COLOUR PERCEPTION IN FISH 19 



the same as the impression conveyed to 

 our eyes. We have an example of this 

 always with us in the photograph, where 

 red and blue, in relation to each other, 

 certainly do not produce the same effects 

 on the plate as they do on the eye ; and as 

 we have no accurate knowledge as to the 

 effect of contiguous colours upon a nor- 

 mally monochromatic eye, we could hardly 

 be certain of producing an accurate mono- 

 chromatic imitation of a "multi-coloured 

 object, which would deceive that eye. 



The case of a colour-blind human being 

 is certainly not a normal case, so the shade 

 value of the various " colours to this eye 

 could hardly be taken as a safe standard. 



Even if we assumed that all these diffi- 

 culties had been surmounted, and that 

 the exact relative shade values to this 

 monochromatic eye of every colour were 

 estimated, I think that there can be no 

 doubt that it would be easier to imitate" 

 the colours, with the various shades in 

 these colours, than to calculate out the 

 relative shade values of the different 

 colours, in one particular colour, and that 

 the result of the former and easier, would 



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