22 
Proceedings of Royal Society of Edinhurgh. [dec. 5, 
laminar shearing or distortional motion of which the wave-motion of 
the light consists, the “ plane of the shear ” * (or plane of the 
distortion,” as it is sometimes called), is the plane through the 
direction of the ray and the direction of vibration; and therefore it 
would be the ordinary ray that would have its line of vibration in 
the principal plane, if the ether’s difference of quality in different 
directions were merely the aeolotropy of an unstrained elastic 
solid. t Hence ether in a crystal must have something essentially 
different from mere intrinsic aeolotropy ; something that can give 
different velocities of propagation to two rays, of one of which the 
line of vibration and line of propagation coincide respectively with 
the line of propagation and line of vibration of the other. 
4. The difficulty of imagining what this “ something ” could 
possibly be, and the utter failure of dynamics to account for double 
refraction without it, have been generally felt to be the greatest im- 
perfection of optical theory. 
It is true that ever since 1839 a suggested explanation has been 
before the world ; given independently by Cauchy and by Green, 
in what Stokes has called their “Second Theories of Double 
Eefraction,” presented on the same day, the 20th of May of that 
year, to the French Academy of Sciences and the Cambridge 
Philosophical Society. Stokes, in his Report on Double Eefraction, J 
has given a perfectly clear account of this explanation. It has been 
but little noticed otherwise, and somehow it has not been found 
generally acceptable ; perhaps because of a certain appearance of 
artificiality and arbitrariness of assumption, which might be 
supposed to discredit it. But whatever may have been the reason 
or reasons which have caused it to be neglected as it has been, and 
though it is undoubtedly faulty, both as given by Cauchy and by 
Green, it contains what seems to me, in all probability, the true 
principle of the explanation, and which is, that the ether in a 
doubly refracting crystal is an elastic solid, unequally pressed or 
* Thomson and Tail's Natural Philosophy, § 171; (or Elements, § 150). 
t The elementary dynamics of elastic solids show that on this supposition 
there might be maximum and minimum velocities of propagation for raj^s in 
directions at 45° to one another, but that the velocities must essentially le 
equal for every two directions at 90° to one another in the principal plane, when 
the line of vibration is in this plane. 
+ British Association Report, 1862. 
