390 ON LIGHT. 



observed, on turning it from right to left ; in plates of 

 the same thickness cut from other crystals the same suc- 

 cession is seen on turning it from left to right. Yet 

 more singular, is the fact that this inversion this right- 

 and-left-handedness in the succession of tints, corresponds 

 to, and is predictable beforehand from, the appearance 

 of certain small obliquely posited facets on the crystal 

 previous to polishing, which lean unsymmetrically in 

 some crystals to the right, in others to the left hand of 

 the axis held up straight before the eye. In all other 

 respects the crystals are identical.* A similar right-and- 

 left-handedness in the external form of their crystals, 

 accompanied with the very same optical phenomena, 

 has been remarked by M. Pasteur in the salts called para- 

 tartmtes and their crystallized acid. 



(166.) The account given by the undulatory theory of 

 these phenomena is this. Quartz (to adhere to our first, 

 chosen instance) is uniaxal, but it differs from Iceland 

 spar and others of that class in a most essential point 

 first noticed by Mr Airy, viz. : that the sphere and sphe- 

 roid representing the simultaneous surfaces of the ordi- 

 nary and extraordinary waves propagated within them, 

 though having a common axis, do not touch each other 

 internally. Hence, in the direction of that axis, though 

 there is, at a perpendicular incidence, no double refraction, 

 there is a difference of velocity in the two rays. Now 

 the theory at present adopted is, that owing to some 

 peculiarity at present not understood ; when a polarized 



* Amethyst consists of thin alternate layers of right-handed and 

 left-handed quartz superposed, parallel to their axes. 



