John Ruskin (1873). “The Eagle's Nest: Ten Lectures on the Relation of Natural Science to Art, Given Before the University of Oxford in Lent Term, 1872”, p.34
John Ruskin “Two paths. Lectures on art, delivered before the University of Oxford in Hilary Term, 1870. Aratra Pentelici. Time and tide. Pre-Raphaelitism. The eagle's nest. King of the Golden River. The relation between Michael Angelo and Tintoret”
John Ruskin (1873). “The Eagle's Nest: Ten Lectures on the Relation of Natural Science to Art, Given Before the University of Oxford in Lent Term, 1872”, p.80