Thoreau writes in his journal:
There are no skaters on the pond now. It is cool today and windier. The water is rippled considerably. As I stand in the boat, the farther off the water, the bluer it is. Looking straight down, it is a dark green. Hence, apparently, the celestial blueness of those distant river-reaches, when the water is agitated, so that their surfaces reflect. the sky at the right angle . . .