LECTIO FIFTEEN

             § 427. After treating of colour and the transparent medium and luminosity, the Philosopher now proceeds to explain how the medium is related to colour. It is clear, from the foregoing, that the transparent medium is receptive of colour; for colour, we have seen, acts upon it. Now what is receptive of colour must itself be colourless, as what receives sound must be soundless; for nothing receives what it already has. The transparent medium is therefore colourless.

             § 428. But, as bodies are visible by their colours, the transparent medium must itself be invisible. Yet since one and the same power apprehends contrary qualities, it follows that sight, which apprehends light, also apprehends darkness. Hence, although the transparent medium of itself possesses neither light nor colour, being receptive of both, and is thus not of itself visible in the way that things bright or coloured are visible, it can, all the same, be called visible in the same sort of way as dark things and scarcely visible things are so called. The diaphanum is therefore a kind of darkness, so long as it is not actually but only potentially transparent: the same thing is the subject, sometimes of darkness, sometimes of light. Thus the diaphanum, while it lacks luminosity and is only potentially transparent, is in a state of darkness.

             § 429. Then at 'Not all', having decided about colour, which is made visible by light, he reaches a conclusion about that other visible object of which he said above that it had no proper name. He observes that not all things depend on light for being seen, but only the colour that is proper to each particular thing. Some things, e.g. certain animals that appear fiery and lucent in the dark, are not visible in the light, but only in darkness. There are many such things, including the fungi of oaks, the horn of certain beasts and heads of certain fish, and some animals' scales and eyes. But while all these things are visible in the dark, the colour proper to each is not seen in the dark. The things are seen both in light and in darkness; but in darkness only as bright objects, in light as coloured objects.

             § 430. The reason why they are seen shining in the darkness is another matter. Aristotle only mentions the fact incidentally, in order to show the relation of the visible to luminosity. This, however, seems to be the reason for their being visible in the dark, that such things have in their constitution something of light, inasmuch as the brightness of fire and the transparency of air and water is not entirely smothered in them by the opacity of earth. But having only a small amount of light, their brightness is obscured in the presence of a greater light. Hence in the light they appear not as bright, but only as coloured. The light in their constitution is so weak that it is unable perfectly to actualise the potentially transparent medium to receive the full effect of the colours which by nature it is fitted to receive. Hence, by this light neither their own colour, nor that of other things, is seen: but only their brightness. For brightness, being a more effective agent upon the medium than colour, and in itself more visible, can be seen with less alteration of the medium than colour requires.

             § 431. Next, at 'At present what is clear', he explains how colour actually affects sight, first pointing out what this necessarily presupposes, and then, at 'The same account holds', indicating a like necessary condition in the case of the other senses. As to the former point, he first decides what is the truth of the matter, and then at 'Democritus etc.', sets aside an error. First, then, he says that we are now clear that what is seen in light is colour, and that colour is invisible without light; and this because, as has been explained, colour of its nature acts upon a transparent medium, and it does this in virtue of light, which is the latter's actuality. Hence light is necessary if colour is to be seen.

             § 432. An indication of this is the fact that if a coloured body is placed upon the organ of sight it cannot be seen; for then there remains no transparent medium to be affected by the colour. The pupil of the eye is indeed some such medium, but, so long as the coloured body remains placed upon it, it lacks actual transparency. There has to be a medium, say air or something of the kind, which, being actualised by colour, itself acts upon the organ of sight as upon a body continuous with itself. For bodies only affect one another through actual contact.

             § 433. Then at 'Democritus etc.', he sets aside an erroneous view. Democritus, he says, was wrong in thinking that if the medium between the eye and the thing seen were a vacuum, any object, however small, would be visible at any distance, e.g. an ant in the sky. This cannot be. For if anything is to be seen it must actually affect the organ of sight. Now it has been shown that this organ as such is not affected by an immediate object--such as an object placed upon the eye. So there must be a medium between organ and object. But a vacuum is not a medium; it cannot receive or transmit effects from the object. Hence through a vacuum nothing would be seen at all.

             § 434. Democritus went wrong because he thought that the reason why distance diminishes visibility was that the medium is of itself an impediment to the action of the visible object upon sight. But it is not so. The transparent medium as such is not in the least incompatible with luminosity or colour; on the contrary, it is proximately disposed to their reception; a sign of which is that it is illumined or coloured instantaneously. The real reason why distance diminishes visibility, is that everything seen is seen within the angle of a triangle, or rather pyramid, whose base is the object seen and apex in the eye that sees.

             § 435. It makes no difference whether seeing takes place by a movement from the eye outwards, so that the lines enclosing the triangle or pyramid run from the eye to the object, or e converso, so long as seeing does involve this triangular or pyramidal figure; which is necessary because, since the object is larger than the pupil of the eye, its effect upon the medium has to be scaled down gradually until it reaches the eye. And, obviously, the longer are the sides of a triangle or pyramid the smaller is the angle at the apex, provided that the base remains the same. The further away, then, is the object, the less does it appear--until at a certain distance it cannot be seen at all.

             § 436. Next, at 'But fire', he explains how fire and bright bodies are seen--which are visible not only, like coloured objects, in the light, but even in the dark. There is a necessary reason for this, namely that fire contains enough light to actualise perfectly the transparent medium, so that both itself and other things become visible. Nor does its light fade out in the presence of a greater light, as does that of the objects mentioned above.

             § 437. Then, at 'The same account . . .', he shows how the case of the other senses is similar to sight. No sound or odour, e.g., is perceived if there is immediate contact with the organ in question. There must be a medium affected by sound or odour, which itself then affects our sense of hearing or of smell. A sounding or odorous body placed upon the organ is not perceived as such. The same is true even of touch and taste, though, for a reason to be given later, this is less evident.

             § 438. Finally, at 'The medium of sound', he states what is the medium in hearing and smelling. That of hearing is air, and that of smelling is something common to air and water--just as both of these provide a medium for colour in so far as each is a transparency. There is indeed no name for the quality in air and water which provides the medium for odour; but it certainly is not transparency. And that both air and water are conductors of smell he shows from the fact that marine animals have a sense of smell. Man, however, and other animals that walk and breathe, only smell by breathing; which proves that air is the medium of smell. This fact will be explained later.

419b 3-419b 33