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
"Classroom Without Walls". Explorations Magazine, Volume 7, 1957; reprinted in "Explorations in Communication" edited by E. Carpenter and M. McLuhan, 1960, and in "McLuhan: Hot and Cool" edited by Gerald Emanuel Stearn, 1967.