354 ON LIGHT. 



referred to in what follows as a " polarizing frame," a 

 similar series set transparently being termed a " polariz- 

 ing pile." 



(135.) Other modes of polarization. There are certain 

 doubly refractive crystals, more or less coloured, which 

 possess the singular property of absorbing or stifling in 

 their passage through them, unequally ', the two oppositely 

 polarized rays into which they divide the incident light. 

 Two bodies especially have been noticed in an eminent 

 degree endowed with this property the one a mineral 

 occurring more or less frequently among the rocks of 

 igneous origin, called the tourmaline, the other an arti- 

 ficial chemical compound, the iodo-sulphate of quinine. 

 The former crystallizes for the most part in long prisms of 

 many sides, terminated by faces, three of which belong 

 to the primitive form, an obtuse rhomboid, whose axis is 

 that of the prism itself. In consequence, like all crys- 

 tals of this class, it is doubly refractive, and if artifi- 

 cially cut and polished into a doubly refracting prism, 

 having its refracting edge parallel to that of the rhom- 

 boid, an object viewed transversely through it will 

 appear double ; provided the eye be held quite close to 

 the refracting edge ; but if ever so little removed, so as 

 to look through a greater thickness of the substance, one 

 of the images will be observed to diminish rapidly in 

 intensity ; and at a certain, usually very moderate, thick- 

 ness to disappear altogether, as if extinguished by a 

 deeper colour or a higher degree of opacity in the 

 medium, the other remaining un diminished. The unex- 

 tinguished ray is, of course, completely polarized, and 



