Christiaan Huygens began a series of observations of little animals replicating and expanding on Leeuwenhoek's

Using his own single-lens microscope, Christiaan Huygens infused rainwater with pepper, ginger, and later coffee to observe the same little animals that Leeuwenhoek had.

Huygens kept a day-by-day and sometimes hour-by-hour record of his observations through 22 December 1678, pick up again in 1679, and finishes with one observation in 1680, on 7 March, when he reports seeing little animals in his own urine.

These observations, with many drawings of little animals, are found in Oeuvres Completes, vol. XIII. Dioptrique

Appendice XI, À la troisième Partie de la Dioptrique ‘De Telescopiis et Microscopiis’. 1678-1692.
Observations microscopiques. Première Partie 1678-1680.