A. van Leeuwenhoek's Microscopes: Upside Down?

Mikx, F.
The Lancet
321, no. 8338, pp. 1387-88

A quotation from the article:

Every time van Leeuwenhoek describes the observation of bacteria he gives an extended explanation of the sample preparation of infusions of pepper seeds in rainwater or suspensions of white matter from between his teeth in rainwater or saliva. Thus van Leeuwenhoek made his observations in wet preparations, most probably a hanging drop. The curvature of this drop and its magnifying effect have been generally neglected, although refraction of the hemispherical surface of a water column has been postulated as the "secret method" of van Leeuwenhoek. A waterdrop can give a magnification factor of x 2, as can be seen by putting a drop of water on the millimetre scale of an ordinary ruler. This magnifying effect turned van Leeuwenhoek’s simple microscope into a true compound microscope, with a magnification factor of x 500 or more. To create a hanging drop the objective screw must have been pointing down, which was probably the way the microscope was used.

I suggest that van Leeuwenhoek in some ofhis microscopes used a combined lens system-namely, the glass lens and the curvature of the fluid drop wherein the bacteria were suspended. Together with dark ground illumination this would explain the detailed observations of spiral shaped and other motile bacteria. For this "particular method of observation" Antoni van Leeuwenhoek most probably held his microscope upside down.

The other possibility is that Leeuwenhoek put the liquid samples in capillary tubes. The curved surface of the tube would have boosted the magnification as much as a water drop.