Source, in full
"The Glass in the Field", The New Yorker (31 October 1939); Fables for Our Time & Famous Poems Illustrated (1940). This is the moral of a fable in which several birds reject a Goldfinch's report that he ran into "crystallized air" while flying across a field, where workmen had left a large plate of glass upright. The Swallow rejects the offer to come along with others and prove the Goldfinch wrong.