184 OPTICAL PRINCIPLES. 



and two in the other ; and, the primary rays of the 

 two sets in each plane being in different stages or 

 phases of undulation, in consequence of the retarda- 

 tion of the extraordinary rays, the undulations of cer- 

 tain coloured rays check and annihilate each other, 

 while the remainder or complementary conspire and 

 pursue their course, producing the appearance of 

 colour, this effect being reversed at each quarter- 

 revolution of the analyzer. 



An idea of what is meant by phases of undulation 

 may be obtained by reference to fig. 29, in which the 

 undulations, , b, are in similar states or phases, and 

 so conspire in action, while the wave c is in a different 

 phase and half an undulation behind the others; 

 hence it would check or interfere with either of the 

 other waves (a, b), the etherial molecules of the two, 

 which vibrate perpendicularly or at right angles to 

 the direction of the wave, acting to the same extent 

 and in opposite directions. 



In fig. 30 the analyzer is represented as composed 

 of a natural crystal (rhomb) of calcareous spar, which 

 transmits both sets of rays; but in the ordinary 

 analyzer or NicoPs prism which is made by dividing 

 a rhomb through the obtuse angles into two wedge- 

 shaped pieces and cementing them together again 

 with balsam, only one set of rays is transmitted at 

 each quarter-revolution, the other being refracted out 

 of the field. In the case of the tourmaline, one of the 

 sets of rays is absorbed ; so that the tourmaline, like 

 the NicoFs prism, is single-imaged. 



Thus the colours produced by polarization are the 

 same as those of the spectrum, but separated in a 

 different way, both arising from the elementary co- 

 loured rays of the compound white light. For while 

 the spectral rays are separated by dispersive refrac- 

 tion, the polarized coloured rays are separated by the 

 interference and annihilation of some rays, the re- 

 mainder passing on to produce the colours. 



