POLARISED LIGHT. 



109 



intelligible, as we are unable to introduce them in tie colours given by 

 the above gentleman, we may state that ordinary light is represented 

 by a cross, which denotes that its vibrations are in planes at right 

 angles to each other ; whereas when one set of such vibrations only is 

 shown, the light is said to be polarised. In fig. 76, abed represent 



B, t> 



Xo 



Te. U Be 



BO 



A V He- 



fig. 76. 



the rectangular vibrations of common light ; e a plate of tourmaline 

 placed with its axis in a vertical direction — this is called the polariser ; 

 fa, beam of polarised light obtained from a 6 cc? by the stoppage of 

 plane cd ; g a, film of selenite of such a thickness as to produce red or 

 green light; A the polarised beam/split into two planes at right angles 

 to each other; i a second tourmaline or analyser, with its axis in the 

 same direction as that of e ; by this all the vibrations that are not 

 inclined at a greater angle than 45° to the axis of the analyser can 

 be transmitted and again brought together; ^ the waves RoRe of 

 red light, meeting in the same state of vibration, and forming a wave 

 of red light of doubled intensity; Im the waves Ye Yo and Bo Be 

 for yellow and blue meeting together, with a difierence of an odd num- 

 ber of half undulations, and thus neutralising each other by interfer- 

 ence ; n the resultant red light. 



In the next line of figures may be seen what takes place by turning 

 the analysing tourmaline one quarter of a circle, h represents the polar- 

 ised beam split into two rays ; i the tourmaline turned so that its axis is 

 at right angles to that in the preceding figure ; h the waves Ro Re oi 

 red light destroying each other by interference ; Im the waves Yo Y e 

 and Bo Be ior yellow and blue, meeting together in the same state of 

 vibration, and by their coincidences, waves of doubled intensity for 

 yellow and blue light ; n green light, resulting from the mixture of the 

 yellow and blue light respectively. By substituting Nichol's prisms for 



poses ; the instrument is described in Elements of Natmal PMloiophy, by J. Hogg, 

 p. 281. 



