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 treated of in a distinct work, which may be regarded as supplementary to the present treatise; and this will explain why the eye and vision are here very briefly alluded to, while particular attention has there been given to the ear and hearing.

visible is that for which vision is the sense, and the visible is both colour and something which is describable by words, although it happens to be without a name; but our meaning will become clear to those who accompany us in the inquiry. The visible is colour, and colour is that which is upon something visible in itself; and this something is visible, not only after its appellation but, because it has in itself the cause of being visible. All colour is motive of the diaphanous, in activity, and to be so motive is the nature of colour. On which account nothing is visible without light, but the colour of each object is visible in the light; and we must, therefore, first say what light is. There is a something diaphanous, and I call diaphanous what is visible, and yet not visible,