Later in the day, if the weather is very hot the air, close above the ground, becomes so heated that it rises very quickly, and we see a shimmer of light upon its shifting patches. I mention this, because I wish you to think of atmosphere, not only as veils of gauze hung between us and objects we are looking at, but also as a moving, palpitating, vibrating fluid. We will talk a little more about this presently. Meanwhile, let us note some of the effects of atmosphere upon form and color.