Perceived world and imaginal world appears together in the phenomenon of blue bird feathers. A vivid blue hue is not a pigment, not a dye, but a reflection of light that bounces off the thin opaque covering of the black physical feather.
Hillman uses the optical physics of blue bird feathers — blue that is visually present yet materially absent — as the paradigmatic demonstration that the imaginal world is real in experience without being materially grounded.
, Alchemical Psychology, 2010thesis