We have seen in Chapter 11 that the magnifying power of a single lens may be increased by decreasing the focal length, but there is a practical limit to this procedure. Then, the lens becomes too small, and during observation, object, lens, and eye must be brought very close together. A practical limit is a magnifying power of approximately 100×, with a focal length of approximately 2 mm. Anthony Van Leeuwenhoek in the seventeenth century in Holland made several microscopes using a minute single lens with very strong curvatures, obtaining magnifying powers of the order of 100. Two problems with this simple design are the low numerical aperture (0.2 NA) and the small field. With these simple microscopes, he discovered the first microorganisms.