"[L]ight ...is the colour of the transparent medium contingently determined; for when anything of the nature of fire is found in the transparent medium its presence constitutes light, its absence darkness. ...[T]he transparent element is nothing which is found exclusively ...in any one of the substances ...it ...finds its existence in these bodies and subsists in varying degrees in the rest of material substances. ...[T]he Pythagorean terminology identified the visible superficies with colour. ...[C]olour exists in the boundary, but it by no means is the boundary of the body ...internally there exists the same constitution as externally displays colour. ...Colour ...is the limit of the transparent element in a determinately bounded body ...both in transparent substances ...and in those which appear to have a surface colour of their own. ...[T]hat, which in air causes light, may be present in the transparent medium or it may not ...[W]hite and black may be juxtaposed in such a way that by the minuteness of the division of its parts each is invisible while their product is visible, and thus colour may be produced. This product can appear neither white nor black ...it must be a sort of compound and a fresh kind of tint. ...[C]olours ...may be produced, and ...their multiplicity is due to differences in the proportion of their composition. ...[C]olours may ...be analogous to harmonies. ...[T]hose compounded according to the simplest proportions ...as is the case in harmonies, will appear to be the most pleasant ...e.g. , ...This is one of the ways in which colours may be produced; a second is effected by the shining of one colour through another. This we may illustrate by the practice ...by painters when they give a wash of colour over another more vivid tint, when, for example, they wish to make a thing look as though it were in the water or in the air. ...[W]e may illustrate by the sun, which in itself appears white, but looks red when seen through mist and smoke. ...[W]e should have to suppose there was some ratio between the superficial and the underlying tints in ...some colours, while in others there would be ...lack of commensurate proportion. ...[Thus ...it is absurd to maintain, with the early philosophers, that colours are effluxes and that vision is effected by a cause of the efflux type. It was in every way binding on them to account for sensation by means of contact, and therefore it was obviously better to say that sensation was due to a movement set up by the sense object in the medium of sensation, and thus account for it by contact without the instrumentality of effluxes.] According to the theory of juxtaposition, just as we must assume that there are invisible spatial quanta, so must we postulate an imperceptible time to account for the imperceptibility of the diverse stimuli transmitted to the sense organ... But on the other theory there is no such necessity; the surface colour causes different motions in the medium when acted on and when not acted on by an underlying tint. Thus it appears to be something different, and neither black nor white. ...But let us premise that substances are mixed not merely in the way some people think by a juxtaposition of their ultimate minute parts ...imperceptible to sense but that they entirely interpenetrate each other in every part throughout ...The former theory accounts for the mixture only of those things which can be resolved into ultimate least parts ...On the other hand, things which cannot be resolved into least parts, cannot be mingled in this way; they must entirely interpenetrate each other; and these are the things which most naturally mix. ...[W]hen substances are mixed their colours too must be commingled, and that this is the supreme reason why there is a plurality of colours; neither superposition nor juxtaposition is the cause. In such mixtures the colour does not appear single when you are at a distance and diverse when you come near; it is a single tint from all points of view. The reason for the multiplicity of colours will be the fact that things which mix can be mixed in many different proportions ...[T]he same account will apply to the juxtaposition or superposition of colours as to their mixture. [T]hey, and likewise tastes and sounds, have definite species limited in number..."