John Ruskin,
Modern Painters, Volume 1 (of 5…
“ In the whole range of Turner's works, recent or of old date, you will not find an instance of anything near enough to have details visible, painted in sky blue. Wherever Turner gives blue, there he gives atmosphere; it is air, not object. Blue he gives to his sea; so does nature;—blue he gives, sapphire-deep, to his extreme distance; so does nature;—blue he gives to the misty shadows and hollows of his hills; so does nature: but blue he gives not, where detail and illumined surface are visible; as he comes into light and character, so he breaks into warmth and varied hue ”
