By touching the tips of the forefingers together and bringing them close to the eyes, a short finger or “sausage” can be seen floating between the fingertips.This optical illusion is discussed in W. L. Sharp's “The Floating-Finger Illusion” in Psychological Review, Vol. 35 No. 2, Mar. 1928, p. 171.
Capitalizing on this optical illusion, Michael Baker developed an acrylic finger held by a black-art wire that can “float” between your fingers. See “Optical Delusion” by Baker in Genii, Vol. 51 No. 6, Dec. 1987, p. 384; also The Linking Ring, Vol. 83 No. 3, Mar. 2003 p. 84, where Baker describes how the gimmick is constructed.