XPost: alt.usage.english, sci.physics   
      
   Jonathan de Boyne Pollard  wrote:   
   > > Our TV sets can show only three colours, [...]   
   > Apart from the ones with the latest gimmick: yellow.   
   > > A hypothetical Martian with four kinds of colour receptors [...]   
   > Of course, one doesn't have to go to Mars to find a creature whose   
   > visual apparatus differs from that of a human.   
      
   Even some humans have four colour receptors, apparently;   
      
      http://en.wikipedia.org/wiki/Cone_cell   
      
   has some references (including a scientific article).   
      
      
   --   
   ---------------------------------+---------------------------------   
   Dr. Paul Kinsler   
   Blackett Laboratory (Photonics)   (ph) +44-20-759-47734 (fax) 47714   
   Imperial College London,          Dr.Paul.Kinsler@physics.org   
   SW7 2AZ, United Kingdom.          http://www.qols.ph.ic.ac.uk/~kinsle/   
      
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