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same amplitude with the quarter wave phase difference the end view in Fig, 

 2.25 would have been a true circle. Now since the vibration, in these cases, 

 is not all in one plane, the light cannot be extinguished by an analyzer; it 

 is not plane polarized light. In the one case it is called " elliptically polar- 

 ized" hght and in the other, "circularly polarized" light. If the slow ray 

 loses an integral number of wave-lengths, it makes no difference; only frac- 



Fig. 2.24 — The recombination of the light components after passing through a 



thin crystal 



tions of wave-lengths count, except that if several wave-lengths are present 

 a thickness that is right to give an integral number of wave-lengths for one 

 color may give an integer plus a fraction for some other color. If the thick- 

 ness is fairly small, this may cause spectral colors from white light. For 

 thick plates the wave-lengths so overlap that the field appears colorless 

 but dark or white according to the value of the angle a; i.e., if a is zero or 



