POLARISED LIGHT. 3 



to the beam, then both the reflected and the transmitted rays 

 will be found to be in peculiar conditions. For if either of 

 them be received upon a second plate of glass at a similar 

 angle of incidence, then it will be found that in certain 

 positions of the latter, or analyser, as I may at once call it, 

 the reflected ray, and in others the transmitted ray, will be 

 wanting. To explain these positions I would remind you 

 that while the glass plates maintain the same angle of inci- 

 dence towards the ray incident upon each, they may each be 

 turned into various positions. Thus either of them may be 

 so directed as to face upwards, or downwards, or towards you, 

 or towards me. Suppose, for example, that the first plate, 

 or polariser, faces upwards, and that we consider only the 

 beam, which having been reflected from the polariser is 

 either reflected from, or transmitted through, the analysing 

 plate ; then it will be seen that when the analyser faces 

 either towards you^ or towards me, the reflected rays will 

 be wanting, but the transmitted rays will be present. 

 "When, on the other hand, the analyser faces upwards or 

 downwards, the transmitted rays will be wanting but the 

 reflected present. And further, as the analyser is turned 

 round from one of these positions to the other about the 

 beam as an axis to which it always remains inclined at the 

 same angle, the relative brightness of the two beams will 

 gradually alter. When the angle of turning amounts to 45, 

 one will have increased and the other diminished in bright- 

 ness, so that both are of the same intensity ; and when the 

 angle reaches 90, the beam which had been first extinguished 

 will have attained its full brightness, while that which had 

 been bright will have been extinguished. When these pecu- 

 liarities are found to appertain to either the reflected or the 

 transmitted beam, the light so examined is said to be polar- 

 ised. And it is to be observed, first, that the direction in 

 which this peculiarity is found depends upon the position of 

 the first glass plate, or polariser. For instance, when the 

 polariser faces upwards the beam which would be reflected 

 from the analyser towards you is found to be extinguished ; 

 then, if the polariser be turned so as to face towards you, the 

 ray which would be reflected from the analyser upwards will 

 be extinguished. And so likewise for any intermediate posi- 

 tion of the polariser. Also, since the effect in question 

 appertains to the entire beam of light, it appertains to every 



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