All the facts which can be gathered from the study of museums or literature point to the conclusion that the sense of light was developed first, then the sense of colour. The tendency has been to regard colour-blindness as “chromic myopia;” but this is not correct unless there is a defective perception of light as well, as shown by the cases which I have recorded. A man may be able to see light of all colours at twice the normal distance, and yet be colour-blind. I specially wish to emphasise the fact that there is no definite relation between light and colour. When light falls upon the eye it sets up a nerve impulse, which is conveyed to the brain. In the impulse itself we have the physiological basis of light, and in the quality of the impulse the physiological basis of colour. My contention is that these two factors are perceived by two entirely different sets of cerebral cells, those devoted to the perception of colour being developed at a later period than those conveying to the mind the sensation of light. All the evidence which can be obtained shows that all objects were first seen as in a photograph, that is, in different degrees of black and white. In the evolution of the colour sense those waves which differ most physically, namely, red and violet, were first recognised as different, the remainder of the spectrum appearing grey. Homer's colour vision was of this class, which represents the degree just preceding total colour-blindness. I have recorded a case of this kind of a man who was colour-blind with one eye, and who was therefore able to tell me exactly how objects appeared with this eye. He said that the spectrum appeared nearly all grey, but with a tinge of red at one end and a tinge of violet at the other; he could see very much better with the colour-blind eye than with the other.
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