350 ELEMENTARY LESSONS ON [CHAP. ix. 



so that when these were placed in connection with the 

 terminals of an induction coil or of a Holtz machine the 

 accumulating charges on the wires subjected the inter- 

 vening dielectric to an electrostatic stress. The slab 

 when placed between two Nicol prisms as polariser and 

 analyser 1 exhibited double refraction. The behaviour of 

 the glass was as if it had been subjected to a pull along 

 the direction of the electric force, i.e., as if it had ex- 

 panded along the lines of electrostatic induction. Later, 

 he found that bisulphide of carbon and other insulating 

 liquids exhibit similar phenomena, but that of these the 

 fatty oils of animal and vegetable origin exhibited an 

 action in the negative direction, as if they had contracted 

 along the lines of induction. It is found that the 

 quantity of optical effect (i.e., the difference of retardation 

 between the ordinary and extraordinary rays) per unit 

 thickness of the dielectric is proportional to the square of 

 the resultant electric force. The axis of double refraction 

 is along the line of the electric force. Qumcke has 

 pointed out that these phenomena can be explained by 

 the existence of electrostatic expansions and contractions, 

 stated in Art. 273. 



387. Magneto-optic Rotation of the Plane of 

 Polarisation of a Ray of Light. A ray of light 

 is said to be polarised if the vibrations take place in one 

 plane. Ordinary light can be reduced to this condition 

 by passing it through a suitable polarising apparatus 

 (such as a Nicol prism, a thin slice of tourmaline crystal, 

 etc.) In 1845 Faraday discovered that a ray polarised 

 in a certain plane can be twisted round by the action 

 of a magnet, so that the vibrations are executed in a 

 different plane. The plane in which a ray is polarised 

 can be detected by observing it through a second Nicol 

 prism (or tourmaline), for -each such polariser is opaque 

 to rays polarised in a plane at right angles to that plane 



1 The student is referred to Prof. Balfour Stewart's Lessons on Element- 

 ary Physics for further information concerning the properties of polarised light. 



