Seeing Through the Sky

In his book Modern Painters, John Ruskin speaks of looking not at a sky, but through it.

“It is not flat dead colour, but a deep, quivering, transparent body of penetrable air, in which you trace or imagine short falling spots of deceiving light, and dim shades, faint veiled vestiges of dark vapour.”

Painting: Thomas Hill (1829-1908), View of Yosemite Valley, 1871. Thanks, Armand.
Wikipedia on John Ruskin and Modern Painters