Leeuwenhoek Measures Red Blood Cell Diameter

In a letter to Constantijn Huygens dated April 1674, Antonie van Leeuwenhoek described measuring human red blood cells using his single-lens microscope. By comparing clusters of cells against sand grains of known size, he estimated each cell at approximately one three-thousandth of an inch, equivalent to roughly 8.5 micrometers. This calculation came close to the modern accepted diameter of 7 to 8 micrometers. The measurement demonstrated that microscopy could yield quantitative data about biological structures.