All this, you may say, has more to do with light than atmosphere. But the two are really united. What we call atmosphere, as you know, is the volume of gases which surrounds the earth. The particles from these gases are lit up by the light. We cannot see the particles, only the reflections of light thrown off by them. But though we cannot see the particles themselves, they can interfere with our seeing of other things. It is the layers or veils of atmosphere that lie between us and a distant hill, that prevent our seeing the bright green grass on the latter and the dark green fir trees. Seen through the atmosphere, the colors of the hill appear subdued, the very form and bulk of the ground flattened and, perhaps, indistinct.