In painting, however, there are two elements to be kept distinct—drawing and colour. Both these are traceable to the centres of sight, but they correspond to different sensibilities. Optical centres, which perceive with particular keenness and delicacy the distinctions between intensities of light, are the real, organic hypothesis of the talent for drawing; for what we perceive with the eye, without the aid of the senses of touch and muscle, as outline or form, is purely distinction of intensity in light. Optical centres, on the other hand, which are particularly sensitive to the variations of undulations of light, form the basis of the sense of colour and the talent for colouring. As a rule the talents for drawing and colouring appear coupled, even if one or the other preponderates, for highly developed or particularly sensitive optical centres are naturally receptive beyond the average of optical impression of every sort, of differences both of intensity and of undulations of light. This is, however, not always the case, and there are dry, sharp draughtsmen without sense of colour, and some who revel in colour without the ability to grasp the idea of form and render it plastically. In a brain that is characterised by a special morphological or functional development or sensitiveness of the optical centres, this dominates all functions of the brain, especially memory and association of ideas. The entire thinking faculty has an optical or visual character; it stands in a dependent relation to sight. Memory clings almost exclusively to reminiscent pictures of the sense of sight, and association of ideas connects mainly pictures of this category. Every perception of form calls up in the consciousness representations of form. The fancy is “inwardly completely full of figures,” as Albert Dürer quaintly and with wonderful intuition expresses it in his “Diaries.” The world-picture of this brain, always disposed to intense observation, is neither loud nor excited, but bright, flashing, and radiant like a mosaic work of precious stones and enamel. All the inner connections between the different domains of the brain are sharpened towards the optic centres, and all the activities of the brain, all emotions, all processes on the threshold of consciousness also release central optical excitements.