jig. 3, normal to 

 the Faces No. 1 . and No. 4. of the Hexahedron and passing through 

 its axis. 



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 cross- 

 ing each other rectangularly, and xy, 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 perpendi- 

 cular to the axis X Y ; 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 jilate 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- 



