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Another experiment by Young-Helmholtz the theory of colour vision
     confirms how we use the Retina cone and rod cells.  Imagine being in a

     darkened room after a few seconds our eyes adjust using our Retina rod

     cells we start to see dark shapes.



     Suddenly three light beams shine on the far wall one red one green and one

      blue.  As they overlap three secondary colours form yellow, magenta and
      cyan.




      Where all six colours overlap the three different types of Retina cone cells
      all become excited at the same time, and the colour white is recognised

      through the human eye.




      (Please see Fig. 9)
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