E.F. Grün Liquid Lens 2.5/6"

Black enamelled English brass lens engraved in white 'Liquid Lens, E.F. Grun's Patent, No.217, 6" Focus' and aperture scale from f/2.5 to f/20, the rear space between the elements filled with a light green coloured oil. In very fine condition. Ex Spira Collection. Four other lenses are known to exist with serial numbers 202, 206, 208 and 239. In 1901, Dr. Edward F. Grün of Brighton, England, patented (Brit. Pat. 6,194/01; U.S. Pat. 695,606) a lens of the Petzval type in which the two rear elements were separated by a layer of oil in place of air. The doctor, knowing little about optics, assumed that this would improve the lens in a way similar to the way the oil-immersion microscope lens functions. It did, however, not perform even as well as a conventional Petzval lens of the same focal length and aperture. Desptie extensive initial publicity, sales were minimal and by 1903 the lens had apparently been discontinued.