Two computers the back of the eye and the brain

It is interesting to study the reaction of the eye to mixtures of photons of different wavelengths. Thus, for example, a mixture of 'red' and 'green' light produces a sensation identical to that produced by 'yellow' light alone. This is quite different to the reaction of the ear to sounds of different wavelengths. A trained ear can recognise a chord of music as a mixture of notes. By 'listening hard' the components of the chord can be distinguished. Not so in the case of light. No matter how hard we look at light which appears yellow, it is impossible to tell whether it is a pure beam of yellow light, or a mixture of red and green. The microcomputer at the back of the eye has sent the signal 'yellow' to the brain. The brain, our 'mainframe computer', accepts this signal, and has no more information on the original input data.

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