SICT. xxn. cmcuLAR POLARIZATION. 183 



compression, dilatation, and induration ; and so little 

 apparatus is necessary for performing 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 wfiich 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 colored rings already described. Regularly crys- 

 talized quartz, however, forms an exception. ID 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 colored rings produced by all 

 crystals having but one optic axis are circular, and 

 traversed by a black cross concentric with the rings ; so 

 that the light entirely vanishes throughout the space 

 inclosed by the interior ring, because there is neither 

 double refraction nor polarization along the optic axis. 

 But in the system of rings produced by a plate of 

 quartz, whose surfaces are perpendicular to the axis of 

 the crystal, the part within the interior ring, instead of 

 being void of light, is occupied by a uniform tint of red, 

 green, or blue, according to the thickness of the plate 

 (N. 209). Suppose the plate of quartz to be ^ of an 

 inch thick, which will give the red tint to th'e space 

 within the interior ring; when the analyzing plate is 

 turned in its own plane through an angle of 17|, the 



