SECT, xxii.] CIRCULAR POLARIZATION. 213 



centric 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 2 ' 3 of an inch thick, which 

 will give the red tint to the space within the interior ring ; 

 when the analyzing plate is turned in its own plane through 

 an angle of 17^, the red hue vanishes. If a plate of rock 

 crystal 5 2 5 of an inch thick be used, the analyzing plate must 

 revolve through 35 before the red tint vanishes, and so on, 

 every additional 25th of an inch in thickness requiring an 

 additional rotation of 17^; whence it is manifest that the 

 plane of polarization revolves in the direction of a spiral 

 within the rock crystal. It is remarkable that, in some 

 crystals of quartz, the plane of polarization revolves from 

 right to left, and in others from left to right, although the 

 crystals themselves differ apparently only by a very slight, 

 almost imperceptible, variety in form. In these phenomena, 

 the rotation to the right is accomplished according to the 

 same laws, and with the same energy, as that to the left. 

 But if two plates of quartz be interposed, which possess 

 different affections, the second plate undoes, either wholly 

 or partly, the rotatory motion which the first had produced, 

 according as the plates are of equal or unequal thickness. 

 When the plates are of unequal thickness, the deviation is 

 in the direction of the strongest, and exactly the same with 

 that which a third plate would produce equal in thickness 

 to the difference of the two. 



M. Biot has discovered the same properties in a variety 

 of liquids. Oil of turpentine, and an essential oil of laurel, 

 cause the plane of polarization to turn to the left, whereas 

 the syrup of sugar-cane, and a solution of natural camphor, 



