PHYSICAL PROPERTIES OF SOLARISED LIGHT. 141 



condition called polarisation has been produced, and 

 by experiment we discover that the single ray has 

 properties different from those of the compound or 

 ordinary ray. 



It is somewhat difficult to explain what is meant by, 

 and what are the conditions of, polarised light. In the 

 iirst instance let us sec by what methods this peculiar 

 state may be brought about. 



If we reflect a ray of light from the surface of any 

 body, fluid or solid, but not metallic, at an angle between 

 53 and 68 it undergoes what has been called plane 

 polarisation. It may also be produced by the refraction 

 of light from several refracting surfaces acting upon the 

 pencil of light in succession ; as by a bundle of plates 

 of glass. Each surface polarises a portion of the pencil, 

 and the number of plates necessary to polarise a whole 

 beam depends upon the intensity of the beam and the 

 angle of incidence. Thus, the light of a wax candle is 

 wholly polarised by forty-seven plates of glass at an 

 angle of 40 41'; while at an angle of 79 11' it is 

 polarised by eight plates. Again, plane polarisation 

 may be produced by the double refraction of crystals. 

 Each of the two pencils is polarised, like light re- 

 flected from glass at an angle of 56 45', but in opposite 

 planes. 



Non-scientific readers will still ask, What is this 

 mysterious condition of light which is produced by re- 

 flection and refraction at peculiar angles to the incident 

 ray. It is one of the most difficult of problems to 

 express in popular language. The conditions are, 

 however, these : 



An ordinary ray of light will be reflected from a re- 

 flecting surface at whatever angle that surface may be 

 placed in relation to the incident beam. 



A polarised ray of light is not reflected in all positions 

 of the reflecting surface. 



An ordinary ray of light is freely transmitted through 



