Engraving of a Wilson screw barrel microscope (1720), designed by James Wilson, an English optical instrument maker. The microscope is about 1 inch (2.54cm) in diameter & 2 inches (5.08cm) long. This was a single-lens microscope, a development on Antoni van Leeuwenhoek's earlier idea. The lens (L) fitted inside the plate (K), which was then screwed into the end of the microscope at (G). The screw mount was for focusing. The specimen was viewed by holding the handle (I) & looking at a light through the lens (K). The other end (C) held the condenser lens. (M) is the slider holding four specimens mounted between bits of mica. Reference: MICROCOSMOS, pages 186-187.