Even if we assumed that all these difficulties 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 be much more likely to be accurate than the latter and more difficult attempt.
Besides this, possibly, as the eyes of some families of fish are more highly developed than those of others, the relative shade values of colour might be different to the different families, so that if we eliminate colours from our lures, we must have different shading for different fish.
Having considered all these things carefully, I have come to the conclusion that it will be much safer and easier to keep on using colours in our imitations, even if we do present these imitations to a monochromatic eye.
Since writing the above article, I have been able to collect some further information with regard to the probable power of the trout’s eye to discriminate between colours.
These researches, though I have not yet had time to carry them as far as I had hoped, have led me to believe more firmly than ever that I am right in recommending the use of colours in our imitation flies. I have prepared some sections of the retina of the trout, and examined them carefully in comparison with the retinæ of several other fish. A short account of what is known at present of colour-vision is, I think, advisable to make my meaning clear to those of my readers who may not be sufficiently well versed in this particular subject.
The sensation of an individual colour is produced by rays of light of a particular wave-length falling upon the retina. A sensation of “white” is produced by rays containing all the wave-lengths which are able to affect it. When, on looking at an object, we find that neither a colour nor white sensation is produced, this sensation is called “black.”
The white sensation may be mixed with the sensation of any colour of the spectrum, as also may the sensation of black, and when these two are mixed they produce a sensation of “grey.” Some colours of the spectrum are probably produced by a mixture of various wave-lengths of different primary colours, and many colours in nature do not exist in the spectrum.
The word “tone” expresses variations of wave-lengths within a named colour, and “brightness” is used to indicate the intensity of the sensation produced upon the retina.
The enormous difficulty of working out into a monochrome the shade-values of a collection of colours, with several tones and shades of brightness in each of the variously coloured parts of the object we wish to imitate, can be imagined on considering these facts only; but there are more facts which lead me to believe that to do this is not only difficult, but impossible.