An anamorphic amusement dating from the 16th and 17th century is described and discussed from a mathematical point of view. The Channel Anamorphosis displays two images which are cut into strips and arrayed on the alternate faces of long triangular prisms. The separate images are resolved when the array is viewed from one side or the other. Perspective corrections and finite-distance viewing of the images are discussed and analysed with examples. The 3-image channel anamorphosis is also briefly described.