I will now show on the screen some representations of pictures painted by colour-blind persons. The upper picture is the copy, and the one below is the one which has been painted by the colour-blind artist. He has been given a selection of colours on plates, and from them has selected the one which he thought appropriate in each case. It will be noticed that the mistakes made are characteristic of the colour-perception of the person painting them. Whenever I show these pictures, I am asked why it is that these characteristic mistakes should be made, and that the true colour of the object is not used instead? This undoubtedly would be the case if the artist were allowed to match the colours by directly comparing them. But he is not able to do this; he looks at the copy and decides upon the colour of an object, and then looks for the colour with which to paint it. A man rarely uses a hue which he does not see as a definite colour, and therefore it has been quite possible for me to pick out those who are more or less colour-blind in the exhibitors of the picture gallery. For instance, if a trichromic have to paint a yellow object he will decide, after looking at it, whether it be a red or green in his estimation, and represent it accordingly. He will be greatly influenced by the nature of colours in its immediate proximity, because simultaneous contrast is increased in the colour-blind. Thus he will certainly represent a yellow which is adjacent to a red as green, and a yellow which is adjacent to a green as red. THE EVOLUTION OF THE COLOUR-SENSE There can be no doubt that an evolution of the colour-sense has taken place: the only point is how and when did this occur. It is obvious that in those low forms of animal life in which the most rudimentary sense of sight exists there can be no sense of colour. The animal which can only perceive light and shade can only discriminate in a rough way between varying intensities of the stimulus. It is obvious, therefore, that the sense of light must have been developed first and then the sense of colour. The sense of sight must have been first developed for those waves which produce their maximum effect upon the sensitive protoplasm. The next process of development would be for the protoplasm to become sensitive to the waves above and below those which produced the primary stimulus. In the physical stimulus which produces the sensation of light there are two factors to be considered, the length of the wave and its amplitude: the greater the amplitude within certain limits the greater the intensity of the sensation. The wave-length of the physical stimulus is the physical basis of the sensation of colour. How did the sensations of colour first arise? Let us suppose that the physiological effect of the physical stimulus differed according to the wave-length of the physical stimulus.