Formerly no one seems to have noticed how atmosphere affects every object that is seen through it.

The painters had to show us that it is so.

After we had seen the effect of atmosphere in pictures we began to be able to see for ourselves in nature, and thus a whole group of new pleasures in views of nature was opened up to us.
Away back in the Middle Ages, six hundred and more years ago, folks had far less educated eyes than we possess to-day.

They looked at nature more simply than we do and saw less in it.

So they were satisfied with pictures that omitted a great many features we cannot do without.
But painting does not only concern itself with representing the world we actually see and the people that our eyes actually behold.