CONSTRAINT : ϕ limited to the invisible spectrum.

INVENTION : ultraviolet “prismography” (1801). A prism on a sheet of light-sensitive material splits a beam of white light and casts a spectrum that oxidises the silver salts in the ultraviolet section.

WORKS : black ϕs of pure ultraviolet light. W.H. Fox TALBOT also took up “prismography”, and imagined lighting a room full of people using ultraviolet light only (obtained by prism filters at all the windows) and snapping them unawares (The Pencil of Nature, 1844-1846) :

“[…] the eye of the camera would see plainly where the human eye would find nothing but darkness. Alas! that this speculation is somewhat too refined to be introduced with effect into a modern novel or romance, for what a dénouement we should have, if we could suppose the secrets of the darkened chamber to be revealed by the testimony of the imprinted paper!”