Chap, xxx.] POLARISATION OF LIGHT. 401 



at right angles to the former it is extinguished. The 

 ray, therefore, which exhibits these phenomena, when 

 examined by a Nicol's prism, has peculiar characters. 

 It is said to be plane polarised. 



Polarisation of light. Ordinary light, ac- 

 cording to the wave theory, is due to vibrations 

 occurring transversely to the direction of propaga- 

 tion of the wave, but the vibrations take place in all 

 planes across the direction of the wave. Light is said 

 to be plane polarised when the vibrations take place 

 all in one plane. To put it in another way. The par- 

 ticles of ether, whose vibrations produce light, all 

 move in directions transverse to the direction of 

 propagation, but in their vibrations they may de- 

 scribe figures of various forms, straight lines, circles, 

 etc. When light is polarised, however, the particles 

 of ether are all made to vibrate in the 

 same direction, e.g. in straight lines in 

 the same plane. In. Fig. 181 let BA 

 represent a ray of ordinary light. The ^ 

 velocity of a body along the line BA 

 may be decomposed into two velocities 

 at right angles, one, namely, in the 

 direction BY, the velocity in that 



, . , . -, V i j Fig.181. Decoin- 



direction being represented by BA , and position of a 

 the other in the direction BX, the ^o^^ht 

 velocity being represented by BB'. Angles to one 

 Similarly the velocity of a body along 

 BC may be considered as compounded of a velocity 

 BC' and BD, BC being, in short, the resultant of the 

 two velocities. So, letting BA represent a ray of 

 ordinary light, it may be considered as compounded 

 of vibrations occurring in the direction By and B.T, 

 with different velocities represented by BA' and BB'. 

 BA' and BB' will represent polarised rays. An 

 ordinary ray of light may then be decomposed into 

 two rays polarised in planes at right angles to one 

 A A 7 



B D 13' 3G 



