On Leeuwenhoek's Method of Seeing Bacteria

Abstract

In his famed letter of October 9th, 1676, to the Royal Society of London, Leeuwenhoek wrote: "My method for seeing the very smallest animalcules and minute eels, I do not impart to others; nor how to see very many animalcules at one time. That I keep for myself alone." His trick of manipulation has remained something of a mystery; although Robert Hooke, who independently repeated some of the observations, seems to have left no record of any unusual technique. Clifford Dobell (1932) has expressed a conviction that the pioneer microscopist must have hit upon some method of dark-field illumination. It will be recalled that Leeuwenhoek's "microscope" was a single, biconvex magnifying glass which he ground and then mounted between small apertures in a metal holder. The instrument was held close to the eye and the object on the other side of the lens was adjusted to proper focus. The fluids to be observed were usually held in thin glass capillary tubes. With this apparatus , he obtained sufficient contrast between floating bacteria and their background to see and describe them, often recognizably. The following experiments may perhaps throw some light on Leeuwenhoek's procedure. The first series was done with fused glass spheres about 1.6 mm. in diameter, set into bevelled 0.9 mm. apertures in sheet copper. Their constants, calculated approximately , are: magnification, 230; working distance, 0.3 mm.; numerical aperture, 0.4. These simple lenses have amazingly high power, for they can resolve, without detectable distortion, the rulings of a Wallace replica grating with 25,000 lines to the 343