as to observation of the phenomenon. If the interval were small, the fact might, indeed, escape us; but, extended as it is from the East to the West, the postulate is too extravagant to be admitted.
Now that which is without colour is receptive of colour, as that which is without sound is receptive of sound; and that which is without colour is the diaphanous and the invisible or scarcely visible, such as darkness seems to be. Such too is the diaphanous; but then it is the diaphanous, not in potentiality but, in reality; for the same nature is sometimes darkness and sometimes light. But all objects are not visible in light, as there are some of which the peculiar colour only of each is visible; for some, not visible in light, produce sensation in the dark, as certain fiery brilliant appearances (which have no special appellation,) which emanate from fungi, horn, scales and eyes of fishes, but the peculiar colour is not seen of any one of those objects. It is foreign to our present purpose to explain how such objects become visible; but this much is manifest, that it is colour which is visible in light. Therefore, without light colour is not visible; for it is an essential property of colour to be motive of the diaphanous in activity, and the reality of the diaphanous is light. As proof of this, if any coloured object be placed over the sight, the object will not be seen, and yet there is colour, which is motive of the diaphanous, the air, that is, and, by
