From: astroelectronic@t-online.de   
      
   Phil,   
      
   thanks for your answer.   
      
   > Tip it a bit so it effectively isn't a full wave plate anymore, put it   
   between crossed polarizers, and turn to get a dark fringe.   
      
   That sounds like a good idea, but my FWP doesn't behave as expected. When I   
   turn it between crossed polarizers, it changes color between pink and green.   
   Tilting doesn't change much. Then it's more pink than green.   
      
   > To find out whether you've found the fast or slow axis is a little more   
   involved, but you can do it by putting your two QWPs nearly orthogonal, so as   
   to make an almost-zero wave plate, and adjust to cancel out the retardation of   
   the tipped FWP.    
      
   If the fast axis of the first QWP is at 0° and the other at 80°, then I think   
   the resulting fast axis of the almost-zero wave plate should be at 40°. Is   
   this correct?   
      
   I did upload a picture of a sample in the circular polariscope:    
   http://www.directupload.net/file/d/3999/ys9pi7th_jpg.htm     
   The sample is a lid of an acrylic box. When I rotate the sample in the   
   polariscope, the colors do not move on the sample. I think this is correct.   
   On the left side I did hold 4 layers of 1/4 wave foil in front of the sample.   
   The fast axis is horizontal. I don't understand why the FWP doesn't change the   
   colors. I would have expected that the colors are shifted by a full order.   
      
   Michael    
      
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