Illumination

For Leeuwenhoek, more light was not always better. Early in his career (June 1674), he wrote to Oldenburg:

You then hold the microscope towards the open sky, within doors, and out of the sunshine, as though you had a telescope and were trying to look at the stars in the sky through it.

And later:

The instrument may be held within doors and in the shade, yet held to the free Air, as is with a Telescope you would look upon the Stars in the Firmament.

A quarter-century later, in June 1699, he wrote:

Above all things you must have a care, not to make your view in the Sunshine, for if you do so, the Circumference of each Animal, will have almost as many Colours, as we see in the Rainbow

Was the specimen transparent? Then he used transmitted light -- more light from behind and a smaller aperture.

Was the specimen opaque? Then he used reflected light -- more light from the sides and a larger aperture.

After Leeuwenhoek's death, the Royal Society's Henry Baker wrote (p. 518):

Mr. Leeuwenhoek says ... that sometimes, to throw a greater Light upon his Objects, he used a small convex Metal Speculum.

Baker was probably referring to what Leeuwenhoek wrote on June 9, 1699:

To have still more light, I use sometimes a metal Concave Looking glass.

He could have polished the bronze plate around the lens to direct light back onto his side of an opaque specimen. He could also have placed a thin mirror or white paper on the bronze plate to reflect light back on the specimen. Baker continues:

He had filed the Brass which was round his Microscope, as bright as he could, that the Light, while he was viewing Objects, might be reflected from it as much as possible.

He tried microscopes of silver perhaps because, polished, they would reflect light onto the specimen better than bronze. He made so few microscopes of gold perhaps because, not reflecting any better than bronze, they were not worth the expense.

Was the specimen hard to distinguish? To increase the contrast, he manipulated it against a dark background; later in his career, he once tried a saffron stain.

Both Dobell and Cohen speculate about the potential of dark-field or dark-ground illumination. Through decades of using his instrument, Leeuwenhoek may have found methods to compensate for the transparency and low optical contrast of many of his specimens. Shining light on the specimen from the side while pointing the instrument toward a dark background would create a good-enough dark field illumination effect, as would slowly moving a finger or stick just off the centerline between the light source and the scope and then slowly moving it back towards the center. Think about how the dust in a room, dancing in the sunlight, can be made visible by viewing it from the side against a dark curtain or piece of furniture.

visibility Drawing by Robert Hooke for Micrographia shows his illumination system (click to enlarge). On the right are two photos of the same piece of metal, dark field illumination on top and bright field illumination on bottom.

What about aberrations? The way Leeuwenhoek used his microscopes, he created an aperture by mounting the sphere behind a slightly smaller hole in a metal plate. With an aperture smaller than the lens, the microscope had fewer problems with spherical aberrations. And with the eye so close to the lens, the chromatically aberrant edges were off to the periphery of his vision.

Thomas Molyneux's report to the Royal Society refers directly to Leeuwenhoek's "reflective specula" as well as "secrets":