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 phenomena 
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 phenomena 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. 
Seconp Series. Plates cut round the Edge a b, fig. 1, and according 
to the different Azimuths of the Plane mn X op Y, fig. 3, normal to 
the Faces No. 1. and No. 4. of the Hexahedron and passing through 
ats 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 ay, 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 was 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- 
