Do you remember, it was as patches we saw the cows which we met early in our talk? The reason then given for their appearance was that our eyes were not strong enough to distinguish their details at such a distance. And this reason also holds good in the case of the wagon. But it is not only the distance that reduces our power of seeing, but also the layers, or veils of atmosphere that hang between us and the object. We are sure of this on a foggy day, when the mist lies low over the country or city, and trees and tall buildings loom up like blurs, and everything beyond the distance of a few hundred paces is blotted from sight. But the fog or mist is only the atmosphere more moist than usual and with its moisture condensed by cooling.