"So the atmosphere appears blue because of the darkness which is beyond it; and if you look towards the horizon of the sky, you will see that the atmosphere is not blue, and this is due to its density; and so at every stage as you raise your eye up from this horizon to the sky which is above you, you will find that the atmosphere will seem darker, and this is because a lesser quantity of air interposes between your eye and this darkness. And if you are on the top of a high mountain the atmosphere will seem darker above you just in proportion as it becomes rarer between you and the said darkness; and this will be intensified at every successive stage of its height, so that at the last it will remain dark."