CHAP, ix.] ELECTRICITY AND MAGNETISM. 349 



CHAPTER IX. 



ELECTRO-OPTICS. 



LESSON XXXV. General Relations between Light 

 and Electricity. 



385. Of late years several important relations have 

 been observed between electricity v and light. These 

 relations may be classified under the following heads : 



(i.) Production of double refraction by dielectric stress. 



(ii.) Rotation of plane of polarisation of a ray of light 



on traversing a transparent medium placed in a 



magnetic field, or by reflection at the surface of a 



magnet. 



(iii.) Change of electric resistance, exhibited by 



selenium and other bodies during exposure to light. 



(iv.) Relation between refractive index and dielectric 



capacity of transparent bodies. 



It was announced by Mrs. Somerville, by Zantedeschi, and 

 others, that steel needles could be magnetised by exposing 

 portions of them to the action of violet and ultra-violet rays 

 of light ; the observations were, however, erroneous. 



386. Electrostatic Optical Stress. In 1875 Dr - 

 Kerr of Glasgow discovered that glass when subjected to 

 a severe electrostatic stress undergoes an actual strain, 

 which can be observed by the aid of a beam of polarised 

 light. In the original experiment two wires were fixed 

 into holes drilled in a slab of glass, but not quite meeting, 



