Additional magnification

While we always think of Leeuwenhoek as working with a single magnifying lens rather than the system or two or more lenses in a microscope, his methods of using this lens could have introduced another lens.

In addition to the lenses themselves, he wrote in one of his first letters, September 1674, that he blew:

very slender hollow glass pipes, of which some were not thicker than a mans-hair; and the slenderer they are, the clearer will they make the red Globuls of the Blood appear.

The thin glass tube containing the liquid could further magnify the specimen, as could an air bubble, creating in just the right lateral light a dark background that Leeuwenhoek compared to "sand-grains on black taffeta." Leeuwenhoek wrote in Letter 30 of April 5, 1680:

The reason why I caused it to be drawn hanging in water is that the filaments hang better separated thus than without water, and so can better be distinguished, especially if one allows the light of a candle to shine through the glass. But knowing that everything in water shows bigger than outside it, we must not take the filaments to be as thick as they are drawn here.

Birch's History of the Royal Society records three meetings, November 1 and 8, 1677, and March 14, 1678, when Robert Hooke, defending Leeuwenhoek, suggested ways from his own experience that Leeuwenhoek could have used to achieve results that members of the Royal Society doubted.

Hooke started with thin glass pipes, some ten times thinner than a human hair (my emphasis).

The discoveries, affirmed to be made by Mr. LEEWENHOECK, were made by help of viewing with a good microscope such small pipes containing the liquor or water, in which those multitudes of exceedingly small insects or animals wriggling among each other are discovered;

for that he alledged, that the said pipes being filled with liquors became themselves as it were magnifying glasses, augmenting such bodies, as swim in the said liquor, on those parts of the said pipes, which are farthest from the eye-glass;

for the pipes themselves being looked on by the help of a very good microscope, are made very large and conspicuous; and they again augmenting the opposite parts by the refraction on their cylindrical surfaces double the effect of a single microscope, as was very evident.

A week later:

Mr. HOOKE suggested same farther improvement of that instrument by making use of the convexity of the surface of the liquor itself (put upon the plates of Muscovy glass) for augmenting the body within the liquor.

At the March meeting, Hooke read part of what he would publish later that year in Microscopium about his own work with grains and seeds steeped in water (aka infusoria):

[Hooke] discovered the several ways and contrivances by which he made those observations; and therein shewed how easily and apt such persons are to be deceived by the appearances of these transparent bodies through a microscope, who are not aware of certain properties of transparent bodies, especially such as are peculiar to substances of such small bulk.

And for the avoiding and preventing all these inconveniences, he shewed several ways and expedients, without which no true discovery could be made, and by the help of them they were very easily made.

Some of those mentioned by him were glass plates, and plates of Muscovy glass, particular kinds of light, the immersing the bodies in waters and other liquors, the squeezing bodies between two glass plates, the stretching and squeezing others with a kind of tongues, etc. whilst they are looked upon in a convenient light by the eye.