SECT. XXII. EFFECTS OF COMPRESSION. 189 



quartz and one of beryl, both cut perpendicularly to their axis, 

 were compressed thus by MM. Moignot and Soleil, They 

 found that the single system in the quartz, which is a positive 

 crystal, was doubled in the direction of the compression, while 

 in the beryl, which is a negative crystal, the duplication was 

 perpendicular to the compression. In the quartz the axis of the 

 double system coincided with the line of pressure, but in the 

 tourmaline, which is a negative crystal, the line which joins the 

 centres of the rings was perpendicular to the pressure. 



If a positive crystal be compressed in the direction of its axis 

 the tint of the rings descends, and that of a negative crystal 

 rises. But if the ^crystals be dilated in the direction of their 

 optic axis, the tints in positive crystals rise, and negative 

 descend. 



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 sym- 

 metry 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 described. Regularly crystallized quartz, however, forms 

 au 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 concentric with the rings ; so that the light entirely 

 vanishes throughout the space enclosed 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. 214). Suppose the 



