590 Proceedings of the Royal Society 
perties of the minute crystal ultimately obtained; and as crystals 
of quartz and other substances have been obtained of almost micro - 
scopic dimensions, but still possessing all the properties of large 
crystals of these substances, we see that the motion of the ether at 
any point of a crystal is only affected by the material molecules 
which are within extremely minute distances of that point. 
Again, there is no dispersion of light in vacuo, or in space. In 
order that this may be the case, that is, in order that rays of all 
wave lengths may be propagated with the same velocity, it can he 
shown that the action exerted by the parts of the ether on each 
other can only be sensible at very small distances. 
In obtaining the equations of motion, it is supposed that the 
motion constituting light is transversal to the direction of propaga- 
tion, which is equivalent to supposing that the ether is incom- 
pressible with respect to the forces called into action in the pro- 
pagation of light, or that the motion of the ether takes place without 
change of density. 
The arguments in support of the hypothesis of transversal vi- 
brations, to which Fresnel was led by physical considerations, 
founded on the non-interference of rays polarised in planes at 
right angles to each other, are so well known, that it is not neces- 
sary to enter into their discussion. Suffice it to say, that “if the 
simplicity of a theory which conducts us through a multitude of 
curious and complicated phenomena, like a thread through a laby- 
rinth, be considered to carry the stamp of truth, the claims of the 
theory of transverse vibrations seem hut little short of those of the 
theory of universal gravitation” (Stokes “On the Dynamical 
Theory of Diffraction,” Cam.. Phil. Trans., vol. ix. p. 2). As in 
other theories, the squares of the displacements of the elements of 
ether from their positions of equilibrium are neglected. 
