Mercury and Argos

Here is a passage from Modern Painters, illustrating some of Ruskin's ideas on colour as described in the third part of the Elements of Drawing:

"In the Mercury and Argus, the pale and vaporous blue of the heated sky is broken with grey and pearly white, the gold colour of the light warming it more or less as it approaches or retires from the sun; but, throughout, there is not a grain of pure blue; all is subdued and warmed at the same time by the mingling grey and gold, up to the very zenith, where, breaking through the flaky mist, the transparent and deep azure of the sky is expressed with a single crumbling touch; the keynote of the whole is given, and every part of it passes at once far into glowing and aerial space. (Modern Painters v. I, pp. 292-293)"

J.M.W Turner: Mercury and Argos (c.1836) National Gallery of Canada