140 THE MICROSCOPE. 



/, light polarised by e, by stopping the vibrations c d, 

 and transmitting those of a b. 



g, a piece of selenite of such a thickness as to produce 

 red light, and its complementary colour green. 



h, the polarised light / bifurcated, or divided into ordi- 

 nary and extraordinary rays, and thus said to be de- 

 polarised by the double refractor g, and forming two planes 

 of polarised light, o and e, vibrating at right angles to 

 each other. 



i, a second plate of tourmaline, here called the analyser, 

 with its axis in the same direction as that of e, through 

 which the several systems of waves of the ordinary and 

 extraordinary rays /*, not being inclined at a greater 

 angle to the axis of the analyser than that of 45 degrees, 

 are transmitted and brought together under conditions 

 that may produce interferences. 



k, the waves R o and R e, for red light of the ordinary 

 and extraordinary systems meeting in the same state of 

 vibration, occasioned by a difference of an even number 

 of half undulations, and thus forming a wave of doubled 

 intensity for red light. 



I m, the waves Y o and t e and B o and b e for yellow and 

 blue of the ordinary and extraordinary systems respec- 

 tively meeting together, with a difference of an odd 

 number of half undulations, and thus neutralising each 

 other by interferences. 



n, red light, the result of the coincidence of the waves 

 for red light, and the neutralisation by interferences of 

 those for yellow and blue respectively. 



h, fig. 87 b, depolarised light, as fig. 87 A. 



i, the analyser turned one quarter of a circle, its axis 

 being at right angles to that of i in fig. 87 A. 



k, the waves R o R e, for red light of the ordinary and 

 extraordinary systems meeting together with a difference 

 of an odd number of half undulations, and thus neutral- 

 ising each other by interference. 



I m, the waves y o Y e and bo Be, for yellow and blue of 

 the two systems severally meeting together in the same 

 state of vibration, occasioned by the difference of an even 

 number of half undulations, and forming by their coin- 

 cidences waves of doubled intensity for yellow and blue 

 light. 



