4i2 PHYSIOLOGICAL PHYSICS. [Chap. xxx. 



the vibrations of each ray being all in the same direc- 

 tions. The two rays into which the quartz plate 

 splits the ray which enters it are polarised circularly, 

 but in opposite directions, one right-handed, the other 

 left-handed. These rays travel through the quartz 

 plate with unequal velocities. They unite on issuing 

 from the quartz plate. That is to say, the ether at 

 the surface of the quartz plate receives an impulse to 

 vibrate in the direction of the first circularly polarised 

 ray which reaches it, the ray which travels through 

 the plate with greatest velocity ; and it has just 

 begun to vibrate in that direction when the retarded 

 ray reaches it and gives it an impulse to vibrate in 

 the opposite direction. The result of the impulse to 

 vibrate in the direction of two opposite circularly- 

 polarised rays is a plane polarised ray, but with the 

 plane rotated to right or left, according as the right 

 circularly-polarised ray, or the left, proceeded through 

 the quartz with the greater velocity. 



Besides quartz various other substances rotate 

 the plane of polarisation, some to the right, others to 

 the left. Thus, crystals of chlorate, bromate, and 

 iodate of sodium, of sulphate of strychnine, crystallised 

 with thirteen equivalents of water, and of other sub- 

 stances, rotate the plane of polarisation. Essences of 

 turpentine, lemon, lavender, peppermint, rosemary, 

 aniseed, fennel, caraway, etc., also possess a similar 

 property. Of these, essence of turpentine, aniseed, 

 and peppermint rotate to the left, the others to the 

 right. 



Solution of cane sugar rotates the plane to the 

 right, through an angle of 33 64' if the solution 

 contain 50 per cent, sugar. 



The following table gives a number of substances 

 which in solution exercise a rotatory power, the 

 amount of which is stated in degrees, right or left 

 being indicated by the + or sign : 



