Explain the metaphor in this quote from "Walden". "It is far more glorious to carve and paint the very atmosphere and medium through which we look?"

Thoreau is using the metaphor of the artist in this quote.  He is explaining that the artist can carve and paint a scene to make something look beautiful, such as a bowl of fruit.  He is comparing that process to humanity and the human perspective.  He is saying that the way we look at things can be "carved and painted" by our perception and our attitude.  Things will look beautiful we we look at them with beauty in our mind and heart.

He takes this idea a little further in order to promote morality.  He says that if we are morally righteous - meaning free from sin - then our perception will be clean.  Thus, the world around us with be clean and "glorious".

"Every man is tasked to make his life, even in its details, worthy of contemplation of his most elevated and critical hour."

The details of the world we carve out for ourselves should be representative of our ideals.