having reached the disc behind, shines brightly thereon; and this is the reason why the image appears only in the glass which has lead at the back. From this pupil the visual spirit, which is continued from it to the part of the Brain, the anterior, where the sensitive power is, suddenly, without loss of time, depicts it as in the clear spring of a fountain; and thus we see. Wherefore, in order that its vision be truthful, that is, such as the visible thing is in itself, the medium through which the form comes to the eye must be without any colour, and so also the humour of the pupil; otherwise the visible form would be stained of the colour of the medium and of that of the pupil. And this is the reason why they who wish to make things appear of a certain colour in a mirror interpose that colour between the glass and the lead, the glass being pressed over it. Plato and other Philosophers said, indeed, that our sight was not because the visible came into the eye, but because the visual virtue went out to the visible form. And this opinion is confuted by the Philosopher in that book of his on Sense and Sensation. Having thus considered this law of vision, one can easily perceive how, although the star is always in one way bright, clear, and resplendent, and receives no change whatever except that of local movement, as is proved in that book on Heaven and the World, yet from many causes it may appear dim and obscure; since it may appear thus on account of the medium, the atmosphere, that changes continually. This medium changes from light to darkness, according to the presence or absence of the Sun; and during the presence of the Sun the medium, which is transparent, is so full of light that it overpowers the star, and therefore it no longer appears brilliant. This medium also changes from rare to dense, from dry to moist, because of the vapours of the Earth which rise continually. The medium, thus changed, changes by its density the image of the star, which passes through it, makes it appear dim, and by its moisture or dryness changes it in colour. In like manner it may thus appear through the visual organ, that is, the eye, which on account of some infirmity, or because of fatigue, is changed into some degree of dimness or into some degree of weakness. So it happens very often, owing to the membrane of the pupil becoming suffused with blood, on account of some corruption produced by weakness, that things all appear of a red colour; and therefore the star appears so coloured. And owing to the sight being weakened, there results in it some dispersion of the spirit, so that things do not appear united, but scattered, almost in the same way as our writing does on a wet piece of paper. And this is the reason why many persons, when they wish to read, remove the paper to some distance from the eyes, in order that the image thereof may come within the eye more easily and more subtly, and thereby the lettering is left impressed on