Book 20
11.26
More than this, rainbows were constantly seen; and how that phenomenon is wont to occur, a brief explanation will show. The warmer exhalations of the earth and its moist vapours are condensed into clouds; these are then dissipated into a fine spray, which, made brilliant by the sun’s rays that fall upon it, rises swiftly and, coming opposite the fiery orb itself, forms the rainbow. And the bow is rounded into a great curve, because it extends over our world, which the science of natural philosophy tells us rests upon a hemisphere.The meaning seems to be that the vault of the heavens is therefore a hemisphere.
11.27
Its first colour, so far as mortal eye can discern, is yellow, the second golden or tawny, the third red, the fourth violet, Purple varied from scarlet to violet. and the last blue verging upon green.
