Note 821 — Chapter VIII · THE WATERS

436William of Conches went on to explain in this connection (De phil. mundi, II, 4) why the heavens are blue, a phenomenon which some observers had attributed to the presence of waters. “What do we see up there, dense and the color of water? It is not fire, for if the air is invisible because of its great rarity (subtilitas), so also must fire be invisible, fire which is so much more rare than air. Furthermore, it is not the color of fire.” William asserted that you see nothing at all and that the impression of seeing water is an optical illusion. Unless some other color interposes, a ray of light on entering the eye takes the color of water from the aqueous humor contained in the eye.