Visibility

The constraints of his design on visibility, listed on the table below, created problems that Leeuwenhoek had to solve with his viewing techniques and specimen preparation methods.

Constraint Problem
nature of the specimen How to see through transparent specimens? How to light opaque specimens?
narrow angle of view How to see all of the specimen?
close proximity to lens How to light the specimen at all?
tiny aperture in plate How to get enough light on the specimen?
medium How to distinguish the specimen from its context, the surrounding structures, creatures, and fluids?

Magnification of Leeuwenhoek's specimens

 Leeuwenhoek made all his own lenses, and there was no way to calibrate their magnification exactly.

Looking back, we can measure these old lenses. The best double-lens microscopes of the seventeenth century magnified from 20 to 40 times with reasonable resolution.

Leeuwenhoek regularly used lenses that we now know magnified between 100 and 200 times. The strongest of the dozen or so surviving lenses exceeded 260 times, though to see bacteria Leeuwenhoek may have found ways to boost that even further.

20x 40x 100x 200x

40x 100x 400x

Chart showing relative sizes

 After Leeuwenhoek gained some fame, people would visit him hoping to see some of his animalcules and other discoveries. Not having sufficiently strong microsopes of their own, they wanted to look through his. At his house, Leeuwenhoek had microscopes and specimens waiting for them. One of the more popular was the one that showed the capillaries in the tail of an eel, in the red section of this chart.

In 1673, Leeuwenhoek was able to see single red blood cells circulating through the eel's capillaries, in the yellow part of this chart. Since then, people have claimed that Leeuwenhoek could not have seen things this small. However, human erythrocytes are about 5 um in diameter, well within the resolving power of Leeuwenhoek's best surviving microscope.

Resolution

These images are identical except for resolution. The one on the left is like the eye chart; you know the letters are bigger but you can't tell a B from an E.

Here, the farther away you stand, the more similar the two images will appear to you. The oval structure in the top left corner will begin to resolve first.

Of Leeuwenhoek's dozen surviving lenses, nine were measured by J. van Zuylen in the early 1980's. Their resolving power range from 1.16 to 4 microns. In other words, it will clearly show structures that small, the yellow area in the chart above.
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