Claude lorrain glass - diminishing glass
A device used to reduce and simplify views of landscapes, the Claude Glass reflects the scene through a dark, convex lens, reduces and isolates color tones, and in the process, loses definition. The term is named for French artist, Claude Lorrain, who is said to have devised such an instrument. Many of the prints and drawings produced with the aid of the Claude Glass are monochrome. This device, also known as Diminishing Glass, reduces dazzle and allows the eye to dwell on the motif, which helps the artist to make the analysis of the scene in tonal terms, un-distracted by color. Source: Ralph Mayer, "A Dictionary of Art Terms and Techniques"; Kimberley Reynolds and Richard Seddon, "Illustrated Dictionary of Art Terms"