Johannes Swammerdam wrote to Melchisedec Thévenot, saying that Leeuwenhoek "is biased, and reasons in a very barbarical way"

April 28, 1678

Swammerdam used a glass capillary tube (invented by Leeuwenhoek in 1674) to withdraw human blood from a louse and to observe red blood cells. In a January 1678 letter to Melchisedec Thévenot, he refers to these "small globuls" and states "nothing more beautiful is to be seen than that, especially if one lets it run to and fro, as when every globulus separately is revolving like a circle." 

In a letter in April of the same year (1678), Swammerdam noted that he began using glass capillary tubes for observing liquids after seeing Leeuwenhoek use them. Swammerdam said of Leeuwenhoek, "It is impossible to get into a discussion with him, as he is biased, and reasons in a very barbarical way, having no academic education."