212 COLOURED IMAGES. [SECT. xxii. 



that have been described will be seen by reflection on its 

 surface. 



Coloured rings are produced by analyzing polarized light 

 transmitted through glass melted and suddenly or unequally 

 cooled; also through thin plates of glass bent with the 

 hand, jelly indurated or compressed, &c., &c. In short, all 

 the phenomena of coloured rings may be produced, either 

 permanently or transiently, in a variety of substances, by 

 heat and cold, rapid cooling, compression, dilatation, and 

 induration; and so little apparatus is necessary for per- 

 forming the experiments, that, as Sir John Herschel says, 

 a piece of window glass or a polished table to polarize the 

 light, a sheet of clear ice to produce the rings, and a broken 

 fragment of plate-glass placed near the eye to analyze the 

 light, are alone requisite to produce one of the most splendid 

 of optical exhibitions. 



It has been observed, that when a ray of light, polarized 

 by reflection from any surface not metallic, is analyzed by 

 a doubly refracting substance, it exhibits properties which 

 are symmetrical both to the right and left of the plane of 

 reflection, and the ray is then said to be polarized according 

 to that plane. This symmetry is not destroyed when the 

 ray, before being analyzed, traverses the optic axis of a 

 crystal having but one optic axis, as evidently appears 

 from the circular forms of the coloured rings already de- 

 scribed. Regularly crystallized quartz, however, forms an 

 exception. In it, even though the rays should pass through 

 the optic axis itself, where there is no double refraction, 

 the primitive symmetry of the ray is destroyed, and the 

 plane of primitive polarization deviates either to the right 

 or left of the observer, by an angle proportional to the 

 thickness of the plate of quartz. This angular motion, or 

 true rotation of the plane of polarization, which is called 

 circular polarization, is clearly proved by the phenomena. 

 .The coloured rings produced by all crystals having but one 

 optic axis are circular, and traversed by a black cross con- 



