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.