in Thoreau’s Journal:

To Walden, to learn the temperature of the water….This is a very mild, melting winter day, but clear and bright. Yet I see the blue shadows on the snow at Walden. The snow lies very level there, about ten inches deep, and, for the most part, bears me as I go across with my hatchet.  I think I never saw a more elysian blue than my shadow.  I am turned into a tall blue Persian from my cap to my boots, such as no mortal dye can produce, with an amethystine hatchet in my hand. I am in raptures with my own shadow. Our very shadows are no longer black but a celestial blue.